TY - JOUR
AU1 - Arunachalam, Lakshmanan,
AB - Abstract CaSO4:Dy is a reliable high-sensitive themoluminescent phosphor useful for low-level and high-level radiation measurements as it exhibits fading free linear dose response with a single glow peak at ~230°C in these dose regions. For large-scale radiation protection dosimetry service, it is embedded in Teflon matrix with varying thicknesses. Extensive studies have been carried out with such CaSO4:Dy Teflon discs in individual and environmental radiation monitoring applications including its capability to measure International Commission on Radiation Units and Measurements operational quantities. The review highlights their development and application in high-energy photon measurements, thin wafers and graphite-loaded Teflon discs for beta-dosimetry, phosphor-filled aluminium discs for high-dose applications, 6LiF-mixed CaSO4:Dy Teflon discs for thermal and albedo or moderated fast neutrons, sulphur-mixed CaSO4:Dy pellets for fast-neutron exposure even in the presence of gamma-rays and polyethylene-mixed CaSO4:Dy discs for fast neutrons. INTRODUCTION Radiation is present everywhere on the Earth’s surface since its origin. According to United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR)(1), ~87% of the radiation dose received by mankind is from natural sources and the remaining is due to anthropogenic sources. The dose received by the population in a region comprises of (1) external gamma radiation dose due to cosmic rays and primordial radionuclides, (2) inhalation dose due to radon, thoron and their progeny and (3) ingestion dose due to the intake of radionuclides through the consumption of food, milk, etc. Environmental gamma ray background generally refers to the gamma radiation from radioactivity in the environment, i.e. from terrestrial sources and building materials. The assessment of external exposure due to terrestrial radiations is possible and in a given place its distribution is found to be dependent on the geographical characteristics of that place. There are several international studies reported for measurement of terrestrial gamma radiation background levels using the technique of thermoluminescence (TL) to assess the dose to the population. Loose powder requires careful weighing and is cumbersome to handle especially while handling dosemeters in large numbers required in routine personnel and environmental radiation dosimetry. Solid form of TL dosemeters (TLDs) such as sintered pellets, extruded ribbons or rods, Teflon discs and glass capillaries, is preferred for such applications. Some of the special features of the Dy or Tm doped CaSO4 developed originally by Yamashita et al.(2) are (1) low cost; (2) TL sensitivity 40 times higher than that of LiF:Mg,Ti; (3) inertness to heat treatments below 700°C, unlike that of LiF:Mg,Ti; and (4) very high storage stability in ambient climatic conditions(3). India is using CaSO4:Dy loaded in Teflon for individual and environmental radiation monitoring in nuclear and other industries and medical institutions(4, 5). It exhibits fading free linear TL response with dose in the low level (10 μGy–0.1 Gy) as well as at high levels (0.1–7 kGy)(6) with a single glow peak at ~230°C. However, in the intermediate dose range (0.1 Gy–0.1 kGy), CaSO4:Dy exhibits supralinear TL response with gamma-dose due to trap competition(7). Other advantages of the CaSO4:Dy phosphor include exposure-rate independence, very low self-irradiation rate, which is a necessary requirement for low level such as environmental radiation dosimetry and its low neutron sensitivity, which is a necessary requirement for photon dosimetry in mixed radiation fields This phosphor is widely used in the dosimetry of X- and gamma-rays, beta rays, neutrons and high-energy cosmic rays. From the data of several international intercomparisons of environmental dosemeters(8), it can be seen that CaSO4:Dy/Tm is one of the TLDs widely used for environmental accumulative dose monitoring. For example, CaSO4:Dy TLDs with a copper compensation holder are mainly used for a nation wide survey of natural background in China. In Bulgaria, CaSO4:Dy TLDs covered with 2 mm Sn filter are routinely used for environmental gamma background measurements in the area around a research reactor as well as several other sites located in various parts of the country. A Reuter Stokes pressurised ionisation chamber RS111 was also used for measurements at some of these sites. The data obtained by the two methods were in satisfactory agreement(9). However, CaSO4 is a very soft (hardness = 3.5) material. CaSO4-based TLDs are therefore not amenable for making sintered pellets without a binding material which usually interferes with its TL properties. Therefore, other solid forms are usually in vogue as described below. LOOSE POWDER Loose powder, i.e. small grains of phosphor, encapsulated in plastic containers, was the first form of TLD and is still widely used. It has the advantage of flexibility, since a variety of shapes and sizes can be contrived by altering the container. The disadvantages of TLD powders, however, are the general inconvenience of handling powdered materials and a sensitivity variation with particle size due to light scattering and other effects. Furthermore, for low-dose measurements, it is essential to use inert gas flow to suppress large non-radiation induced luminescence (triboluminescence and chemiluminescence). The grain sizes generally used for routine measurements are in the range 75–210 μm, since they are free running and convenient to handle. Smaller grains tend to stick to each other and to their container due to electrostatic charges, while larger grains result in uneven heating and too few particles per sample for adequate averaging out in sensitivity. The particle size distribution changes with re-use, as smaller grains are produced by handling. Generally ~30 mg powder is used per readout, though the optimum weight may vary from 10 to 40 mg, depending on the design of the heating tray. After correcting individual readings for weight, coefficients of variation of as low as 0.5% can be obtained. The limiting factor on precision is then the reproducibility with which the powder can be distributed in the heating tray. Although most accurate results are obtained by individually weighing each sample, vibrating powder dispensers can be used to dispense weights with a coefficient of variation of <1%. Hence, loose powder is still generally a first choice when accuracy is of paramount importance such as medical dosimetry in radiotherapy. GLASS CAPILLARIES Since for routine applications, handling of loose powder is quite cumbersome, Pradhan et al.(10) introduced the idea of using sealed glass capillaries containing equal amount of phosphor powder. Glass capillaries encapsulating CaSO4:Dy and Li2B4O7:Mn TL powders have been developed using phosphors prepared in Bhabha Atomic Research Centre (BARC), Trombay, India. Their suitability for dosimetry and environmental radiation surveillance was investigated. The minimum gamma ray exposure measurable with CaSO4:Dy dosimeters was 1.29 μC kg−1 (5 mR) and Li2B4O7:Mn was 25.8 μC kg−1 (100 mR) with a precision of ±5%. Various parameters such as the accuracy, precision, sensitivity, energy dependence, effect of light and reusability were investigated. Selected dosimeters of this type can be used for accurate determination of radiation levels if certain precautions on light exposure and annealing, etc., are taken into account. Iga et al.(11) from Japan describes a TLD system developed initially by Matsushita, Japan, based on CaSO4:Tm powder contained in glass capsules of 1.6 mm diameter and 6 mm in length. For the dosimetry of photons above 70 keV, the above dosemeter covered with a tin shield of 0.9 mm thickness was used. For thermal neutron dosimetry, a glass capsule containing a mixture of CaSO4:Tm powder and a 6Li compound was used. The glass capsules were heated one by one by a hot-air mechanism. A drawback of this system is that it cannot be used for beta dosimetry since glass attenuates the soft beta radiation. Furthermore, glass contains 40K which is radioactive and hence the dosemeter can receive unacceptably high self-dose depending upon the potassium content of the glass which will make it unfit for environmental dosimetry. Therefore, Yasuno et al.(12) from Matsushita developed the 4-element TLD powder mounted on a plastic plate. This is one of the most popular personnel monitoring system in the market before other solid forms of TLDs were developed In India, the west coast is a high background radiation area due to monazite content in the sand. As Chhatrapur region in coastal Odisha in India is known as a high background area, a detailed TLD study in the adjoining Gopalpur–Chhatrapur–Rushikulya sectors was carried out by Rao et al.(13) CaSO4:Tm powder filled in glass capillaries of 2 mm diameter and 12 mm length and again placed in a plastic capsule for complete protection was used for this purpose (Figure 1). A total of nine villages from the three sectors, viz. Gopalpur, Chhatrapur and Rushikulya, have been selected for the study. The average external gamma dose to people residing in the three sectors was 3.77, 4.47 and 3.57 mSv y−1, which is about three to four times the international limit of 1 mSv y−1 (Table 1). These dose values are high compared to other background radiation areas like Tamilnadu along the east coast of India, but are comparable to the high radiation areas in Kerala along the west coast of India. Table 1. External gamma dose rate in nine villages along southern coastal Orissa in India(13). Sector Village Dose rate (mSv y−1) Avg. dose rate (mSv y−1) Gopalpur Garampeta 3.8 3.77 Dhableswari 2.2 Gopalpur 5.3 Chhatrapur Mattikhalo 3.2 4.47 Aryapalli 7.7 Nuapalli 2.5 Rushikulya Parumpeta 3.9 3.57 Batteswar 3.8 Proyagi 3.0 Sector Village Dose rate (mSv y−1) Avg. dose rate (mSv y−1) Gopalpur Garampeta 3.8 3.77 Dhableswari 2.2 Gopalpur 5.3 Chhatrapur Mattikhalo 3.2 4.47 Aryapalli 7.7 Nuapalli 2.5 Rushikulya Parumpeta 3.9 3.57 Batteswar 3.8 Proyagi 3.0 Table 1. External gamma dose rate in nine villages along southern coastal Orissa in India(13). Sector Village Dose rate (mSv y−1) Avg. dose rate (mSv y−1) Gopalpur Garampeta 3.8 3.77 Dhableswari 2.2 Gopalpur 5.3 Chhatrapur Mattikhalo 3.2 4.47 Aryapalli 7.7 Nuapalli 2.5 Rushikulya Parumpeta 3.9 3.57 Batteswar 3.8 Proyagi 3.0 Sector Village Dose rate (mSv y−1) Avg. dose rate (mSv y−1) Gopalpur Garampeta 3.8 3.77 Dhableswari 2.2 Gopalpur 5.3 Chhatrapur Mattikhalo 3.2 4.47 Aryapalli 7.7 Nuapalli 2.5 Rushikulya Parumpeta 3.9 3.57 Batteswar 3.8 Proyagi 3.0 Figure 1. View largeDownload slide CaSO4:Tm powder filled glass capillary placed in a plastic capsule(13). Figure 1. View largeDownload slide CaSO4:Tm powder filled glass capillary placed in a plastic capsule(13). CaSO4:Dy EMBEDDED EXTRUDED LiF RIBBONS Using the patented technology procedure of making extruded ribbons of LiF, Harshaw Chemical Co., has made CaSO4:Dy ribbons (3.2 × 3.2 × 0.9 mm3). In these ribbons, the CaSO4:Dy material is embedded in ~90% of nonluminescent LiF which acts as a binder. It is commercially known as TLD-900. Their dosimetric properties have been studied by Bacci et al.(14) and Furetta and Gennai(15). Figure 2 shows the slight damage to these ribbons on prolonged exposure to humid environment which indicates the presence of humidity sensitive compound. Hence, the long-term storage stability of these ribbons in humid conditions is doubtful. TLD-900 has an unstable TL peak at 140°C with comparable intensity to that of the dosimetric main peak at 210°C. This could be the result of diffusion of Li in CaSO4 host crystal lattice. Li2SO4 is a water soluble compound and hence humidity sensitive. When reading 20 h after irradiation, a thermal fading of ~20% was observed as shown in Figures 3 and 4. A post-irradiation annealing at 100°C for 5 min was found necessary to remove the 140°C peak considerably. With such a procedure, thermal fading becomes negligible up to 20 h after irradiation and ~10% after 1 month. Figure 2. View largeDownload slide CaSO4:Dy embedded in ~90% of nonluminescent LiF which acts as a binder. It is commercially known as TLD-900 before Figure 1 and after Figure 2 storage in a humid environment for 30 days. The pellets got slightly damaged on prolonged exposure to humid environment(15). Figure 2. View largeDownload slide CaSO4:Dy embedded in ~90% of nonluminescent LiF which acts as a binder. It is commercially known as TLD-900 before Figure 1 and after Figure 2 storage in a humid environment for 30 days. The pellets got slightly damaged on prolonged exposure to humid environment(15). Figure 3. View largeDownload slide TL glow curve of TLD-900 along with deconvolution(14, 15). Figure 3. View largeDownload slide TL glow curve of TLD-900 along with deconvolution(14, 15). Figure 4. View largeDownload slide TL glow curves of TLD-900 as a function of post-irradiation interval(14, 15). Figure 4. View largeDownload slide TL glow curves of TLD-900 as a function of post-irradiation interval(14, 15). SINTERED PELLETS For sintered pellet preparation, usually, the phosphor powder is intimately mixed with a suitable binder and the sintering is carried out at a higher temperature. Since the solid state reaction route poses problems in the TL glow curve structure, development of pellets based on CaSO4:Dy dosemeters was not very successful, the only exception being the product made and marketed by Prokic which is patented(16, 17). Other studies made in this direction were not successful because of the interference of the binder material with the TL properties of CaSO4:Dy. Drazic and Trontelj(18) made two kinds of CaSO4:Dy pellets of diameter 6.5 and 0.5 mm thick. In one type, phosphor powder made by the usual method(2) were mixed with 5 wt% of the binder. The binder was prepared by mixing 90 wt% Mg3(BO3)2 and 10 wt% Li2SO4, and reacting them at 900°C for 1/2 h. The product was milled to a fine powder. Pellets were formed by pressing at 100 MPa and sintering at temperatures between 650 and 1250°C in air for 1–2 h. They named these dosemeters as ‘crystalline’. In the second type, the CaSO4:Dy phosphor itself was made during pelletization. In this technique, pellets were formed from a mixture of CaSO4.2 H2O and Dy2O3 with added binder to the amount of 5 wt%. Samples were sintered at temperatures between 650 and 1150°C for 1–8 h. These were named as ‘sintered’. TL glow curves of the ‘crystalline’ dosemeters prepared from AR grade chemicals and ultra pure chemicals as well as ‘sintered’ dosemeters prepared from AR grade chemicals were compared after exposure to 60Co gamma radiation (Figures 5 and 6). The latter technique was adopted to avoid the time consuming acid evaporation method. However, only the ‘crystalline’ dosemeters prepared from ultra pure chemicals gave the desired glow peak structure. While crystalline dosemeters have their main peak at 200°C, sintered dosemeters show a very pronounced peak at 300°C. The glow curve of a pellet composed of CaSO4 and Dy2O3 (0.2 mol% Dy) showed only one peak at 300°C. When Mg3(BO3)2 was added as an inorganic binder, low-temperature peaks appeared. These low-temperature peaks were very much intensified by the addition of Li2SO4. The influence of Na2SO4 (1 mol% Na) on the glow curve of sintered dosemeters showed that the intensities of low-temperature peaks drop, while the 300°C peak remains uninfluenced. Figure 5. View largeDownload slide Glow curve changes of ‘sintered’ CaSO4:Dy dosemeters as a function of the sintering temperature. Sintering time was 3 h. Dosemeter prepared at: (I) 900°C, (II) 1000°C, (III) 750°C and (IV) 1150°C(18). Figure 5. View largeDownload slide Glow curve changes of ‘sintered’ CaSO4:Dy dosemeters as a function of the sintering temperature. Sintering time was 3 h. Dosemeter prepared at: (I) 900°C, (II) 1000°C, (III) 750°C and (IV) 1150°C(18). Figure 6. View largeDownload slide Glow curves of CaSO4:Dy dosemeters. I. ‘Sintered’ dosemeter made from AR grade chemicals. II. ‘Crystalline’ dosemeters made from ultra pure chemicals. III. ‘Crystalline’ dosemeter made from AR grade chemicals. All samples were prepared at 900°C, 2 h(18). Figure 6. View largeDownload slide Glow curves of CaSO4:Dy dosemeters. I. ‘Sintered’ dosemeter made from AR grade chemicals. II. ‘Crystalline’ dosemeters made from ultra pure chemicals. III. ‘Crystalline’ dosemeter made from AR grade chemicals. All samples were prepared at 900°C, 2 h(18). Microstructure of these samples observed on polished surfaces by optical and scanning electron microscopes revealed that in contrast to the ‘crystalline’ pellets, the distribution of Dy is not homogenous in the ‘sintered’ pellets. Besides being distributed within CaSO4 grains, Dy is also concentrated in inclusions, a few μm in size. The intensities and the peak to peak ratios depend on the sintering temperature. The maximum height for the 200°C peak is obtained at a sintering temperature of 900°C. In samples prepared at 1000°C, the 150°C peak is higher than the 200°C peak, while samples prepared at 900°C show a lower peak at 150°C than that at 200°C. At a temperature of 1150°C, poor TL sensitivity was obtained and only peaks at 150 and 300°C were found. Sintering of the pellets at higher temperature resulted in the corrosion of grain boundaries. The boundaries have a porous structure due to the evaporation of the binder phase. In some places at the boundaries a new phase is formed which is richer in Ca than the bulk of the grains and does not contain sulphur. It is believed that at elevated temperature, decomposition of CaSO4 takes place and a layer of CaO is formed. On the basis of these studies, it was concluded that the optimum sintering temperature for sintered pellets is 900°C, at which diffusion of Dy in CaSO4 lattice is fast enough to obtain a solid solution over the entire grain and there is, as yet, no influence from a CaO layer between the grains of CaSO4, which could be the reason for the poor TL sensitivity of dosemeters prepared at 1150°C. However, the ‘sintered’ dosemeters had a TL sensitivity four times lower than dosemeters prepared by the crystallisation in ultra pure chemicals. It is not clear why Li2SO4 binder when added along with Mg3(BO3)2 increases the intensity of low temperature TL peak in ‘sintered’ pellet while in ‘crystalline’ pellet no such effect was observed. The pronounced low and high-temperature peaks in ‘sintered’ pellets will be a hindrance for radiation dosimetric applications. Katona and Zarand(19) have investigated the effect of annealing on the reproducibility of CaSO4:Dy pellets. Their studies have shown that these dosemeters cannot be used without external thermal treatment in high precision therapy level dosimetry. A 600°C, 1 h oven annealing resulted in a better reproducibility than a 400°C, 1 h treatment. The background of the dosemeters increased considerably even in a 10 × 1 mGy dose-readout cycle as a result of irradiation history if no external regeneration was used. Prokic(20) produced hard and durable sintered CaSO4:Dy pellets (4.36 mm diameter and 0.95 mm thickness) with an addition of minimal content of a binder. They were reportedly prepared by adding a small amount of Mg3(BO3)2, only ~2–3%, and the same amount of SiO2, of the phosphor content with a minimal addition of water to make a compact paste; after pressing the powder reportedly at a pressure of a few thousand pounds per square inch (psi) at RT, the pellets were sintered for 1 h at 750°C. The CaSO4:Dy phosphor used in pellets was prepared following the recrystalline method of Yamashita et al.(2) but the starting material CaSO4.2 H2O was precipitated from Ca(NO3):4 H2O (Carlo Erba’s make) and H2SO4 (Merck) chemicals. The method of preparation of these pellets is patent protected. Attempts to reproduce them by others were not very successful. The unique feature of this pellet is that its TL glow curve structure and sensitivity are the same as that of the phosphor powder obtained by the usual technique(2). Prokic claims that the unspecified binding substance used by her has the role of sensitiser of basic TL emission. The sensitivity of CaSO4:Dy pellet is claimed to be 1.25 times higher than that of CaSO4:Dy powder. The sintered CaSO4:Dy solid dosemeter showed a glow curve identical to the glow curve of powder sample. No change in dosemeter response was observed after repeated use. A high reproducibility was claimed. Environmental agents did not affect solid CaSO4:Dy. Fading of the main peak was not observed over a storing period of at least 3 months. Subsequently Prokic(21) has reported a 2-fold improvement in the TL sensitivity of these pellets by incorporating 63–125 micron sized phosphor grains instead of the previously used 0–45 µ grains. The CaSO4:Dy pellet was claimed to be an ideal TLD for environmental monitoring. They are reported to be successfully used in a few laboratories for environmental monitoring in Yugoslavia and in other countries(21–23). Recently, Prokic(24) added several Li compounds (Li2CO3, LiCl, LiF, Li2B4O7 and Li2SO4) during the pressing and sintering of CaSO4:Dy and CaSO4:Tm pellets. Wang et al.(25) have reported that the addition of Li compounds increases the luminescence efficiency of CaSO4:Dy but shifts the TL peak to low temperature near 135°C. Such a major lowering of trap stability emphasizes that there is an intimate and close connection between the charge trap and the lithium dopant. In that work samples were x-irradiated at 300 K (room temperature) and at 20 K. Radiation dose was 5 Gy in each case. The phosphors used were prepared by recrystallisation method in Prokic’s laboratory. But, unusually, the lithium co-dopant was not added during the initial growth and crystallization stage. Instead, lithium was successfully introduced at a concentration of 0.06% during a subsequent step of pressing and sintering of the TLD pellets. It is interesting to note that Lithium compounds could be introduced into CaSO4:Dy host during sintering stage. A number of alternative lithium compounds have been used in this role and they include Li2CO3, LiCl, LiF, Li2B4O7 and Li2SO4. Figures 7 and 8 show the TL peaks of CaSO4:Dy,Li at high (above RT) and low (below RT) temperatures, respectively. The presence of lithium has however, had a dramatic effect in reducing the high-temperature glow peak from around 200°C down to 120°C which indicate the incorporation of Li compound into CaSO4 lattice during pellet sintering, which is undesired for dosimetric applications. These studies confirmed that the traps giving rise to TL must be based on a very complex set of defect complexes which involve the RE, Li and an intrinsic feature of the host lattice, all packaged within a single overall defect complex. Figure 7. View largeDownload slide High temperature glow curves recorded at two Dy emission wavelengths from CaSO4:Dy,Li pellets. The intensities are normalized at 120°C(25). Figure 7. View largeDownload slide High temperature glow curves recorded at two Dy emission wavelengths from CaSO4:Dy,Li pellets. The intensities are normalized at 120°C(25). Figure 8. View largeDownload slide TL glow curve of CaSO4:Dy,Li pellets at low temperature(25). Figure 8. View largeDownload slide TL glow curve of CaSO4:Dy,Li pellets at low temperature(25). The emission spectra shown in Figure 9 for low- and high-temperature peaks are characteristic of Dy3+ ion. However, there is general agreement that SO4−-related ions could play an important role in the TL. It was suggested by Huzimura et al.(26) that the 120°C TL peak may be caused by the recombination of charge carriers from SO4−, whereas the 220°C peak may be produced by a stimulated relaxation of SO3− ions (i.e. the lithium has altered the charge state of the intrinsic site). There are several variations on models for the SO4− centers, which can be stabilised by a nearest-neighbour Ca vacancy, and formation of Ca vacancies can be prevented by co-doping with monovalent cations. Figure 9. View largeDownload slide TL emission spectra of CaSO4:Dy,Li at two temperatures(25). Figure 9. View largeDownload slide TL emission spectra of CaSO4:Dy,Li at two temperatures(25). Shastry et al.(27) made sintered pellets of 1 cm diameter down to a thickness of 100 mg cm−1 by homogeneously mixing the CaSO4:Dy powder (0–74 µ size) with minimal (3%) of binders (magnesium borate and silicon oxide powders—same as those used by Prokic(16, 17)) moistened with water and applying a pressure of 2 tons cm−2. To impart strength, these cold pressed pellets were sintered by them at a temperature of 750°C for 1 h in air before use. The glow curves of these samples are shown in Figure 10. This work, especially pellet strength, integrity and TL glow curve structure and sensitivity need to be substantiated. Figure 10. View largeDownload slide TL glow curves of CaSO4:Dy powder powder (___, 12.5 mg/cm2); and sintered pellets of various thickness (----, 100 mg/ mg/cm2; -- - -, 225 mg/cm2; ….., 345 mg/cm2; - O -, 480 mg/cm2)(27). Figure 10. View largeDownload slide TL glow curves of CaSO4:Dy powder powder (___, 12.5 mg/cm2); and sintered pellets of various thickness (----, 100 mg/ mg/cm2; -- - -, 225 mg/cm2; ….., 345 mg/cm2; - O -, 480 mg/cm2)(27). Yang et al.(28) from Korean atomic energy research institute (KAERI) reported the development of CaSO4:Dy pellets (known as KCT 300) with 10 mol% NH4H3PO4 binder. The binder was wet mixed with CaSO4:Dy powder (63–100 μm) made by recrystallisation route, dried and cold pressed before sintering at high temperatures. When the concentration of NH4H3PO4 binder was increased, the pellet strength increased but the TL sensitivity decreased as shown in Figure 11. So a compromise of 10 mol% was reached to obtain pellets with good sensitivity and strength. The dimensions of the pellet, 4.5 mm dia, 25 mg weight and 0.8 mm thickness were chosen from the points of view of strength and TL sensitivity. Figure 12 shows that the sensitivity of main TL peak increased beyond 500°C and reached maximum at 700°C but the ratio of main TL peak to low-temperature peak was highest after 600°C sintering treatment for 30 min and hence the latter was chosen as optimal sintering temperature. Figure 11. View largeDownload slide The TL sensitivity of CaSO4:Dy pellet as a function of NH4H3PO4 binder concentration(28). Figure 11. View largeDownload slide The TL sensitivity of CaSO4:Dy pellet as a function of NH4H3PO4 binder concentration(28). Figure 12. View largeDownload slide TL sensitivity of CaSO4:Dy pellet as a function of sintering temperature(28). Figure 12. View largeDownload slide TL sensitivity of CaSO4:Dy pellet as a function of sintering temperature(28). Figure 13 shows that the TL glow curves of CaSO4:Dy pellet and phosphor powder are identical except for the fact the TL glow peak of pellet occurs at a slightly higher temperature than that of the powder, as expected from the point of thermal lag of a thick sample. Figure 14 shows that the TL sensitivity of CaSO4:Dy pellet is three times higher than that of CaSO4:Dy Teflon disc made by KAERI and six times higher than that of Teflon disc made by Teledyne. In view of the high sensitivity, the CaSO4:Dy pellet (KCT 300) was recommended for personnel dosimetry. Figure 13. View largeDownload slide TL glow curve of CaSO4:Dy pellet (___) and phosphor powder (…….)(28). Figure 13. View largeDownload slide TL glow curve of CaSO4:Dy pellet (___) and phosphor powder (…….)(28). Figure 14. View largeDownload slide TL glow curves of CaSO4:Dy pellet (____, KCT 300), Teflon disc made by KAERI (…….) and Teledyne (….)(28). Figure 14. View largeDownload slide TL glow curves of CaSO4:Dy pellet (____, KCT 300), Teflon disc made by KAERI (…….) and Teledyne (….)(28). EFFECT OF PRESSURE ON CaSO4:Dy During the manufacture of pellets, the intimate mixture of CaSO4:Dy and the binder (calcium borate + calcium silicate mixture 20% by wt of CaSO4) has to be cold pressed into pellets prior to sintering. The application of static pressure on the above mixture or on the phosphor powder alone in a hydraulic press was found to reduce the TL as well as photoluminescence (PL) sensitivities of CaSO4:Dy drastically(29). Figure 15 shows the effect of pressure on the TL glow curve structure of the above phosphor binder mixture. TL sensitivity decreased and the major TL peak shifted to low temperature with increase in the applied pressure from 200 to 1000 psi. Pressure applied prior to or subsequent to gamma-irradiation had similar effect on the TL glow curve. Figure 16 shows that a 700°C, 1 h anneal restored the TL sensitivity only partially. These changes were attributed to the pressure induced destruction of traps and luminescent centres in CaSO4:Dy as the PL from Dy3+ ions also decreased with pressure. No pressure (up to 1000 psi) induced change in the X-ray diffraction (XRD) pattern was seen which indicate the stability of the CaSO4 crystal structure. Therefore, pressure applied during the preparation of pellets should be kept minimum. But from the point of view of mechanical strength, higher pressure is preferred. As a compromise 500 psi was recommended for the preparation of CaSO4:Dy pellets. Figure 17 shows that the TL sensitivity of CaSO4:Dy sintered pellets is three times higher than that of CaSO4:Dy (25%) embedded polytetrafluoro ethylene (PTFE—75%) discs. An additional advantage of pellets over PTFE discs is the former can be self annealed in the TLD reader at higher temperatures. The residual TL peak obtained on a higher temperature readout seen in Figure 18 could be used to check an accidental exposure. However, no detailed studies on the properties of these pellets were subsequently made. Binders such as SiO2 when mixed alone without any other binder was found to shift the TL glow curve of CaSO4:Dy to low temperatures as shown in Figure 19 and hence must be avoided. This could be due to the formation of calcium silicate phase during pellet preparation. Therefore, a proper choice of binder is essential in the preparation of CaSO4:Dy sintered pellets. Figure 15. View largeDownload slide The effect of pressure applied on phosphor binder mixture during the preparation of pellets on the TL glow curve of CaSO4:Dy pellets. 200 (1), 500 (2), 800 (3) and 1000 psi (4) followed by sintering at 700°C, for 1 h in air before irradiation. Gamma dose = 1 Gy. Pressure applied before or subsequent to gamma irradiation showed similar effect(29). Figure 15. View largeDownload slide The effect of pressure applied on phosphor binder mixture during the preparation of pellets on the TL glow curve of CaSO4:Dy pellets. 200 (1), 500 (2), 800 (3) and 1000 psi (4) followed by sintering at 700°C, for 1 h in air before irradiation. Gamma dose = 1 Gy. Pressure applied before or subsequent to gamma irradiation showed similar effect(29). Figure 16. View largeDownload slide TL glow curves of CaSO4:Dy phosphor powder (<120 µ) before (a) and after the application of 1000 psi pressure (b) as well as after the 700°C, 1 h anneal subsequent to pressurization (c). Gamma dose: 1 Gy. A partial recovery in TL sensitivity on anneal is seen(29). Figure 16. View largeDownload slide TL glow curves of CaSO4:Dy phosphor powder (<120 µ) before (a) and after the application of 1000 psi pressure (b) as well as after the 700°C, 1 h anneal subsequent to pressurization (c). Gamma dose: 1 Gy. A partial recovery in TL sensitivity on anneal is seen(29). Figure 17. View largeDownload slide Typical TL glow curves of CaSO4:Dy pellet (P) and CaSO4:Dy Teflon discs (T) (both 6 mm dia and 0.9 mm thickness) at the dose level of 1 Gy recorded with 300°C clamped readout(29). Figure 17. View largeDownload slide Typical TL glow curves of CaSO4:Dy pellet (P) and CaSO4:Dy Teflon discs (T) (both 6 mm dia and 0.9 mm thickness) at the dose level of 1 Gy recorded with 300°C clamped readout(29). Figure 18. View largeDownload slide A typical glow curve of the CaSO4:Dy pellet during the first readout with clamping at 300°C (1), then subsequent readout with clamping at 400°C (II), repeated twice (III) at the dose level of 1 Gy. The TL sensitivities obtained during second and third readouts are magnified by a factor of 10. The heating profiles used during the first (I) and second (II) readouts are shown. The third readout heating profile is the same as that of the second(29). Figure 18. View largeDownload slide A typical glow curve of the CaSO4:Dy pellet during the first readout with clamping at 300°C (1), then subsequent readout with clamping at 400°C (II), repeated twice (III) at the dose level of 1 Gy. The TL sensitivities obtained during second and third readouts are magnified by a factor of 10. The heating profiles used during the first (I) and second (II) readouts are shown. The third readout heating profile is the same as that of the second(29). Figure 19. View largeDownload slide Effect of SiO2 binder on the TL glow curve shape of CaSO4:Dy sintered pellet(29). Figure 19. View largeDownload slide Effect of SiO2 binder on the TL glow curve shape of CaSO4:Dy sintered pellet(29). ALUMINIUM BASED CaSO4:Dy Aluminium (Al)-based solid CaSO4:Tm dosemeters were used by Burgkhardt et al.(30). They reported that these dosemeters could not be annealed at sufficiently high temperatures (i.e >400°C) and hence exhibit residual reading and sensitisation effects after the first evaluation subsequent to high gamma exposures. Hence, CaSO4:Tm on Al base was found to exhibit a ‘permanent’ sensitisation up to a factor of 2.7 (at the pre-dose level of 3 × 103 Gy) even after annealing (<400°C). Our studies, however, indicate that CaSO4:Dy filled Al discs are stable up to 500°C as the melting point of Al is 660°C(31). Figure 20 shows the photographs of pure Al disc, 50% CaSO4:Dy loaded Al disc and 50% LiF:Mg loaded Al disc after cold pressing without any sintering treatment all having the same dimensions of 6 mm diameter and 1 mm thickness. Table 2 shows that the TL sensitivity of 50% CaSO4:Dy loaded Al discs increases with sintering temperature up to 300°C and then decreases slowly up to 700°C. But Figure 21 shows that their TL glow curve shape changes drastically beyond the sintering temperature of 500°C. At 600 and 700°C sintering temperatures, the low-temperature glow peak shoots up at the expense of high-temperature glow peaks perhaps due to the softening and diffusion of aluminium into the CaSO4:Dy crystal lattice. In the sintering temperature range 200–500°C, apart from sensitivity enhancement, the TL glow peak shifts gradually to low temperature with increasing sintering temperature. However, the TL sensitivity of phosphor loaded Al discs is considerably reduced due to the opacity of aluminium which limit their application to medium and high-dose measurements. Table 3 shows that the TL sensitivity of 25% CaSO4:Dy loaded Al disc is hardly 1% of that of the 25% CaSO4:Dy loaded PTFE disc. Since Al mixed phosphors possess good mechanical strength even after cold pressing, they retain the same dosimetric properties as that of the phosphor powder. Hence, there is no need to sinter them at high temperatures. Their poor TL sensitivity is a blessing in disguise for high-dose dosimetry where saturation of the light detection system is an issue. By varying the Al content, the TL sensitivity can be reduced to the desired extent so saturation of the detector system at high doses can be avoided. This is an alternate to the use of neutral density attenuation filters in the TLD reader system. Table 2. Relative TL sensitivity (area of glow curve) of 50% CaSO4:Dy loaded aluminium disc—6 mm dia and 1 mm thick as a function of sintering temperature in air for 30 min duration after cold pressing(31). S. no. Sintering temperature (°C) Relative TL sensitivity 1 Unsintered 1.00 2 200 1.13 3 300 1.88 4 400 1.47 5 500 1.59 6 600 1.54 7 650 1,39 8 700 0.998 S. no. Sintering temperature (°C) Relative TL sensitivity 1 Unsintered 1.00 2 200 1.13 3 300 1.88 4 400 1.47 5 500 1.59 6 600 1.54 7 650 1,39 8 700 0.998 Table 2. Relative TL sensitivity (area of glow curve) of 50% CaSO4:Dy loaded aluminium disc—6 mm dia and 1 mm thick as a function of sintering temperature in air for 30 min duration after cold pressing(31). S. no. Sintering temperature (°C) Relative TL sensitivity 1 Unsintered 1.00 2 200 1.13 3 300 1.88 4 400 1.47 5 500 1.59 6 600 1.54 7 650 1,39 8 700 0.998 S. no. Sintering temperature (°C) Relative TL sensitivity 1 Unsintered 1.00 2 200 1.13 3 300 1.88 4 400 1.47 5 500 1.59 6 600 1.54 7 650 1,39 8 700 0.998 Table 3. Relative TL sensitivity (glow curve area) of CaSO4:Dy loaded Al disc (6 mm dia and 1 mm thick) as a function of Al %. TL sensitivity of standard 25% CaSO4: Dy loaded PTFE disc of the same dimensions is also shown(31). S. no. Sample Relative TL sensitivity 1. 75% PTFE + 25% CaSO4:Dy 1.000 2. 30% Al + 70% CaSO4:Dy 0.084 3. 40% Al + 60% CaSO4:Dy 0.035 4. 50% Al + 50% CaSO4:Dy 0.031 5. 60% Al + 40% CaSO4:Dy 0.018 6. 70% Al + 30% CaSO4:Dy 0.014 7. 80% Al + 20% CaSO4:Dy 0.008 8. 90% Al + 10% CaSO4:Dy 0.005 9. 95% Al + 5% CaSO4:Dy 0.003 S. no. Sample Relative TL sensitivity 1. 75% PTFE + 25% CaSO4:Dy 1.000 2. 30% Al + 70% CaSO4:Dy 0.084 3. 40% Al + 60% CaSO4:Dy 0.035 4. 50% Al + 50% CaSO4:Dy 0.031 5. 60% Al + 40% CaSO4:Dy 0.018 6. 70% Al + 30% CaSO4:Dy 0.014 7. 80% Al + 20% CaSO4:Dy 0.008 8. 90% Al + 10% CaSO4:Dy 0.005 9. 95% Al + 5% CaSO4:Dy 0.003 Table 3. Relative TL sensitivity (glow curve area) of CaSO4:Dy loaded Al disc (6 mm dia and 1 mm thick) as a function of Al %. TL sensitivity of standard 25% CaSO4: Dy loaded PTFE disc of the same dimensions is also shown(31). S. no. Sample Relative TL sensitivity 1. 75% PTFE + 25% CaSO4:Dy 1.000 2. 30% Al + 70% CaSO4:Dy 0.084 3. 40% Al + 60% CaSO4:Dy 0.035 4. 50% Al + 50% CaSO4:Dy 0.031 5. 60% Al + 40% CaSO4:Dy 0.018 6. 70% Al + 30% CaSO4:Dy 0.014 7. 80% Al + 20% CaSO4:Dy 0.008 8. 90% Al + 10% CaSO4:Dy 0.005 9. 95% Al + 5% CaSO4:Dy 0.003 S. no. Sample Relative TL sensitivity 1. 75% PTFE + 25% CaSO4:Dy 1.000 2. 30% Al + 70% CaSO4:Dy 0.084 3. 40% Al + 60% CaSO4:Dy 0.035 4. 50% Al + 50% CaSO4:Dy 0.031 5. 60% Al + 40% CaSO4:Dy 0.018 6. 70% Al + 30% CaSO4:Dy 0.014 7. 80% Al + 20% CaSO4:Dy 0.008 8. 90% Al + 10% CaSO4:Dy 0.005 9. 95% Al + 5% CaSO4:Dy 0.003 Figure 20. View largeDownload slide Photographs of cold pressed pure Al disc (a), 50% CaSO4:Dy loaded Al disc (b) and 50% LiF:Mg loaded Al disc (b)(31). Figure 20. View largeDownload slide Photographs of cold pressed pure Al disc (a), 50% CaSO4:Dy loaded Al disc (b) and 50% LiF:Mg loaded Al disc (b)(31). Figure 21. View largeDownload slide TL glow curves of 50% CaSO4:Dy loaded Al disc as a function of sintering temperature for 30 min in air after cold pressing (a—unsintered, b—200°C, c—300°C, d—400°C, e—500°C, f—600°C, g—650°C, h—700°C)(31). Figure 21. View largeDownload slide TL glow curves of 50% CaSO4:Dy loaded Al disc as a function of sintering temperature for 30 min in air after cold pressing (a—unsintered, b—200°C, c—300°C, d—400°C, e—500°C, f—600°C, g—650°C, h—700°C)(31). CaSO4:Dy FILM Films of CaSO4:Dy were deposited by spray pyrolysis method using stoichiometric spray solutions of calcium acetate [Ca(CH3–COO–)2] with dysprosium sulphate [Dy2SO4] (3%) and ammonium sulphate [(NH4)2SO4] in ethanol water environment by Roman et al.(32). The films were deposited on three different substrates of glass, aluminium and quartz at different temperatures from 450 up to 600°C. The spray solutions were sprinkling on the substrates at the spray rate of 7.5 L/min for each deposit temperature. The deposit time on the substrates was of 5 min. After this processes, CaSO4:Dy films deposited on quartz substrate were submitted at thermal treatment at 1000°C for 1 h. The morphology and chemical composition of the films deposited on glass, aluminium and quartz substrates were analysed by scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) (Figures 22 and 23). All CaSO4:Dy films were exposed with ultraviolet light (200–300 nm). The irradiations of the samples were carried out using a monochromator. The experimental conditions of irradiation were at room temperature and using dark light. TL glow curves of the films were obtained immediately after the irradiation. In the SEM photographs of CaSO4:Dy films with glass substrate, the formation of a homogeneous film for a temperature of 450°C are observed. The homogeneity in the films is modified when the deposit temperature is increased up to 600°C. CaSO4:Dy films deposited on quartz substrate exhibited a TL glow curve with a very prominent peak in intensity at a temperature of 81.5°C and a little perceptible peak at 231.5°C. Similar glow curve structure has been observed in CaSO4:Dy precipitation routes involving ammonium sulphate. But such a low temperature glow peak is unsuitable for radiation dosimetric applications. Figure 22. View largeDownload slide SEM photographs of CaSO4:Dy films deposited on glass substrates at different temperatures: (a) 450°C, (b) 500°C, (c) 550°C and (d) 600°C(32). Figure 22. View largeDownload slide SEM photographs of CaSO4:Dy films deposited on glass substrates at different temperatures: (a) 450°C, (b) 500°C, (c) 550°C and (d) 600°C(32). Figure 23. View largeDownload slide SEM photographs of CaSO4:Dy films deposited on quartz substrates (a) and deposited on aluminium substrates (b) at 500°C of temperature of deposit(32). Figure 23. View largeDownload slide SEM photographs of CaSO4:Dy films deposited on quartz substrates (a) and deposited on aluminium substrates (b) at 500°C of temperature of deposit(32). TL response of CaSO4:Dy film as a function of UV wavelength is shown in Figure 24. TL response decreased as wavelength is increased. The inset in Figure 24 shows the formation of TL curve when the CaSO4:Dy films on quartz substrate are irradiated at wavelengths of 280 and 300 nm. Finally, CaSO4:Dy films deposited on aluminium and glass substrate showed a TL response when these films were kept in normal conditions and at room temperature for 8 days. The TL response presented a peak around 287.5°C for the aluminium substrates and 300°C for the glass substrates (Figure 25). This response in the CaSO4:Dy films with glass and aluminium substrates could be induced by the environmental radiation. The intensity and the shape of the TL glow curve of CaSO4:Dy films were analysed as a function of the substrate. TL response of CaSO4:Dy films deposited on quartz substrate were the most sensitive. Also, thermal treatment (1000°C for 1 h) applied to the CaSO4:Dy films deposited on quartz substrate induce an increase in sensitivity. The shapes of the glow curve in all films were similar. Figure 24. View largeDownload slide TL response of CaSO4:Dy films deposited on quartz substrates irradiated from 200 to 300 nm(32). Figure 24. View largeDownload slide TL response of CaSO4:Dy films deposited on quartz substrates irradiated from 200 to 300 nm(32). Figure 25. View largeDownload slide TL response of CaSO4:Dy films deposited on glass and aluminium substrates without exposure to radiation(32). Figure 25. View largeDownload slide TL response of CaSO4:Dy films deposited on glass and aluminium substrates without exposure to radiation(32). CaSO4:Dy EMBEDDED TEFLON DOSEMETERS Production techniques Among the CaSO4:Dy dosemeters in solid form, phosphor-embedded Teflon dosemeters are the most popular and have been used highly successfully for personnel and environmental radiation dosimetric applications in many countries notably by Bhabha Atomic Research Centre (BARC) in India. In general, the TL properties are not changed by the incorporation of TLDs into PTFE (Poly Tetra Fluoro Ethylene also known as Teflon), which is chemically inert. The emission spectra are unchanged, since PTFE does not have any absorption bands in the region of visible light. Also PTFE by itself does not give any significant TL above RT following irradiation. Other advantages of phosphor-filled PTFE discs are wide variety of available formats—rods, discs, sheets, etc., and they can be conveniently identified by writing on the surface placed away from the light detector during readout. This is extremely useful when individually calibrating dosemeters for high precision applications. The disadvantages of Teflon impregnated dosemeters are, however, the following. Their low thermal conductivity and flexible shape makes them more difficult to heat reproducibly, and can also lead to changes in the efficiency of light collection. Various background signals associated with the PTFE can cause difficulty when making low-dose measurements. In contrast to pellets, CaSO4:Dy-embedded Teflon dosemeters were prepared at BARC relatively easily from a homogenous mixture of CaSO4:Dy powder (<74 μm size) and Teflon powder (Dupont 7 A grade, average grain size ~50 μm) mixed in cryogenic temperature usually in the ratio of 1:3, prior to cold pressing at RT and then sintering them at 400°C in an air-circulation oven. The die and plunger used for cold pressing should be made of tungsten carbide to prevent wear and tear during repeated use. Otherwise burrs appear on the circumference of the cold pressed disc. This is due to the high compressibility of Teflon (compression ratio ~4–5). During mixing, cryogenic temperature is essential to remove the electrostatic force which otherwise keeps the Teflon grains sticking together. Since Teflon softens around 327°C, the discs are to be kept in between glass plates (microslides) during sintering at 400°C(33). The purpose of this sintering is to impart strength to Teflon which otherwise is very brittle. Discs down to 0.8 mm thickness can be made with this manual technique. The cooling rate from the sintering temperature has to be very slow to achieve a uniform optical density. For large-scale production, especially of discs with reduced thickness (<0.4 mm), long rods or thick billets are cold pressed from Teflon–phosphor mixture. After a suitable heat treatment, the rods are sliced using a microtome and the billets are skieved to desired thickness using suitable machines. The optical transmission through PTFE is dependent on the rate of cooling, from the sintering temperature of 365°C, through a crystalline transition point at 327°C. It is necessary to cool the bar slowly, at a rate of ~0.6°C/min through the transition temperature and down to at least 250°C, to stress-relieve the PTFE. The phosphor/PTFE bars are made slightly oversize (1) to allow for shrinkage during sintering and (2) to enable the sintered bar to be turned down to an accurately known diameter, removing surface debris and discolouration in the process. The diameter of a bar can be quite easily machined to within ~ ±10 μm of the desired diameter, so that variations in the diameter of a disc are negligible. It is more difficult to part off discs of a reproducible thickness, and the problem becomes particularly acute with thinner discs. Discs of thickness greater than ~0.1 mm are generally parted off on an automatic lathe, and their reproducibility of thickness, therefore, depends on the accuracy with which the lead screw has been cut. However, greater uniformity is obtained by this procedure than by the alternative method of extruding sheet and stamping out discs from it. The parting-off tool is ground to a fine edge to avoid wastage, and discs are produced somewhat bowed as a result. But they become flatter with repeated readouts or anneals. Handling practices may also cause changes in shape. The surface condition degrades with re-use due to dust and other debris burnt into the surface during readout and annealing. When discs are passed through a temperature cycle similar to that used in the sintering process, they appear to be more transparent than before. The increased transparency seems to be due to a change in the surface texture, which is much smoother than the comparatively coarse surface produced during manufacture by the action of veneering the disc off a bar. Another interesting effect of the ‘sintering cycle’ is that the discs are completely stress relieved of the distortion incurred during veneering. The characteristic ‘bowing’ disappears, and the discs become quite flat. These techniques can be standardised only with the help of expensive equipment available normally in industries manufacturing Teflon products. The final product should (1) be white in colour to avoid being opaque to light emanating from it, and (2) have a smooth surface, otherwise it will pick up dust during handling which will burn and give a black colour to the dosemeter on heating. The CaSO4:Dy embedded Teflon discs and tape are very popular and used widely for various applications in radiation dosimetry. During TL readout, the maximum temperature reached should, however, be kept well below the softening temperature of Teflon (327°C). As a result of this limitation, Teflon-based dosemeters cannot be heated to higher temperatures during TL readouts. Since Teflon is a thermal insulator, there is a temperature lag between the upper and lower layers of Teflon-based dosemeters during heating. Thus, the time taken for reading such dosemeters is usually long, 20–40 s with a Kanthal strip heater for 0.4 and 0.8 mm thick dosemeters respectively and 10 s with a solid block heater for 0.4 mm thick dosemeter. Because of the fact that Teflon has a relatively large thermal expansion coefficient, Teflon-based dosemeters of lower thickness (<0.5 mm) buckle during readout unless special precautions are taken. If held tightly, they buckle during heating as a result of thermal expansion. Hence, these dosemeters are to be held loosely in the card holders used in personnel dosimetry, thereby allowing them to expand freely during heating. In any case, thin Teflon dosemeters (<0.8 mm thick) buckle during heating which can be avoided only with a thin insulating but transparent top support such as mica(34). During repeated use, Teflon-based dosemeters, sometimes, get discoloured. The greying of CaSO4:Dy embedded Teflon discs on repeated use in the badge used on a large scale in the countrywide individual radiation monitoring services in India has remained an unsolved problem so far as pointed by Lakshmanan et al.(35, 36). However, LiF-embedded Teflon TLDs used in NRPB, UK or even CaSO4:Dy embedded TLDs manufactured by others (e.g. Teledyne Isotopes) are not reported to pose any such problem. The problem may as well be with the organic impurities entering the CaSO4 lattice during phosphor preparation or with the organic impurities in commercial grade PTFE (7 A grade supplied by Dupont) used in the manufacture of these discs. It is worthwhile to take seriously the claims made by Kasa(37) that an addition of a small quantity of H2O2 to H2SO4 used in the synthesis of CaSO4:Dy/Tm hinder their greying caused by carbonization of the organic matter accidentally getting into the recrystallisation vessel and compare the performance of CaSO4:Dy/Tm embedded Teflon dosemeters made by both techniques. Alternately, PTFE manufactured from different companies could be tried. Effects of photon energy and irradiation history on dose re-estimation In LiF:Mg,Ti TLD, it has been shown that the efficiency of electron transfer using the phototransferred TL (PTTL) technique is somewhat dependent on the linear energy transfer (LET) of the original incident radiation; the re-estimation efficiency for X-rays below 100 keV can be up to 35% higher than that for radium gamma rays. This results from the well known fact that the relative proportions of electrons trapped in 200 and 270°C TL peaks in LiF are LET dependent; the reasons for this have been analysed(38). As Douglas et al.(39) pointed out, the variable transfer efficiency can lead to some uncertainty in the re-estimation of original dose (though it should be recognised that in most cases one is merely trying to confirm or discount a high reading, and high precision is not required). The ability to re-estimate low absorbed doses, especially with simple techniques, removes the validity of an argument against the use of TLDs for personnel monitoring. Lakshmanan and Bhatt(40) have examined the effects of photon energy and irradiation history on the efficiency of dose re-estimation in CaSO4:Dy Teflon discs using repeated readouts. Figure 26 shows typical TL glow curves (gamma-ray dose = 0.1 Gy) of a CaSO4:Dy Teflon disc fixed onto the personnel monitoring card during three consecutive readouts (I, II and III). The glow peak temperature of the residual signal shifted to high temperatures with increasing readouts while their intensities decreased progressively. The second and third readouts (integrated TL intensities during 0–42 s interval) are ~5.5 and 3.5% of the first readout, respectively. Hence, the second and third readouts should be multiplied by a factor of 18.2 and 33.3, respectively, to obtain the re-estimated absorbed dose. These factors are, however, highly dependent on the heating cycle used and hence each reader should be calibrated separately and periodically. Figure 26. View largeDownload slide Typical TL glow curves (gamma-ray dose = 0.1 Gy) of a CaSO4:Dy Teflon disc fixed onto the personnel monitoring card during three consecutive readouts (I, II and III)(40). Figure 26. View largeDownload slide Typical TL glow curves (gamma-ray dose = 0.1 Gy) of a CaSO4:Dy Teflon disc fixed onto the personnel monitoring card during three consecutive readouts (I, II and III)(40). Table 4 shows the effect of irradiation history on the re-estimated absorbed dose of a single CaSO4:Dy TLD card. The first readout directly gives the absorbed dose. A 10-fold increase in the dose delivered could be erased by annealing at 240°C. The second readout during cycle-3 as compared to that during cycle-1 is only marginally higher (by ~1%) after 240°C, 1 h annealing treatment; and the marginal increase in TL signal during second readout is only 0.5% after 240°C, 3 h annealing treatment, which is well within the fluctuations in the TL from disc to disc. These results show that the TLD card does not remember its irradiation history after the 240°C annealing treatment at least up to a dose of 0.1 Gy. For doses exceeding 1 Gy, however, care should be exercised before reuse because the CaSO4:Dy phosphor exhibits sensitization(7). Table 4. Effect of irradiation history on the re-estimated absorbed dose in a CaSO4:Dy embedded Teflon TLD card(40). Cycle No. Anneal Gamma-dose (10−2 Gy) TL sensitivity (counts) Readout Ia II III 1 400°C, 1 h 1 1000 43 ± 15 4.3 ± 1.5 2 240°C, 1 h 12 12 000 550 ± 100 4.5 ± 0.83 3 240°C, 1 h 1 1000 54 ± 20 5.4 ± 2 3 240°C, 3 h 1 1000 48 ± 15 4.8 ± 1.5 Cycle No. Anneal Gamma-dose (10−2 Gy) TL sensitivity (counts) Readout Ia II III 1 400°C, 1 h 1 1000 43 ± 15 4.3 ± 1.5 2 240°C, 1 h 12 12 000 550 ± 100 4.5 ± 0.83 3 240°C, 1 h 1 1000 54 ± 20 5.4 ± 2 3 240°C, 3 h 1 1000 48 ± 15 4.8 ± 1.5 aThe fluctuations in TL measurements during the first readout are within ±105 (1σ). Table 4. Effect of irradiation history on the re-estimated absorbed dose in a CaSO4:Dy embedded Teflon TLD card(40). Cycle No. Anneal Gamma-dose (10−2 Gy) TL sensitivity (counts) Readout Ia II III 1 400°C, 1 h 1 1000 43 ± 15 4.3 ± 1.5 2 240°C, 1 h 12 12 000 550 ± 100 4.5 ± 0.83 3 240°C, 1 h 1 1000 54 ± 20 5.4 ± 2 3 240°C, 3 h 1 1000 48 ± 15 4.8 ± 1.5 Cycle No. Anneal Gamma-dose (10−2 Gy) TL sensitivity (counts) Readout Ia II III 1 400°C, 1 h 1 1000 43 ± 15 4.3 ± 1.5 2 240°C, 1 h 12 12 000 550 ± 100 4.5 ± 0.83 3 240°C, 1 h 1 1000 54 ± 20 5.4 ± 2 3 240°C, 3 h 1 1000 48 ± 15 4.8 ± 1.5 aThe fluctuations in TL measurements during the first readout are within ±105 (1σ). The photon energy dependence of the CaSO4:Dy Teflon discs during the first three readouts was found to be identical. This indicates that the energy dependence of the re-estimated TL is still a characteristic of the dosimetry traps unlike the case with LiF. The changes in glow curve shape with photon energy reported by Srivastava and Supe(41) in CaSO4:Dy at higher dose levels are not probably significant in the dose region of interest in personnel monitoring. Annealing and repeated readout of TLD cards Pradhan et al.(42) studied the annealing and repeated readout of TLD cards based on CaSO4:Dy Teflon discs used for personnel monitoring in India. Annealing of the dosemeters is necessary because a significant portion of TL is left out after the readout which will interfere with subsequent readout. In the case of personnel monitoring TLD cards based on Teflon, the annealing procedure has to be such that the discs do not come out of the mechanical grip during annealing. From the nature of the glow curve seen in Figure 26, it is obvious that significant TL is left out after the readout. Figure 27 shows the TL output of the card exposed to 2.58 mC kg−1 (10 R) of gamma-exposure and readout repeatedly without subjecting to annealing or exposure. The residual TL after the first readout is ~11% which decreased exponentially with the number of readouts. The repeated readout calibration curves of the cards exposed to various exposures are plotted for the first, second and third readouts in Figure 28. The fluctuation in the measurements was found to increase with repeated readout, and the standard deviation of 10 cards for first, second and third readouts were ~ ±10, ±22 and ±30%, respectively. Figure 27. View largeDownload slide TL output of a card exposed to 2.58 mC kg−1 (10 R) of 60Co gamma-rays and readout repeatedly without subjecting to annealing or exposure(42). Figure 27. View largeDownload slide TL output of a card exposed to 2.58 mC kg−1 (10 R) of 60Co gamma-rays and readout repeatedly without subjecting to annealing or exposure(42). Figure 28. View largeDownload slide Exposure versus TL output of the cards. (I) First readout; (II) second readout; and (III) third readout. II and III show that the residual TL is proportional to the exposure(42). Figure 28. View largeDownload slide Exposure versus TL output of the cards. (I) First readout; (II) second readout; and (III) third readout. II and III show that the residual TL is proportional to the exposure(42). In order to re-use the TLD cards, a lower temperature (200°C) and longer duration (16 h) of annealing was used since the discs came out of the card when the annealing temperature exceeded 240°C. As seen in Figure 29, the 200°C annealing treatment was found to erase exposures up to 2.58 mC kg−1 (~10 R). No change in TL sensitivity was observed for phosphor loading from 1 to 50% in Teflon up to eleven cycles of exposure 1.55 mC kg−1(6 R), readout and annealing. These results are shown in Figure 30. However, other independent studies show a slow deterioration in the TL sensitivity of CaSO4:Dy embedded Teflon discs with reuse(35). The absolute TL sensitivity was found to increase with the percentage of CaSO4:Dy in the Teflon disc as shown in Table 5. Figure 31 shows the relative increase in TL sensitivity of CaSO4:Dy Teflon discs with different phosphor content to 60Co gamma rays(42). But the increase in TL sensitivity is not linear because of the increased opacity (due to the slight yellowish tint produced by Dy when doped in CaSO4 lattice) at higher percentage of CaSO4:Dy. Since 75% CaSO4:Dy embedded Teflon (25%) discs were fragile, they were not included in the re-use study. Table 5. Relative TL sensitivity of the cards having various proportions of Teflon and CaSO4:Dy(42). Percentage of CaSO4:Dy TLD powder in Teflon discs Relative TL output 1 1.0 5 5.0 15 14.6 20 23.8 50 38.3 75 42.8 Percentage of CaSO4:Dy TLD powder in Teflon discs Relative TL output 1 1.0 5 5.0 15 14.6 20 23.8 50 38.3 75 42.8 Table 5. Relative TL sensitivity of the cards having various proportions of Teflon and CaSO4:Dy(42). Percentage of CaSO4:Dy TLD powder in Teflon discs Relative TL output 1 1.0 5 5.0 15 14.6 20 23.8 50 38.3 75 42.8 Percentage of CaSO4:Dy TLD powder in Teflon discs Relative TL output 1 1.0 5 5.0 15 14.6 20 23.8 50 38.3 75 42.8 Figure 29. View largeDownload slide Residual TL of the cards exposed to 6 R of60Co gamma-rays and annealed at various temperatures for 16 h(42). Figure 29. View largeDownload slide Residual TL of the cards exposed to 6 R of60Co gamma-rays and annealed at various temperatures for 16 h(42). Figure 30. View largeDownload slide Reusability of TLD cards having discs of various phosphor proportions by weight (x—1%, ●—5%, o—15%, −25%, ▲—50%) subjected to 6R of 60Co gamma-rays, readout and annealing (200°C, 16 h) cycle(42). Figure 30. View largeDownload slide Reusability of TLD cards having discs of various phosphor proportions by weight (x—1%, ●—5%, o—15%, −25%, ▲—50%) subjected to 6R of 60Co gamma-rays, readout and annealing (200°C, 16 h) cycle(42). Figure 31. View largeDownload slide Relative TL sensitivity/mg and optical density of CaSO4:Dy Teflon discs with different phosphor content to 60Co gamma rays(42). Figure 31. View largeDownload slide Relative TL sensitivity/mg and optical density of CaSO4:Dy Teflon discs with different phosphor content to 60Co gamma rays(42). Precision in dose measurements Due to poor thermal conduction in the Teflon and non-uniform thermal contact with the heating element can give rise to differing emitted light patterns. Mathews and Stoebe(43) studied the quantitative effects of thermal patterns on TL measurement precision in these dosemeters. In total, 20 such dosemeters (1.2 cm diameter and 0.4 mm thick) were exposed to 1.29 mC kg−1 (~5 R) and readout repeatedly. They were annealed at 300°C for 3 h after each irradiation and readout procedure. The random error in the readout results was found to be 3–5% nominally, while the standard error for a run, σ/mean, was 5–7% and reached as high as 10% in some cases. Systematic error in the placement of the dosemeter disc and variation in anneal cycles did not contribute significantly to these errors. Glow curves observed in the introductory experiments revealed deviations from expected glow curve shape. Further experiments showed that this was caused by flexing of the dosemeter during heating, resulting in poor thermal contact. This occurred even though the dosemeter was held under the metallic screen in the standard planchet. To show this graphically, photographic studies were conducted on another set of dosemeters, irradiated with 48 Gy of 60Co gamma-rays, then heated on an aluminium block to 290°C. In another study, the light output was integrated for a period of time equivalent to one readout cycle; inhomogeneous light patterns are evident in the second study even though the integration is well past the glow peak. Dosemeters with different histories were chosen for this study, from virgin to well-used; dosemeters subjected to prior use presented the most obvious patterns in photographs of the integrated light output; the patterns coincided with the curvature these dosemeters assumed (saddle shape) during use. In later experiments, a modified heating planchet was employed to ensure uniform heating of the dosemeters and to ensure the same geometry for successive readouts. The modification consisted of placing a solid metal cover (cut down from another planchet) with an aperture of diameter 0.9 cm over the screened area of the planchet; the dosemeters were placed between the cover and the planchet, centred in the aperture during readout. Two groups of dosemeters were tested using this modified planchet: Set 1 consisted entirely of virgin dosemeters; Set 2 had received five doses of 1 rad (0.01 Gy) each at various X-ray energies from 30 to 100 kVp and one dose of 0.8 Gy (80 rad) of 60Co gamma-rays. Each of them had been annealed for 3 h at 300°C between cycles. With the revised planchet, random error in all groups was typically <1%. However, the set of dosemeters which had received a larger number of long-term anneal cycles, showed consistently higher variations from the mean. This result suggests that the recommended 3 h, 300°C anneal for CaSO4:Dy Teflon dosemeters may be detrimental to precision in the use of these dosemeters. It is not clear, however, that this thermal damage is due to the phosphor since optical changes in the Teflon substrate due to thermal damage may account for the results obtained in this study. Thermal history and reusability The dependence of TL response of CaSO4:Dy Teflon dosemeters (13.5 mm diameter and 0.8 mm thick) on the maximum readout temperature as well as on the annealing temperature and their reusability aspects were studied in detail by Lakshmanan et al.(35). While no reduction in TL sensitivity nor discoloration was seen with reuse when the readout temperature was 240°C (Figure 32), a 300°C readout resulted in reduction in sensitivity with reuse due to discoloration of CaSO4:Dy Teflon discs (Figure 33), The reason for above discoloration is not exactly known but it may be related to the production of carbon at high temperatures. Figure 32. View largeDownload slide Results of the reusability study when the CaSO4:Dy Teflon discs were annealed at 240°C for 1 h between readouts. Maximum temperature reached during readout was 240°C(35). Figure 32. View largeDownload slide Results of the reusability study when the CaSO4:Dy Teflon discs were annealed at 240°C for 1 h between readouts. Maximum temperature reached during readout was 240°C(35). Figure 33. View largeDownload slide Results of the reusability study when the CaSO4:Dy Teflon discs were annealed at 240°C for 1h between readouts. Maximum temperature reached during readout was 300°C(35). Figure 33. View largeDownload slide Results of the reusability study when the CaSO4:Dy Teflon discs were annealed at 240°C for 1h between readouts. Maximum temperature reached during readout was 300°C(35). Isothermal annealing at 400°C (sandwiched between glass plates during annealing) removed the discolouration and restored the original colour of these discs. However, their TL sensitivity decreased gradually (over 15% in 20 cycles) with the number of cycles (300°C readout + 400°C, 1 h anneal) as seen in Figure 34(35). Bakshi et al.’s(43) subsequent studies have shown that the above reduction in TL sensitivity is mainly due to the loss in TL sensitivity of the CaSO4:Dy phosphor itself on prolonged anneal at 400°C. Their results depicted in Figure 35 confirmed that the TL sensitivity of CaSO4:Dy phosphor decreased by ~20% after 20 cycles of annealing at 400°C for 1 h. accompanied with a reduction in the 240°C/140°C peak height ratio. No such change was observed on annealing at 300°C or 650–700°C. The reduction in TL could be restored on annealing at 650–700°C. These two experimental results which showed a loss in the TL sensitivity of CaSO4:Dy TLD phosphor during repeated annealing at 400°C(35, 44) call into doubt the original claim of Yamashita et al.(2) that the TL efficiency of CaSO4:Dy remains constant for more than 100 readout cycles with 400°C anneal for several minutes after each readout cycle. It was shown that such a loss in TL sensitivity probably occurs due to lattice damage as a result of thermal migration of defects causing the major TL peak in CaSO4:Dy crystal lattice at 400°C(45) rather than due to any chemical reaction induced by the fluorine gas generated due to the break-down of Teflon at high temperatures(35, 46) or due to lattice changes caused by the loss of water molecules associated with Dy2(SO5)3(47). Bhatt et al.(48) have pointed out that the plausible explanation for decrease in TL sensitivity in CaSO4:Dy on prolonged anneal at 400°C is due to defect or impurity aggregation which may lead to non-radiative energy transfers. The 650°C anneal followed by quenching will disperse the impurity ion and hence enhance the sensitivity of the phosphor. Figure 34. View largeDownload slide Results of the reusability when the CaSO4:Dy Teflon discs were annealed at 400°C for 1h between readouts. Maximum temperature reached during readout was 300°C(35). Figure 34. View largeDownload slide Results of the reusability when the CaSO4:Dy Teflon discs were annealed at 400°C for 1h between readouts. Maximum temperature reached during readout was 300°C(35). Figure 35. View largeDownload slide Changes in TL sensitivity (area) and 240°C/140°C peak height ratio of CaSO4:Dy after 20 cycles of pre-irradiation annealing (for 1 h duration in each cycle) at different temperatures(44). Figure 35. View largeDownload slide Changes in TL sensitivity (area) and 240°C/140°C peak height ratio of CaSO4:Dy after 20 cycles of pre-irradiation annealing (for 1 h duration in each cycle) at different temperatures(44). Teflon softens around 327°C. Hence, Teflon-based TLDs are normally heated in the TLD reader/annealed below 327°C even though TLD materials such as CaSO4:Dy can be annealed up to 700°C without loss in their TL sensitivity. The thermal regime (during readout) which induces the darkening in CaSO4:Dy Teflon discs is 250–300°C and that which removes the darkening is 395–405°C. The discs which have undergone a 400°C, 1 h annealing treatment during production do not discolour during subsequent annealing between 220 and 400°C in the air-circulation oven. Teflon sheets skived out from billets which have been under heated (<400°C sintering) pose serious discolouration problem during readout/anneal. However, when such cut-out tape pieces were given additional annealing treatment at 400°C for 1 h, the discolouration problem vanished. It is, however, important to note that the 400°C pre-treatment has to be given to the processed tape before the discs are punched out from it. The discs would otherwise tend to distort in shape during annealing at 400°C. Break-down of Teflon polymer at high temperatures At annealing temperatures exceeding 400°C, the Teflon polymer breaks down with carbon (lamp black) production and the emanation of fluorine and CO gas and eventually should disappear altogether as CO or CO2 and F2 gases in the presence of air. But there is no systematic study on this subject. Moreover, the fate of residual TLD phosphor that should remain after the disappearance of Teflon is also not known. It will be worth knowing if the phosphor can be recovered without any loss in its TL sensitivity, since a large number of these Teflon-based dosemeters have been discarded due to discolouration and buckling with their repeated usage in the individual monitoring services in India. Therefore, Lakshmanan and Madhusoodanan(46) studied for the first time the behaviour of CaSO4:Dy Teflon TLDs in the annealing temperature range of 400–800°C. CaSO4:Dy embedded Teflon discs (13.5 mm diameter, 0.8 mm thickness, phosphor weight 70 mg, Teflon weight 210 mg) were used in this study. Since the Teflon polymer was found to completely break down at annealing temperatures exceeding 500°C, the TL sensitivities of high-temperature treated CaSO4:Dy Teflon discs were compared with that of CaSO4:Dy powder (<74 μm size) after a pre-irradiation annealing at 700°C, 1 h. In the Teflon-based TLDs also, the CaSO4:Dy phosphor used has the same grain size range. To find out the products present after the high-temperature annealing treatment, X-ray diffraction studies of CaSO4:Dy powder as well as the CaSO4:Dy Teflon discs were carried out using a Philips make Xpert multipurpose diffractometer. After 1 h annealing at 500°C, the Teflon disc turned into a fine powder indicating the complete break down of Teflon polymer strength. The residual powder was slightly blackish in colour which indicated that the release of carbon in air is not complete at this temperature. Surprisingly the weight of this residual powder was only 36 mg indicating nearly 50% loss in the CaSO4:Dy powder itself, even if we assume that the Teflon is nearly evaporated. At the annealing temperature of 700°C, the same result was obtained. The only exception was that the residual powder was completely white (but not free flowing; it was flowing in lumps similar to that of the pure Teflon powder) this time indicating that the carbon release from PTFE is complete at 700°C. At 800°C also, the residual powder was white in colour. Additionally it was free flowing similar to that of CaSO4:Dy powder. But at all these annealing temperatures nearly the same amount of residual powder was found to remain. While the disappearance of Teflon was anticipated, the disappearance of a fixed portion of the phosphor powder is a surprising result. It was suspected that in the presence of fluorine and perhaps CO gas atmosphere generated during the break down of Teflon, the CaSO4 powder has reduced to CaF2 and perhaps also to CaS. Ratio of molecular weight of CaF2 to CaSO4 = 0.57 and CaS to CaSO4 = 0.53 both of which agree very well with the observed results summarised in Table 6. The X-ray diffraction study of the residual powder after the 700°C annealing treatment indeed revealed the presence of CaF2 as well as DyF3. No diffraction line corresponding to CaS was observed. The diffraction lines corresponding to CaSO4 (e.g. d ~ 3.50) were still present but with very much reduced intensity (~15%) when compared with the X-ray diffraction data of CaSO4:Dy powder. Diffraction lines corresponding to CaF2 or DyF3 were however completely absent in the XRD data of CaSO4:Dy powder heat treated at 700°C for 1 h. The study thus confirms the conversion of CaSO4:Dy to CaF2 and DyF3 during the break down of Teflon polymer at high temperatures (>500°C). Table 6. Weight of residual powder after annealing of CaSO4:Dy Teflon discs at different temperatures for 1 h duration. The disc originally had 70 mg of CaSO4:Dy powder and 210 mg of Teflon powder(46). Annealing temperature (°C) (mg) Weight of residual powder 500 36 700 39 800 38 Annealing temperature (°C) (mg) Weight of residual powder 500 36 700 39 800 38 Table 6. Weight of residual powder after annealing of CaSO4:Dy Teflon discs at different temperatures for 1 h duration. The disc originally had 70 mg of CaSO4:Dy powder and 210 mg of Teflon powder(46). Annealing temperature (°C) (mg) Weight of residual powder 500 36 700 39 800 38 Annealing temperature (°C) (mg) Weight of residual powder 500 36 700 39 800 38 Figure 36 shows the TL glow curves of the virgin CaSO4:Dy powder and of the residual powder obtained after the high-temperature pre-irradiation annealing treatments at 700 and 800°C of CaSO4:Dy Teflon discs. After both these annealing treatments, the 230°C dosimetric peak in the residual powder had a lower TL sensitivity than that of the virgin CaSO4:Dy powder. The TL sensitivity of this peak decreases with increasing annealing temperature in the range 500–700°C. This indicates that at higher temperatures, due to the chemical reaction of the gases liberated mainly F2 and CO, and consequent conversion of CaSO4:Dy to CaF2 and DyF3, the TL sensitivity of the residual powder is reduced. XRD data confirmed the presence of these phases in high-temperature annealed sample. It is worth studying if a better sensitivity of the residual phosphor can be achieved if the Teflon discs are annealed in inert atmospheres or in air-circulation oven which could prevent the conversion/reduction of CaSO4. The change in TL glow curve shape of the residual powder at least up to 700°C annealing temperature is an indication that the reduction in TL sensitivity is related to chemical reactions and consequent conversion of the host lattice. It is known that annealing of CaSO4:Dy at 800°C, increases the intensity of 110°C peak at the cost of 230°C TL peak. This is related to the formation of CaO phase and hence is not normally recommended. Figure 36. View largeDownload slide Typical TL glow curves of virgin CaSO4:Dy powder (__O__) and of the residual powder obtained after pre-irradiation annealing of CaSO4:Dy Teflon discs at 700°C (_ __ _) and 800°C (_ _ _) for 1 h duration. Gamma dose = 1 Gy(46). Figure 36. View largeDownload slide Typical TL glow curves of virgin CaSO4:Dy powder (__O__) and of the residual powder obtained after pre-irradiation annealing of CaSO4:Dy Teflon discs at 700°C (_ __ _) and 800°C (_ _ _) for 1 h duration. Gamma dose = 1 Gy(46). As a result of this study, it was concluded that the recovery of CaSO4:Dy powder from Teflon-based dosemeter is fruitless. Radiation damage of Teflon The CaSO4:Dy Teflon discs were found to loose their strength and become brittle beyond the dose of 10 KGy which limit their use at high doses(49). Among all plastics, Teflon has the least stability against ionizing radiation. High dose of irradiation breaks carbon–carbon bond and reduces the molecular weight of Teflon thereby forming micro powder which is commercially used as a lubricant(50). Photon energy dependence of TLDs embedded in Teflon Among the many dosimetric characteristics of TLDs, the photon energy dependence is important. Tissue-equivalent (effective atomic number, Z = 7.5) phosphors such as LiF are often the preferred choice for radiation dosimetry especially if low energy photons are present in the radiation field. Theoretical calculations by Bassi et al.(51) have shown that the energy dependence of high atomic number (Z) materials such as CaF2 and CaSO4 could be reduced by embedding them into lower effective Z matrix such as Teflon. Their calculated results for CaSO4 embedded in Teflon matrix in different proportion are shown in Figure 37. The calculation has been carried out on taking into account the atomic compositions of the TLD materials as well as the binding materials in the weight fractions more commonly used and considering the composite as a homogeneous compound. Main impurities in the detector (e.g. RE) were taken into consideration on account of their high Z values. Calculations were made according to the hypothesis of electronic equilibrium, without self-absorption and with a light yield per absorbed dose independent of LET (linear energy transfer) of the incident radiation. The calculations did not take into account the fluorescent X-radiation. Another most important omission in this calculation is that the grain size of the TLD material was not considered Figure 37. View largeDownload slide The calculated photon energy dependence of CaSO4:Dy embedded Teflon dosemeters for different weight fractions. For details of calculations, see text(51). Figure 37. View largeDownload slide The calculated photon energy dependence of CaSO4:Dy embedded Teflon dosemeters for different weight fractions. For details of calculations, see text(51). The energy dependence was calculated as the ratio between the mass energy absorption coefficients of the detector and of the air respectively, in the energy range 10 keV–50 MeV. If S is this ratio, we have S(E)=(μen/ρ)detector(μen/ρ)air Since (μen/ρ)detector refers to a compound or a mixture of different elements, Bassi et al.(51) used the following additivity rule to calculate it at every energy value: (μen/ρ)detector=(μen/ρ)1×w1+(μen/ρ)2×w2+⋯+(μen/ρ)i×wi+⋯ where (μen /ρ)i is the mass energy absorption coefficient of the ith element and wi is its fraction by weight. These calculations predict a significant reduction in the photon energy dependence of CaSO4 with increasing weight proportion of Teflon. In order to find out the usefulness of the above data, Pradhan et al.(52) have experimentally investigated the energy dependence of CaSO4:Dy phosphor-embedded Teflon discs (13 mm diameter and 0.8 mm thickness) with 1, 5, 15, 25, 30 and 50% CaSO4:Dy phosphor (grain size: 0–74 μm) by weight. However, the experimentally observed energy dependence was found to be almost independent of phosphor percentage embedded in Teflon, in contradiction to the calculated energy dependence shown in Figure 38. The calculations of Bassi et al.(51) have been carried out by considering the mixture as a homogeneous compound and not taking into account the grain size of the TL phosphor. For example, the results of Jun and Becker(53) show that the energy response of Mg2SiO4:Tb can be reduced by embedding the phosphor powder of average grain size 4 μm into Teflon matrix. Hence, the effect of binding matrix in practice can be realised only if the grain sizes of the TL phosphor are of the order of a few μm (i.e. <range of secondary electrons produced by low energy photons). Under this condition, charged particle equilibrium is established and the energy deposited in the phosphor grain due to secondary electrons originated in binding material becomes significant compared to energy deposited by its own secondary electrons. For larger grain sizes/grain size ranges such as the one used by Pradhan et al.(52), the TL at low photon energies is mainly induced by the secondary electrons produced by the phosphor grains themselves and hence the effect of low Z binder such as Teflon is not felt. Figure 38. View largeDownload slide A Comparison of calculated and experimental energy dependence of CaSO4:Dy powder and CaSO4:Dy phosphor embedded in Teflon matrix(52). Figure 38. View largeDownload slide A Comparison of calculated and experimental energy dependence of CaSO4:Dy powder and CaSO4:Dy phosphor embedded in Teflon matrix(52). In a classical study, Chan and Burlin(54) applied the cavity theorem and studied theoretically the influence of grain size and the photon energy on the response of a few TLD materials (LiF, Li2B4O7, CaF2 and CaSO4). Their results on CaSO4 is described below. Consider a grain of TL powder situated in a medium of different atomic number and density. Assume that the thickness of the medium in all directions round the grains is sufficient to establish electronic equilibrium for the particular photons with which they are being irradiated. The energy which is actually deposited in a grain depends on the energy balance between the sum of the energies of the electrons generated within the grain plus the energies of the electrons generated in the case of the smallest grains in which the surface to volume ratio is the greatest. The total electron energy entering a grain will depend on the mass energy absorption coefficient of the material surrounding the grain, while the total electron energy generated within a grain will depend on the mass energy absorption coefficient of the grain. The energy removed from these electrons and absorbed by the grains and by the surrounding medium will depend upon the mass stopping powers of the two materials. Since the grain of TL material constitutes a ‘cavity’ in the medium, the problem lends itself to treatment by cavity theory. The basic equation for evaluating the stopping power ratio, fm (To,Δ) is as follows: fm(To,Δ)=(Z/A)cTo(Z/A)mΔ{1+d/To[∫Rm(To,T)×(Bc(T)/Bm(T)−1)dT+Δ.Rm(To,Δ)×(Bc(Δ)/Bm(Δ)−1)]+(1−d)[(μen/ρ)c(Z/A)m/(μen/ρ)m(Z/A)c−1]} (1) where To is the initial energy of electrons, Z is the atomic number, A is the atomic weight, R(To, T) is the ratio of the total electron flux to the primary electron flux at an energy T when the initial energy of the electrons is To, B(T) is the stopping number of electrons of energy T, Δ is the energy of an electron which will, on average, just cross the cavity, d is a weighing factor, μen/ρ is the mass energy absorption coefficient, the suffixes ‘c’ and ‘m’ refer to the cavity and the medium, respectively. The weighing factor, d, is the most critical parameter in the equation in that it determines the relative contributions to the dose in the grain from electrons generated in the surrounding medium and from electrons generated in the grain itself. It is the average attenuation of the electron spectrum in crossing the cavity and is given by the following expression: d=(1−e−βg)/βg where g is the average path length across the cavity in g cm−2 and β is the mass attenuation coefficient for the electrons, exponential absorption being a good approximation for beta rays. The effective mass attenuation coefficient was defined in terms of the range, R, by the following equation: e−βR=0.01 The absorbed dose, De, in the grain of TL material surrounded by a medium of mass energy absorption coefficient, (μen/ρ), is proportional to Deα(μen/ρ)m·fm(To,Δ), When the grain size of the material is very large, the mass stopping power ratio reduces to the ratio of the mass energy absorption coefficients of the cavity material to the medium. In this case, the absorbed dose: De(---∞)α(μen/ρ)e This is in fact the dose that any size of grain would receive if it were surrounded by identical medium (i.e. a ‘matched’ combination). It is therefore convenient to express the TL response, L, of different grain sizes relative to that for a very large grain size: L=(μen/ρ)m(μen/ρ)efm(To,Δ) (2) The response of CaSO4 subjected to photon irradiation was calculated from Equation (2). The media, surrounding it chosen for consideration was air and Teflon. The photon energy range considered was 0.01–0.60 MeV. The electrons generated by 0.60 MeV photons will have an average range of ~0.02 cm in Teflon, which might correspond to the maximum distance between grains embedded in it. Grain diameters of 1, 3, 10, 30 and 100 μm were selected and their TL response when irradiated by monoenergetic photons calculated from equation (2). CaSO4 grains surrounded by air Figure 39 shows that CaSO4 surrounded by air has the most pronounced minimum around 0.06 MeV. At 0.01 MeV photoelectric absorption is the dominant photon interaction but because of their small range the photoelectrons barely penetrate into the dosemeter or escape from it even in the smaller grains. The energy deposited in the grain and hence the response then depends mainly on the mass energy absorption coefficient of the TL material and is almost independent of grain size. As the photon energy increases, the energy of the photoelectrons and their range increases. In consequence they penetrate to a considerable depth into the grain from the surroundings and also escape from the grain from a considerable depth. As the effective atomic number of CaSO4 is greater than that of air, the mass energy absorption coefficient of CaSO4 considerably exceeds that of air and, therefore, the electron energy escaping from the grain exceeds that entering it from the air. This is most marked in the smallest grains where the probability of an electron escaping from the grain is greatest. The effect causes the minimum in the response at a photon energy of 0.05–0.10 MeV depending on the grain size. As the photon energy increases further the proportion of photoelectric interactions declines and the compton interactions become dominant. Thus, at 0.20 MeV, <10% of the interactions are photoelectric and most of the electrons are compton electrons with an average energy of ~0.01 MeV. Hence, at this photon energy the situation is somewhat similar to that at 0.01 MeV and the response is almost independent of grain size. At still higher energies, e.g. 0.60 MeV, all the electrons are compton electrons having much longer ranges but as the mass energy absorption coefficient is almost independent of atomic number, the electron energy entering and leaving the grain is almost balanced and the response is nearly independent of grain size. Figure 39. View largeDownload slide Variation of TL response of CaSO4 grains irradiated in air with photon energy. The diameters of the grains are indicated in the graph(54). Figure 39. View largeDownload slide Variation of TL response of CaSO4 grains irradiated in air with photon energy. The diameters of the grains are indicated in the graph(54). CaSO4 grains in Teflon The TL response of CaSO4 grains in Teflon to monoenergetic photons was similarly calculated and found to be similar to that in air. The reason for this may be understood by examining Equations (1) and (2). The second term in Equation (1) is the product of the weighing factor, d, and an expression, which is small compared to other terms in the equation. Thus, to a close approximation the second term may be neglected and Equation (2) reduces to L=(μen/ρ)m(Z/A)c/(μen/ρ)c(Z/A)m×[1+(1−d){(μen/ρ)c(Z/A)m/(μen/ρ)m(Z/A)c}−1]∴L=(1−d)+d(μen/ρ)m(Z/A)c/(μen/ρ)c(Z/A)m (3) Now at low photon energies the mass energy absorption coefficient of CaSO4 is greater than that of either Teflon or air. Hence, for small grain sizes, where, d is small, the second term is negligible and Equation (3) becomes, L ~ (1−d). Thus, the response, L, is independent of the surrounding medium and is a function of the grain size only for small grains at low photon energies. This is because the relative contribution to the absorbed dose in the grain from electrons entering the grain from the surrounding medium is extremely small. At high photon energies the Compton effect is dominant so that the mass energy absorption coefficients for all materials are nearly equal and Equation (3) becomes L ~ 1. Thus, the response, L, is independent of the surrounding medium and the grain size at high photon energies. This is because the electrons entering and leaving the grain are balanced. The net result is that the radiation induced TL in CaSO4 is almost unaltered when the medium surrounding the grains is changed from air to Teflon. Thus, Chan and Burli’s(54) calculations based on cavity theorem show that if low grain size (~1 μm) is used, the photon energy dependence of CaSO4:Dy in air as well as in Teflon matrix can be decreased by as much as a factor of nearly 5. In order to study experimentally the effects of phosphor proportion and grain size, Pradhan and Bhatt(55) have chosen CaSO4:Dy phosphor grains in the range 53–75, 37–53 and 0–37 μm by grinding and sieving though the standard sieves. The 0–37 μm grain size range was further fractioned to lower grain sizes (8–37 μm, 1–8 μm and <1 μm) by allowing it to suspend in acetone. In 2 min, grains larger than 8 μm settled down. After 2 min, the suspension was poured into another beaker and allowed to stand for 20 min during which the grains in the range 1–8 μm settled. The suspension with grains size <1 μm was poured into a third beaker and allowed to stand for 4 days after which the remaining acetone suspension was discarded. Table 7 shows that the inherent TL sensitivity of the CaSO4:Dy Teflon discs to gamma rays decreases with grain size of the phosphor. Table 8 compares the experimental energy-response for the Teflon discs with phosphor grain size range 1–8 μm in various proportions with the calculated(55) response for similar mixtures not taking into account the grain size of the phosphor as well as that of CaSO4:Dy loose powder. Figure 38 shows that the photon energy dependence observed experimentally (for 5% phosphor proportion) decreases with deceasing grain size range as expected. However, Table 8 shows that the effect of phosphor proportion is insignificant. Figure 40 shows that calculated energy dependence is much lower than the experimental energy dependence observed with grain size <1 μm. This shows that the contribution of secondary electrons from the phosphor grains towards TL continues to be significant even at such low grain sizes. Figure 41 shows that Chan and Burlin’s(54) calculated energy dependence for 1 μm size grains in Teflon matches somewhat with the experimental response for grain size (<1 μm) for 5% phosphor loading. Table 7. Relative sensitivity of Teflon discs having CaSO4:Dy phosphor (5% by weight) in various grain size ranges(42). Phosphor grain size (μm) Relative TL sensitivity 53–75 1.00 8–53 0.85 1–8 0.38 <1 0.14 Phosphor grain size (μm) Relative TL sensitivity 53–75 1.00 8–53 0.85 1–8 0.38 <1 0.14 Table 7. Relative sensitivity of Teflon discs having CaSO4:Dy phosphor (5% by weight) in various grain size ranges(42). Phosphor grain size (μm) Relative TL sensitivity 53–75 1.00 8–53 0.85 1–8 0.38 <1 0.14 Phosphor grain size (μm) Relative TL sensitivity 53–75 1.00 8–53 0.85 1–8 0.38 <1 0.14 Table 8. Experimental and calculated energy dependence factors of CaSO4:Dy Teflon discs(52). Effective photon energy (keV) Experimental Calculated considering effective Z of mixture CaSO4:Dy phosphor (1–8 μm grain size) in Teflon by weight Photon energy (keV) CaSO4:Dy loose powder CaSO4:Dy in Teflon (by weight) 25% 5% 1% 30% 5% 30 7.0 7.0 6.5 30 11.08 4.36 1.86 56 4.2 3.8 3.5 60 6.74 2.89 1.46 78 2.5 2.3 2.2 80 3.83 1.92 1.22 1250 1.0 1.0 1.0 1000 1.00 1.00 1.00 Effective photon energy (keV) Experimental Calculated considering effective Z of mixture CaSO4:Dy phosphor (1–8 μm grain size) in Teflon by weight Photon energy (keV) CaSO4:Dy loose powder CaSO4:Dy in Teflon (by weight) 25% 5% 1% 30% 5% 30 7.0 7.0 6.5 30 11.08 4.36 1.86 56 4.2 3.8 3.5 60 6.74 2.89 1.46 78 2.5 2.3 2.2 80 3.83 1.92 1.22 1250 1.0 1.0 1.0 1000 1.00 1.00 1.00 Table 8. Experimental and calculated energy dependence factors of CaSO4:Dy Teflon discs(52). Effective photon energy (keV) Experimental Calculated considering effective Z of mixture CaSO4:Dy phosphor (1–8 μm grain size) in Teflon by weight Photon energy (keV) CaSO4:Dy loose powder CaSO4:Dy in Teflon (by weight) 25% 5% 1% 30% 5% 30 7.0 7.0 6.5 30 11.08 4.36 1.86 56 4.2 3.8 3.5 60 6.74 2.89 1.46 78 2.5 2.3 2.2 80 3.83 1.92 1.22 1250 1.0 1.0 1.0 1000 1.00 1.00 1.00 Effective photon energy (keV) Experimental Calculated considering effective Z of mixture CaSO4:Dy phosphor (1–8 μm grain size) in Teflon by weight Photon energy (keV) CaSO4:Dy loose powder CaSO4:Dy in Teflon (by weight) 25% 5% 1% 30% 5% 30 7.0 7.0 6.5 30 11.08 4.36 1.86 56 4.2 3.8 3.5 60 6.74 2.89 1.46 78 2.5 2.3 2.2 80 3.83 1.92 1.22 1250 1.0 1.0 1.0 1000 1.00 1.00 1.00 Figure 40. View largeDownload slide Experimental results on effect of phosphor grain size: (1) 53–75 μm, (2) 8–37 μm, (3) 1–8 μm and (4) <1 μm on photon energy dependence of Teflon TLD discs having 5% CaSO4:Dy phosphor by weight(55). Figure 40. View largeDownload slide Experimental results on effect of phosphor grain size: (1) 53–75 μm, (2) 8–37 μm, (3) 1–8 μm and (4) <1 μm on photon energy dependence of Teflon TLD discs having 5% CaSO4:Dy phosphor by weight(55). Figure 41. View largeDownload slide Photon energy dependence of CaSO4:Dy Teflon discs, (a) experimental curve of Teflon disc shaving 5% CaSO4:Dy phosphor (grain size <1 μm) for X-ray spectra with an effective energy equal to the energy for which the points are plotted, (b) calculated curve obtained from calculations(54) for CaSO4 grains of 1 μm size in Teflon, (c) calculated(50) curve for CaSO4 (5%) + Teflon (95%)(55). Figure 41. View largeDownload slide Photon energy dependence of CaSO4:Dy Teflon discs, (a) experimental curve of Teflon disc shaving 5% CaSO4:Dy phosphor (grain size <1 μm) for X-ray spectra with an effective energy equal to the energy for which the points are plotted, (b) calculated curve obtained from calculations(54) for CaSO4 grains of 1 μm size in Teflon, (c) calculated(50) curve for CaSO4 (5%) + Teflon (95%)(55). The above experimental results, i.e. the decrease in photon energy dependence of CaSO4:Dy with decreasing grain size but the insignificant effect phosphor proportion in Teflon matrix at low grain sizes (1–8 μm), are completely in agreement with the theoretical prediction of Chan and Burlin. At such low grain sizes, the probability of an electron escaping the grain is greatest which causes the minimum in the response at a photon energy range 0.05–0.10 MeV. Since the mass energy absorption coefficient of CaSO4 is considerably greater than that of air, the electron energy escaping from the grain exceeds that entering it from the air. This reduces the effect of Teflon proportion on the energy dependence of CaSO4:Dy. However, the benefits of such reduced photon energy dependence at low grain sizes can be realised only if such low grain size phosphor can be manufactured with high TL sensitivity. This requires the development of new phosphor preparation techniques which have not yet been achieved. BETA RADIATION DOSIMETRY As per International Commission on Radiation Units and Measurements (ICRU) definition, the appropriate operational quantity to be measured for photons ≥15 keV is Hp(10) and that for photons <15 keV, Hs(0.07). The application of the ICRU operational quantities to the dose measurement of beta radiation has shown the importance of replacing Hp(10) by Hp(3) for electrons with energies ≥1 MeV so that the risk to gonads can be taken care of separately from that of the lens of the eye. Due to its low penetrating power, low energy (<1 MeV) beta radiation is of potential detriment to the skin only. In determining Hp(10) only electrons above 2 MeV have to be considered as lower energy electrons cannot penetrate a 10 mm thick layer of tissue-equivalent material. For neutrons, Hs(0.07) is never limiting and hence Hp(10) is always used(56). In the nuclear power industry the skin may be subjected to irradiation by a range of energies and types of radiation and in many situations, particularly where low energy beta-radiation is predominant the skin dose may be limiting. There remains the formidable task of implementing the International Commission on Radiological Protection (ICRP) recommendation on the dose limits to skin regions since it infers the use of a 5 mg cm−2 detector beneath a 5 mg cm−2 window. Such a device, because of its thickness, will exhibit low sensitivity, high detection threshold and will probably be unsuitable for routine use because of its fragility. The lack of availability of such dosemeters has been pointed out by the Commission of the European Communities (CEC)(57). Figure 42 shows the TL response of CaSO4:Dy Teflon discs of different thicknesses to beta rays from 32P (Emax = 1.71 MeV) and 204Tl (Emax = 0.77 MeV) relative to 90Sr/90Y (Emax = 2.27 MeV), The TL sensitivity decreases with increasing dosemeter thickness much faster in the case of low energy beta radiation (204Tl) as compared to that of high-energy beta rays (32P). This essentially means that the detection efficiency of thicker dosemeters (>4 mm) to soft beta rays is poor in the presence of high-energy beta rays (90Sr/90Y) or X- and gamma-rays. This is due to the fact that soft beta-rays do not traverse fully the thick solid dosemeter unlike the other radiations mentioned above. Figure 42. View largeDownload slide TL response of CaSO4:Dy Teflon discs of different thicknesses to beta rays from32P (Emax = 1.71 MeV) and 204Tl (Emax = 0.77 MeV) relative to 90Sr/90Y (Emax = 2.27 MeV)(34). Figure 42. View largeDownload slide TL response of CaSO4:Dy Teflon discs of different thicknesses to beta rays from32P (Emax = 1.71 MeV) and 204Tl (Emax = 0.77 MeV) relative to 90Sr/90Y (Emax = 2.27 MeV)(34). A thin CaSO4:Dy Teflon TLD card Several techniques have been proposed in the literature to reduce the beta ray energy dependence of TLDs. These include (1) the deposition of phosphor powder onto a non-TL base, (2) the direct use of a thin phosphor mixed Teflon wafer and (3) addition of graphite to a thick TLD so as to limit the emission of light to that from the surface layer. In the first case very fine grains were deposited on thin aluminium discs using acetone suspension. With this device beta rays as low as 10 keV could be measured. Such dosemeters are however too delicate to handle in routine dosimetry. Many attempts have been made to make handy and thin dosemeters. Examples of such a device are a thin layer dosemeter based on high-temperature self-adhesive Kapton tape and dosemeter films of phosphor mixed polythersulphone. Phosphor powder (5 mg cm−2) layer fixed to a polyimde tape are a few other examples. The last of these is now commercially available (Vinten Instruments, UK), but it has to be discarded after one use. Teflon-based thin TLD poses handling problems in routine use as they curl during readout and make poor contact with the heater strip during heating. To solve the problem of buckling of thin (<0.8 mm thick) CaSO4:Dy Teflon discs several top supports—glass, quartz, meshes made of aluminium, stainless steel, Teflon, Bakelite, Tufnol, etc. were tried by Lakshmanan et al.(34). The perforated 1 mm thick aluminium top support and stainless steel wire mesh acted as a heat sink and hence resulted in a broadened glow curve. A 1.5 mm thick quartz plate and glass plate also acted in a similar fashion. They also broke after a few cycles of use. Top supports of perforated Teflon also gave unsatisfactory results. Perforated Bakelite and Tufnol top supports gave fairly sharp glow curves but they got blackened and emitted fumes during readout. Hence, none of these materials was found to be satisfactory as top supports. A proper device which prevents the buckling of the Teflon dosemeters of any thickness and which at the same time does not act as a heat sink or attenuate the TL appreciably was discovered by Lakshmanan et al.(34). This appears to be the ideal device for routine beta dosimetric applications. The device consists of a natural transparent ruby mica sheet of 0.1 mm thickness interposed permanently in between the metal card and the Teflon dosemeters. While heating, the strip heater makes contact with the TLD disc and presses it against the mica sheet + aluminium card support. The heater strip does not touch the mica sheet since the latter is situated on the side facing the photomultiplier and the light is collected through the mica sheet and hence is attenuated only marginally. The attenuation of TL emission from CaSO4:Dy by this antibuckling device is <15%. However, in the NRPB (National Radiological Protection Board) system, the top support blocks the TL light by ~50%. The top supports used by Teledyne Isotopes, USA and NRPB, UK are incorporated permanently in the reader systems. In these readers, during TL measurement, the Teflon dosemeter along with the TLD card is lifted by a suitable device such that the dosemeter is pressed against the top support. However, in the TLD card developed by Lakshmanan et al.(34), the mica sheet is incorporated in the personal monitoring TLD card which is somewhat inconvenient because each TLD card has to have one mica sheet. The mica sheets used in all the cards should have uniform transparency. Only then the TL sensitivity of different cards will have uniform sensitivity. Since mica is a naturally occurring substance, it contains many impurities. Furthermore, it grows naturally one layer over another. Therefore, large-scale procurement of mica sheets of uniform thickness and transparency is difficult. But the mica sheet in an appropriate holder could be incorporated inside the reader system as in the case of Teledyne Isotopes, USA and NRPB, UK but with better transparency and antibuckling properties than those used in these systems. A cross-sectional and top views of the TLD badge containing a thin CaSO4:Dy Teflon tape with the top mica sheet support is shown in Figure 43. The heater anvil presses the Teflon tape against rigid mica sheet placed on the aluminium card and hence the thin tape does not curl while heating. Mica sheet is transparent to TL light. Mica being an insulator and very thin does not absorb heat and hence the phosphor embedded Teflon tape gets heated up fast resulting in sharp TL glow curves with a readout duration of 30 s as seen in Figure 44a. This device could be used to heat dosemeters down to 50 μm thickness as shown in Figure 44d. In the absence of mica sheet, the 0.4 mm thick TLD tape gets buckled during heating and the glow curve gets distorted due to poor contact with the heater as seen in Figure 44b. The development of an ultra-thin LiF Teflon TLD disc (30 μm) which is thermally bonded to a thick Teflon base (~0.2 mm) has been proposed by Charles(58) for measuring the integral dose (skin dose) between tissue depths 5–10 mg cm−2. This dosemeter has a flat response in the beta-energy range 0.76–2.0 MeV as shown in Figure 45. However, subsequent investigations(59) have shown that the thin layer TLD tends to peel off from the Teflon base since Teflon is basically non sticky and hence does not easily bond with other materials. For beta dosimetry, the phosphor powder layer sandwiched between strips of polyethylene (PE) is marketed by Vinten Instruments, UK, but it has to be discarded after one use. Figure 43. View largeDownload slide Top and side views of the TLD card incorporating a thin (0.05–0.4 mm thick) CaSO4:Dy embedded Teflon tape with a top support made of thin (0.1 mm thick) transparent mica sheet. Figure 43. View largeDownload slide Top and side views of the TLD card incorporating a thin (0.05–0.4 mm thick) CaSO4:Dy embedded Teflon tape with a top support made of thin (0.1 mm thick) transparent mica sheet. Figure 44. View largeDownload slide TL glow curves of 0.4 mm thick CaSO4:Dy tape loaded in the improved aluminium cardholder with (a) and without (b) the antibuckling device. The glow curves of 0.1 mm thick CaSO4:Dy tape (c) and 0.05 mm thick CaF2:Tm tape (d) in the improved cardholder with antibuckling mica device. TL intensities of the glow curves are not to be inter compared. Gamma-dose = 10 mGy(34). Figure 44. View largeDownload slide TL glow curves of 0.4 mm thick CaSO4:Dy tape loaded in the improved aluminium cardholder with (a) and without (b) the antibuckling device. The glow curves of 0.1 mm thick CaSO4:Dy tape (c) and 0.05 mm thick CaF2:Tm tape (d) in the improved cardholder with antibuckling mica device. TL intensities of the glow curves are not to be inter compared. Gamma-dose = 10 mGy(34). Figure 45. View largeDownload slide The energy dependence of TLDs to beta-radiation (A: X) 0.1 mm CaSO4:Dy Teflon disc bonded to 0.7 mm graphite-mixed Teflon base (B:□) 0.1 mm CaSO4:Dy Teflon disc bonded to 0.7 mm pure Teflon base (C:●) 0.2 mm CaSO4:Dy Teflon disc bonded to 0.6 mm graphite mixed Teflon base (D:O) 0.8 mm CaSO4:Dy Teflon disc (E:) 25 μm LiF Teflon disc as reported by Charles(58, 59). Figure 45. View largeDownload slide The energy dependence of TLDs to beta-radiation (A: X) 0.1 mm CaSO4:Dy Teflon disc bonded to 0.7 mm graphite-mixed Teflon base (B:□) 0.1 mm CaSO4:Dy Teflon disc bonded to 0.7 mm pure Teflon base (C:●) 0.2 mm CaSO4:Dy Teflon disc bonded to 0.6 mm graphite mixed Teflon base (D:O) 0.8 mm CaSO4:Dy Teflon disc (E:) 25 μm LiF Teflon disc as reported by Charles(58, 59). While the high inherent TL sensitivity of CaSO4:Dy makes it an attractive dosemeter for beta radiation, its energy dependence to photons make it unsuitable for beta ray dosimetry in mixed fields of beta rays and low energy photons. Since the application of metal filters is not possible in such cases, the photon energy independent TLDs based on LiF or Li2B4O7 remains as the only choice. However, such mixed fields are rarely encountered and hence in most cases CaSO4:Dy TLD has been used successfully for beta ray dosimetry. Extremity dosimetry The monitoring of extremity (e.g. fingers) doses to radiation workers is important for staff handling radiation sources in pharmaceutical and isotope laboratories as well as in nuclear medicine and brachytherapy departments, with special emphasis on finger tip dosimetry. Julius et al.(60) have developed a finger ring dosemeter holder to contain a TLD which also matches their personnel monitoring readout system. For simultaneous measurement of beta/gamma doses in mixed fields, Uchrin(61) has developed a new type of three element dosemeter attached in a ring type holder made of polystyrene. The beta element (LiF TLD cold pressed on an aluminium disc with detector thickness of a few mg cm−2), covered by a thin window (1 mg cm−2 thick) is held at the top of the holder. A hot pressed LiF TLD covered with a 1.5 mm aluminium disc is held below the beta element. In the bottom is another hot-pressed LiF TLD covered with a 0.5 mm thick lead disc. The dosimeter offers a high beta discrimination (factor of 30) and also information about the quality of photons. In nuclear medicine departments, radionuclides (mostly isomers of 99mTc and 113mIn in liquid form) are handled in either glass vials or in injection syringes, and so the dose to the extremities due to beta radiation is not significant. For extremity dosimetry tissue-equivalent dosemeters such as LiF:Mg,Ti or Li2B4O7:Mn/Cu are the most suitable since the above isotopes emit low energy photons. CaSO4:Dy has been used for the measurement of low doses with these isotopes but only after the application of a photon energy-dependent correction factor of 0.5 for 99 mTc and 0.33 for 113mIn(62). The finger tip doses to persons involved in extracting, dispensing and injecting radiopharmaceuticals measured were in the range from a few tens of μGy to a few mGy. Personnel doses to the extremities and the eyes during several types of interventional radiology procedures have been measured by Koukorava et al.(63) using TL pellets. The mean doses per kerma area product were calculated for the monitored anatomic regions and for the most frequent types of procedures. Higher dose values were measured during therapeutic procedures, especially embolisations. The maximum recorded doses during a single procedure were 1.8 mSv to the finger (nephrostomy), 2.1 mSv to the wrist (liver chemoembolisation), 0.6 mSv to the leg (brain embolisation) and 2.4 mSv to the eye (brain embolisation). The annual doses estimated for the operator with the highest work load according to the measurements and the system’s log book were 90.4 mSv to the finger, 107.9 mSv to the wrist, 21.6 mSv to the leg and 49.3 mSv to the eye. There is a consensus in the literature regarding the requirement of regular extremity dose monitoring of the staff in nuclear medicine. Finger dose monitoring is essential for controlling the extremity dose limits for occupational personnel handling unsealed radioactive sources. Some studies showed that the annual dose limit of 500 mSv could be exceeded if the required optimisation is not applied(64, 65). Combined whole-body dual-tracer ((18)F-FDG and (11)C-acetate) PET/CT is increasingly used for staging hepatocellular carcinoma, with only limited studies investigating the radiation dosimetric data of these scans. The radiation dosimetry of combined whole-body dual-tracer PET/CT protocols has been recently characterized(66). Beta dosimetry using thick dosemeters ICRU 39(67) has characterized superficial DE, Hs(0.07) as suitable for shallow organs which will be irradiated by both weakly penetrating and strongly penetrating radiations. Due to its low penetrating power, low energy beta radiation (<1 MeV) is of potential detriment to the skin only. For electrons ≥1 MeV, Hp(10) can be replaced by Hp(3) so that the risks to gonads can be taken care of separately from that of the lens of the eye. In determining Hp(10), only electrons >2 MeV have to be considered as lower energy electrons cannot penetrate 10 mm thick layer of tissue equivalent material(67). The TL response of the 0.8 mm thick CaSO4:Dy Teflon disc used in the open window of the personnel monitoring badge in India to beta rays from 90Sr/90Y (Emax = 2.27 MeV), 32P (Emax = 1.71 MeV) and 204Tl (Emax = 0.77 MeV) was studied by Vohra et al.(4). The primary dosimetry of these beta sources was done with an extrapolation ionisation chamber. Since the thickness of the TLD disc (0.8 mm in this case) is larger than the average range of beta rays, its response decreases considerably at low beta energies. As a result, the TL response ratio for the disc under the open window to that under plastic filter (D3/D2) increases at lower beta energies provided the irradiation consists of only beta radiation, and this ratio is utilised for arriving at the value of an appropriate correction factor for beta-dose estimation. However, the above ratio (D3/D2) = 20 and hence the corresponding correction factor =2.5 (normal incidence) and 5 (45° incidence) at 204Tl beta energy level. In mixed beta–gamma radiation field it is difficult to estimate these correction factors accurately because of the disc to disc sensitivity variation and hence low energy (<2 MeV) beta estimation with the above TLD badge is difficult. Graphite-mixed CaSO4:Dy Teflon discs for beta dosimetry As seen above, the beta energy dependence can be appreciably reduced, particularly in the case of phosphor-embedded Teflon dosemeters, by using very thin discs or tapes (≤0.2 mm). Similar results are obtained by using TLD discs of usual thickness (0.4–0.8 mm) whose optical transparency is reduced by incorporating into them a light-absorbing material. Initial attempts were made to improve the energy response of LiF and Li2B4O7 sintered pellets to beta radiation by adding graphite(68). Subsequently, the incorporation of various amounts of graphite powder into 0.8 mm thick CaSO4:Dy Teflon discs was tried(69). Figure 46 shows that the incorporation of graphite decreases drastically the intrinsic TL sensitivity of normally used 0.8 mm thick CaSO4:Dy Teflon discs (to 60Co gamma-rays) having 25% TL phosphor and 75% Teflon. For example, 10% graphite loading reduces its TL sensitivity by more than a factor of 20. This is due to the reduction in the optical transparency of the CaSO4:Dy Teflon discs with the graphite incorporation. Figure 47 shows the energy dependence of these discs with different graphite content to beta rays. A decrease in the beta energy dependence with increase in graphite content is seen. For example, at 204Tl beta ray energy level (Emax = 0.77 MeV), the TL sensitivity of the normal CaSO4:Dy Teflon discs (0% graphite loading) is just 40% of that at 90Sr/90Y beta ray energy level (2.2 MeV) whereas with 10% graphite-loaded discs more or less a flat TL response is seen in the beta ray energy range 0.77–2.2 MeV. Figure 46. View largeDownload slide Relative TL sensitivity of CaSO4:Dy embedded Teflon discs to gamma-rays as a function of graphite content(69). Figure 46. View largeDownload slide Relative TL sensitivity of CaSO4:Dy embedded Teflon discs to gamma-rays as a function of graphite content(69). Figure 47. View largeDownload slide Energy dependence of CaSO4:Dy embedded Teflon discs to beta-rays as a function of graphite content(69). Figure 47. View largeDownload slide Energy dependence of CaSO4:Dy embedded Teflon discs to beta-rays as a function of graphite content(69). ICRU OPERATIONAL QUANTITIES The ICRU operational quantities for external radiation exposure are needed since the protection quantities equivalent dose in an organ or tissue and effective dose are generally not measurable. Furthermore, exposure limits are given in terms of protection quantities. Measurements need the calibration of instruments in terms of measurable quantities. The operational quantities in current use were defined in the 1980s(67, 70) (ICRU, 1985, 1988), which have been introduced into practice in many countries under radiological protection directives and regulations over the past 30 years. The operational quantities are defined using the quantity dose equivalent, H*(d). H is the product of Q and D at a point in tissue; thus, H′(D), where, D is the absorbed dose and Q is the quality factor at that point. Q is defined as a function of unrestricted LET, L∞ (often denoted as L or LET), of charged particles in water (ICRP, 1996)(71). Nevertheless, the existing system has some limitations and needs further improvement to consider changes in the fields of application of the protection quantities and operational quantities (ICRP, 2007, 2010)(72, 73). ICRU established Report Committee 26 in 2010(74) to discuss the above-mentioned issues and propose an alternative system of operational quantities for external radiations. The protection quantities (organ absorbed dose, DT, organ equivalent dose, HT and effective dose, HE) and operational quantities (ambient dose equivalent, H*(d), directional dose equivalent, H′(D,Ω) and personal dose equivalent, Hp(d)) are related to basic physical quantities (fluence, φ, kerma, k and absorbed dose, D) which are generally used in radiation metrology and radiation dosimetry, and are obtained through primary standards at national standards laboratories. The major change to the currently favoured set of quantities is redefinition of the operational quantities, from being based on doses at specific points in the ICRU sphere and soft tissue, to being based on particle fluence and conversion coefficients for effective dose and absorbed dose to the lens of the eye and local skin. Effective dose is based on Ambient dose equivalent, H* and personal dose equivalent, Hp. Absorbed dose to the lens of the eye is based on Directional absorbed dose, D′lens(Ω) and personal absorbed dose, Dp,lens. Absorbed dose to local skin is based on Directional absorbed dose, D′local skin(Ω) and personal absorbed dose, Dp,local skin. Introduction of the proposed system requires a revision of conversion coefficients, and it is planned to include these conversion coefficients in the forthcoming ICRU/ ICRP report on the operational quantities. Calibration phantoms for personal dosimeters remain unchanged, and there are a few minor changes to be made to the dosimetry of the International Organisation for Standardisation reference fields and calibration procedures. Therefore, the impact of adopting the proposed system on routine measurement practice is not likely to be significant as a whole(75). The following sections describe briefly the measurement of operational quantities defined by ICRU (1985,1988) by CaSO4:Dy BARC TLD badge. The calibration procedures adopted is described elsewhere(76). Estimation of individual dose equivalents, Hp(10) and Hs(0.07) A schematic diagram of the of CaSO4:Dy BARC TLD badge employed in India for countrywide personal monitoring is seen in Figure 48. Its TL response(76) has been studied in terms of Hp(10) and Hs(0.07). and the results are shown in Table 9. Figure 49 shows the dependence of D1 (response under the Cu+Al filter in the TLD badge, type A in Table 10), in terms of D1/Hp (10) and D1/Hs(0.07), as a function of photon energy. As the response D1 differs from that of either Hp(10) or Hs(0.07) by more than 30% in certain photon energy regions, a procedure has been devised to correct its energy dependence through the ratio between the readings of CaSO4:Dy under two different filters. The over-estimation by a maximum factor of 1.5 in HE, i.e Hp(10) by the TLD on phantom for photon energies above 40 keV, is partly due to the backscattered radiation from the phantom and partly due to inadequate compensation of the photon energy dependence of CaSO4:Dy by the Cu+Al filters used. Table 9 gives the energy-dependent correction factors C1 = Hs(0.07)/D1 and C2 = Hp(10)/D1 obtained from a plot between D2/D1 ratio and C1 or C2. D1 and D2 are the TL responses of dosemeters below Cu+Al metal filters and perspex (1.5 mm thickness) respectively for irradiations on phantom. The corresponding HE are to be evaluated from Hp(10) = D1 × C2 and Hs(0.07) = D1 × C1. Table 9. Dependence of D2/D1 (TL response below perspex/TL response below Cu+Al filters) and correction factors C1 and C2 relative to photon energy(76). E (keV) D2/D1 C1 = Hs(0.07)/D1 C2 = Hp(10)/D1 29 14.99 1.32 0.98 46 7.38 0.80 0.78 65 4.31 0.68 0.74 81 2.97 0.64 0.68 105 2.27 0.65 0.69 1250 1.19 1.00 1.00 E (keV) D2/D1 C1 = Hs(0.07)/D1 C2 = Hp(10)/D1 29 14.99 1.32 0.98 46 7.38 0.80 0.78 65 4.31 0.68 0.74 81 2.97 0.64 0.68 105 2.27 0.65 0.69 1250 1.19 1.00 1.00 Table 9. Dependence of D2/D1 (TL response below perspex/TL response below Cu+Al filters) and correction factors C1 and C2 relative to photon energy(76). E (keV) D2/D1 C1 = Hs(0.07)/D1 C2 = Hp(10)/D1 29 14.99 1.32 0.98 46 7.38 0.80 0.78 65 4.31 0.68 0.74 81 2.97 0.64 0.68 105 2.27 0.65 0.69 1250 1.19 1.00 1.00 E (keV) D2/D1 C1 = Hs(0.07)/D1 C2 = Hp(10)/D1 29 14.99 1.32 0.98 46 7.38 0.80 0.78 65 4.31 0.68 0.74 81 2.97 0.64 0.68 105 2.27 0.65 0.69 1250 1.19 1.00 1.00 Table 10. Description of different metal filters(76). Filter type Metal filter used Front side (irradiation side) Back side (phantom side) A 30 × 16 × 1 mm3 Cu 12.5 mm ϕ Cu+ Present 15.7 mm ϕ and 15.7 mm ϕ Al Badge 1 mm thick Al Each 1 mm thick B 30 × 16 × 1.8 mm3 Cu Nil C Same as A 30 × 16 × 1 mm3 Cu D Same as A 30 × 16 × 0.5 mm3 Cu E 30 × 16 × 1.8 mm3 Cu Same as A Filter type Metal filter used Front side (irradiation side) Back side (phantom side) A 30 × 16 × 1 mm3 Cu 12.5 mm ϕ Cu+ Present 15.7 mm ϕ and 15.7 mm ϕ Al Badge 1 mm thick Al Each 1 mm thick B 30 × 16 × 1.8 mm3 Cu Nil C Same as A 30 × 16 × 1 mm3 Cu D Same as A 30 × 16 × 0.5 mm3 Cu E 30 × 16 × 1.8 mm3 Cu Same as A Table 10. Description of different metal filters(76). Filter type Metal filter used Front side (irradiation side) Back side (phantom side) A 30 × 16 × 1 mm3 Cu 12.5 mm ϕ Cu+ Present 15.7 mm ϕ and 15.7 mm ϕ Al Badge 1 mm thick Al Each 1 mm thick B 30 × 16 × 1.8 mm3 Cu Nil C Same as A 30 × 16 × 1 mm3 Cu D Same as A 30 × 16 × 0.5 mm3 Cu E 30 × 16 × 1.8 mm3 Cu Same as A Filter type Metal filter used Front side (irradiation side) Back side (phantom side) A 30 × 16 × 1 mm3 Cu 12.5 mm ϕ Cu+ Present 15.7 mm ϕ and 15.7 mm ϕ Al Badge 1 mm thick Al Each 1 mm thick B 30 × 16 × 1.8 mm3 Cu Nil C Same as A 30 × 16 × 1 mm3 Cu D Same as A 30 × 16 × 0.5 mm3 Cu E 30 × 16 × 1.8 mm3 Cu Same as A Figure 48. View largeDownload slide A cross-sectional view of the personnel dosemeter badge based on CaSO4:Dy (0.5 mol%) Teflon TLD developed at the Bhabha Atomic Research Centre (BARC) in India(4). Figure 48. View largeDownload slide A cross-sectional view of the personnel dosemeter badge based on CaSO4:Dy (0.5 mol%) Teflon TLD developed at the Bhabha Atomic Research Centre (BARC) in India(4). Figure 49. View largeDownload slide Photon energy dependence of CaSO4:Dy Teflon disc below the Cu+Al filter (D1) in the present TLD badge in terns of Hp(10) and Hs(0.07). O: D1/Hp(10) and ●: D1/Hs(0.07)(76). Figure 49. View largeDownload slide Photon energy dependence of CaSO4:Dy Teflon disc below the Cu+Al filter (D1) in the present TLD badge in terns of Hp(10) and Hs(0.07). O: D1/Hp(10) and ●: D1/Hs(0.07)(76). The dose evaluation procedure will, however, become simple, if the dosemeter response, D1 directly simulates Hp(10) response over the entire photon energy range. In order to achieve this, different filter configurations described in Table 10 were tried. Figure 50 shows the TL response (D1) of the CaSO4:Dy Teflon disc beneath these filter(s) in comparison with the response of Hp(10). The over-response of D1 to low energy photons could be reduced by increasing the thickness of the Cu filter. The TL response of CaSO4:Dy sandwiched between a 1.8 mm thick Cu + 1 mm thick Al filters on the irradiation side and 1 mm thick Cu + 1 mm thick Al filters on the phantom side (type E in Table 10) estimates the Hp(10) response within ±30% in the photon energy range studied as required by the ISO/IEC standard (International Organization for Standardisation/International Electrotechnical Commission)(77). Thus, in this case, the D1 reading gives Hp(10) directly. The estimation of Hs(0.07) for photons with the modified badge will, however, still require the application of an energy-dependent correction as above, especially for photon energies below 30 keV. The removal of metal filters on the phantom side (type B in Table 10) enhances the over-response of D1 to low energy photons due to enhanced contribution of backscattered radiation. Figure 50. View largeDownload slide Photon energy dependence of CaSO4:Dy Teflon disc beneath different metal filter configurations (described in Table 8) in terms of Hp(10). ▽, type A; ∆, type B, X, type C, O, type D, ●. type E. The ISO/IEC standards on personal dosemeters require that the calculated dose shall not differ from the conventional true dose by more than 30% for photons in the range 15 keV–3 MeV(76). Figure 50. View largeDownload slide Photon energy dependence of CaSO4:Dy Teflon disc beneath different metal filter configurations (described in Table 8) in terms of Hp(10). ▽, type A; ∆, type B, X, type C, O, type D, ●. type E. The ISO/IEC standards on personal dosemeters require that the calculated dose shall not differ from the conventional true dose by more than 30% for photons in the range 15 keV–3 MeV(76). Table 11 gives the ratio of ion chamber readings on phantom to ion chamber readings in free air at the same location (i.e. backscatter factor, BSF) as well as Cs(E) values at different photon energies. The agreement between the two values is within ±10% despite the fact that the ion chamber readings correspond to doses on the phantom surface whereas Cs(E) values correspond to doses at a depth of 0.07 mm below the phantom surface. Hence, the dosemeters calibrated on phantom (i.e both the ion chamber and the TLD are kept on phantom during calibration irradiations), a practice followed by BARC so far, should estimate Hs(0.07) provided the energy-dependent correction factors are suitably applied as described earlier(76). Table 11. Dependence of backscatter factor (ion chamber + reading on phantom/ion chamber reading in air) and conversion coefficient Cs(E) = Hs(0.07) Ka−1 on the photon energy E(76). E (keV) Backscatter factor Cs (E) (2)/(3) (1) (2) (3) (4) 29 1.26 1.16 1.09 46 1.42 1.40 1.01 65 1.44 1.59 0.91 81 1.50 1.66 0.90 105 1.56 1.63 0.96 E (keV) Backscatter factor Cs (E) (2)/(3) (1) (2) (3) (4) 29 1.26 1.16 1.09 46 1.42 1.40 1.01 65 1.44 1.59 0.91 81 1.50 1.66 0.90 105 1.56 1.63 0.96 Table 11. Dependence of backscatter factor (ion chamber + reading on phantom/ion chamber reading in air) and conversion coefficient Cs(E) = Hs(0.07) Ka−1 on the photon energy E(76). E (keV) Backscatter factor Cs (E) (2)/(3) (1) (2) (3) (4) 29 1.26 1.16 1.09 46 1.42 1.40 1.01 65 1.44 1.59 0.91 81 1.50 1.66 0.90 105 1.56 1.63 0.96 E (keV) Backscatter factor Cs (E) (2)/(3) (1) (2) (3) (4) 29 1.26 1.16 1.09 46 1.42 1.40 1.01 65 1.44 1.59 0.91 81 1.50 1.66 0.90 105 1.56 1.63 0.96 The estimation of photon energy (E) may also become essential in future as different quality factors have been suggested by ICRU report 40. The energy E could be estimated from D2/D1 ratio. Another important factor is the air gap between the dosemeters and the phantom. Lakshmanan(78) has found that for 29 keV X-rays, the response of CaSO4:Dy Teflon TLD sandwiched between copper metal filters decreases with increasing gap. Relative response of LiF TLD-700 between 276 mg cm−2 Dural filters also showed similar results(79). During calibration, it is the normal practice to fix the dosemeters on the phantom without any gap between them. In practice, when the dosemeters are worn by the individual, say on clothing, there exists a certain gap between the dosemeter and the body which can vary (say from 0.5 to 3 cm) depending on the movement of the individual and this can cause an underestimation of BSF and hence the individual HE at low photon energies. In author’s opinion, one way of correcting this error would be to maintain some space interval (say 1 or 1.5 cm) between the dosemeter and the phantom during calibration. The other alternative is to use a belt to tie the dosemeter to body waist. Thus, with a slight modification in the metal filters in the TLD badge, the TL response of CaSO4:Dy Teflon TLD could be made to simulate directly Hp(10) response. Even the so called tissue equivalent LiF:Mg,Ti TLD does not simulate the new ICRU quantities directly. A 7 mm thick plastic dome or 3 mm thick boron +0.1 mm thick Al filter combination over LiF TLD was found to be essential to measure Hp(10) within ±30%(79, 80). Estimation of effective dose equivalent, HE A distinct feature of the ICRU recommendations on individual dosemeters is that its angular response should simulate that of the individual in contrast to earlier recommendations by ANSI, ISO, CEC, etc., which specified more or less an isotropic response to radiations originating in the frontal half sphere of the body. Actually HE decreases with increasing angle of incidence of photons, particularly for E < 100 keV, since most of the critical organs are situated on the anterior side of it, with the consequence that they are shielded by soft tissue against radiation incident from the posterior side. Thus, HE for posterior–anterior (PA) irradiation is considerably smaller than that for AP irradiation. The same argument holds true in the case of lateral irradiation which shows an even more pronounced shadowing effect. However, in some cases, e.g. for lateral irradiation of 10 keV photons, Hp(10) greatly underestimates HE (by a factor of 500). This occurs because of the risk to the female breast. The underestimation would not occur with a dosemeter with an isotropic response for photon fluence. Furthermore, while the response of individual dosemeters could be easily made to simulate HE of the human body in the frontal half sphere, its response to radiation incident in the PA direction is very unsatisfactory. The underestimation of HE by Hp(10) especially to low energy photons (E < 100 keV) originating from the posterior side of the human body (PA direction) is cited as an unsatisfactory property in the definition of Hp(10). Basically in such cases, the radiation has to pass though the human body and hence it is severely attenuated before reaching the dosemeter which is normally worn on the front. It may therefore be necessary to wear an additional dosemeter on the posterior side of the human body if irradiation from that side is anticipated and the response of both the dosemeters could be combined to give a HE response. Alternatively, the filter designs could be modified so as to detect preferentially photons transmitted though the body. For this study, a specially designed CaSO4:Dy Teflon TLD badge having three windows, D1, D2 and D3 (D1, 30 × 18 × 1 mm3 Cu filter on the irradiation side and 16 mm φ and 1 mm thick Cu filter on the 30 cm cuboidal water phantom side; D2, 30 × 18 × 1.5 mm3 perspex filter on both sides; and D3, 30 × 18×1 mm3 Cu filter on the irradiation side and open window on the phantom side) were used(81). In addition D1 and D2 had a 0.5 mm thick Cu filter cover on the two lateral sides along the long axis of the TLD badge. The TLD badges (four on one side) were affixed in the front side, lateral side and back of the water phantom for AP, LAT and PA irradiations, respectively. The studies revealed that the application of two CaSO4:Dy TLD badges, one worn on the front and the other worn at the back of the body, provides a conservative estimate of HE for all types and energies of photon irradiation and hence is recommended when irradiation is anticipated in all directions. Such a procedure is necessary to overcome the drawback of underestimation of HE by the individual DE penetrating Hp(10) during PA irradiations. Estimation of ambient dose equivalent, H* Unlike personnel dosemeters, environmental dosemeters are to be calibrated without phantom, i.e. irradiated in free air conditions similar to the case with ion chambers but TLDs should be calibrated in units of H*(10) similar to the procedure adopted for Hp(10) since in environmental monitoring the dosemeters are always irradiated in free air. The CaSO4:Dy embedded Teflon TLD badge used by BARC as well as the modified one (type E described earlier(76)) were used in this study(82). The studies revealed that the BARC TLD badge containing CaSO4:Dy embedded Teflon discs designed for individual monitoring can be used for environmental monitoring as well. A method for measuring H*(10) using a combination TL response under metal filters (D1) and Perspex (D2) has been developed(82). This can in turn be used to estimate HE the effective DE and Ka, the air kerma from a typical natural background radiation(83). Guelev et al.(84) from Bulgaria also proposed the use of a two-element CaSO4:Dy TLD and a linear combination of their response for environmental monitoring. NEUTRON DOSIMETRY BY PHOSPHOR ACTIVATION Bubble detectors and threshold neutron activation detectors are the two fast-neutron measuring devices which offer complete discrimination against gamma-rays and preserves some information about the neutron energy(85). While the former are passive (integrating) detectors used to measure low neutron doses encountered in personnel exposures, the latter are active detectors (induced radioactivity will decrease with storage time) mostly used for high neutron dose levels encountered in reactor beams, high-energy particle accelerators or in nuclear accidents. Active detectors require information on the time and even the duration of exposure during neutron induced radioactivity measurements to calculate the exact doses received. Conventional foil-activation techniques require sensitive radiation detectors for the estimation of induced activity in the foils. For extensive measurements at low neutron fluences, vast outlays of counting equipment are required. TLDs, could, however, combine into a single device the functions of an activation foil and a detector of radiations emitted by it. TLDs are inexpensive and extremely sensitive radiation detectors. Mayhugh and Watanabe(86) expanded their work on thermal neutron activation in TLDs to fast-neutron exposures. To be certain that the activations produced correspond to the isotopes assigned, a decay of the activity produced by fission neutrons was plotted by them. This was obtained by measuring the TL induced in CaSO4:Dy due to self-irradiation as a function of time after the neutron irradiation in the reactor. This is shown in Figure 51. It is seen that the 165Dy and 32P components are easily separated. The solid lines represent decay with the 343 h half-life of 32P. The discontinuity results from post-irradiation annealing the sample at 600°C for 30 min. Since the self-irradiation times are not identical, each TL reading is divided by exp(Δt/495) − 1. The insert in Figure 51 shows the initial TL due to 30 min self-irradiations. The solid line represents decay with the 2.32 h half-life of 165Dy. The one day delayed dose in the CaSO4:Dy phosphor was found by them to be entirely due to Dy activation. This TL measurement indicated a thermal neutron background of 1.5 × 109 n cm−2 for a 30 min reactor irradiation. TL measurements with CaF2:Dy also gave similar results. The 14-day delayed dose in the CaSO4:Dy dosemeter is essentially due to 32P decay. Using the fission averaged cross-section of 65 mb, a fission spectrum fluence of 2.6 ± 5 × 1010 n cm−2 was calculated which was in good agreement with the 2.1 × 1010 n cm−2 obtained with the conventional sulphur activation and ion chamber measurements. Figure 51. View largeDownload slide The daily TL from self-irradiation as a function of time after irradiation in the reactor. The solid lines represent decay with the 343 h half-life of 32P. The discontinuity results from annealing the sample 30 min at 600°C (see text). Since the self-irradiation times are not always identical, each TL reading is divided by exp (∆t/495) − 1. Inset: The initial TL due to 30 min self-irradiations. The solid line represents decay with the 2.32 h half-life of 165Dy(86). Figure 51. View largeDownload slide The daily TL from self-irradiation as a function of time after irradiation in the reactor. The solid lines represent decay with the 343 h half-life of 32P. The discontinuity results from annealing the sample 30 min at 600°C (see text). Since the self-irradiation times are not always identical, each TL reading is divided by exp (∆t/495) − 1. Inset: The initial TL due to 30 min self-irradiations. The solid line represents decay with the 2.32 h half-life of 165Dy(86). Pearson and Moran(87) expanded the fast-neutron activation to other TLD phosphors such as LiF, CaF2, Mg2SiO4, ZnO and CaSO4. The reactions 19F(n,2n)18F, 32S(n,p)32P, 24Mg(n,p)24Na and 64Zn(n,p)64Cu were found useful by them for fast-neutron activation in commercial TLDs. Table 12 describes the neutron activated nuclides in the TLDs and post-irradiation storage time used subsequent to annealing(86). Table 13 lists the thermal and fast (14.7 MeV) neutron sensitivities of different TLDs(87). Figures 52 and 53 show response functions for the detectors used to determine neutron spectra. The neutron energy response of the TLDs can be given as the product of sensitivity S and the cross-section σ(E). The curves are the energy dependence of σ(E) normalized to the measured value of Sσ(14.7) or Sσ(thermal). Table 12. Description of TLDs, neutron activated nuclides and post-irradiation storage time used subsequent to annealing(87). Phosphor Activated nuclide Storage time Mass (mg) Description LiF:Mg,Ti 18F 1 Day 23 3.2 × 3.2×1 mm3; extruded ribbon CaF2:Dy 18F, 165Dy 1 Day 27 3.3 × 3.3 × 2.5 mm3; hot pressed chip CaF2:Mn 18F, 56Mn 1 Day 50 3.2 × 3.2 × 1 mm3, extruded ribbon Mg2SiO4:Tb 24Na 3 Days 20 2 mm dia × 10 mm, glass capillary encapsulated powder ZnO:Tm 64Cu 3 Days 180 4 mm dia × 10 mm, ungraded powder in a gelatin capsule CaSO4:Dy 32P 14 Days 180 4 mm dia × 10 mm, 80 mesh powder in a gelatin capsule Phosphor Activated nuclide Storage time Mass (mg) Description LiF:Mg,Ti 18F 1 Day 23 3.2 × 3.2×1 mm3; extruded ribbon CaF2:Dy 18F, 165Dy 1 Day 27 3.3 × 3.3 × 2.5 mm3; hot pressed chip CaF2:Mn 18F, 56Mn 1 Day 50 3.2 × 3.2 × 1 mm3, extruded ribbon Mg2SiO4:Tb 24Na 3 Days 20 2 mm dia × 10 mm, glass capillary encapsulated powder ZnO:Tm 64Cu 3 Days 180 4 mm dia × 10 mm, ungraded powder in a gelatin capsule CaSO4:Dy 32P 14 Days 180 4 mm dia × 10 mm, 80 mesh powder in a gelatin capsule Table 12. Description of TLDs, neutron activated nuclides and post-irradiation storage time used subsequent to annealing(87). Phosphor Activated nuclide Storage time Mass (mg) Description LiF:Mg,Ti 18F 1 Day 23 3.2 × 3.2×1 mm3; extruded ribbon CaF2:Dy 18F, 165Dy 1 Day 27 3.3 × 3.3 × 2.5 mm3; hot pressed chip CaF2:Mn 18F, 56Mn 1 Day 50 3.2 × 3.2 × 1 mm3, extruded ribbon Mg2SiO4:Tb 24Na 3 Days 20 2 mm dia × 10 mm, glass capillary encapsulated powder ZnO:Tm 64Cu 3 Days 180 4 mm dia × 10 mm, ungraded powder in a gelatin capsule CaSO4:Dy 32P 14 Days 180 4 mm dia × 10 mm, 80 mesh powder in a gelatin capsule Phosphor Activated nuclide Storage time Mass (mg) Description LiF:Mg,Ti 18F 1 Day 23 3.2 × 3.2×1 mm3; extruded ribbon CaF2:Dy 18F, 165Dy 1 Day 27 3.3 × 3.3 × 2.5 mm3; hot pressed chip CaF2:Mn 18F, 56Mn 1 Day 50 3.2 × 3.2 × 1 mm3, extruded ribbon Mg2SiO4:Tb 24Na 3 Days 20 2 mm dia × 10 mm, glass capillary encapsulated powder ZnO:Tm 64Cu 3 Days 180 4 mm dia × 10 mm, ungraded powder in a gelatin capsule CaSO4:Dy 32P 14 Days 180 4 mm dia × 10 mm, 80 mesh powder in a gelatin capsule Table 13. The thermal and fast (14.7 MeV) neutron sensitivities of different TLDs(87). Dosemeter S(tr) Thermal TL photons σ(th) S(tr) 14.7 MeV TL photons σ(14.7 MeV) mb tr (days) barn n/cm2 barn n/cm2 LiF:Mg,Ti — — 3.3 ± 0.3 × 10−9 65 1 CaF2:Mn 1.08 ± 0.03 × 10−5 13 b 3.9 ± 0.1 × 10−7 65 1 CaF2:Dy 2.1 ± 0.14 × 10−7 2.8 kb 1.14 ± 0.05 × 10−6 65 1 Mg2SiO4:Tb 2.0 ± 0.2 × 10−5 0.535 b 7.2 ± 0.3 × 10−7 175 3 ZnO:Tma 9.6 ± 0.8 × 10−8 1 b 1.6 ± 0.2 × 10−7 200 3 CaSO4:Dyb 9.1 ± 0.4 × 10−9 2.8 kbb 1.4 ± 0.1 × 10−8 210 14 Dosemeter S(tr) Thermal TL photons σ(th) S(tr) 14.7 MeV TL photons σ(14.7 MeV) mb tr (days) barn n/cm2 barn n/cm2 LiF:Mg,Ti — — 3.3 ± 0.3 × 10−9 65 1 CaF2:Mn 1.08 ± 0.03 × 10−5 13 b 3.9 ± 0.1 × 10−7 65 1 CaF2:Dy 2.1 ± 0.14 × 10−7 2.8 kb 1.14 ± 0.05 × 10−6 65 1 Mg2SiO4:Tb 2.0 ± 0.2 × 10−5 0.535 b 7.2 ± 0.3 × 10−7 175 3 ZnO:Tma 9.6 ± 0.8 × 10−8 1 b 1.6 ± 0.2 × 10−7 200 3 CaSO4:Dyb 9.1 ± 0.4 × 10−9 2.8 kbb 1.4 ± 0.1 × 10−8 210 14 aThese are per mg values. bThe readout time for the thermal sensitivity is 1 day. Table 13. The thermal and fast (14.7 MeV) neutron sensitivities of different TLDs(87). Dosemeter S(tr) Thermal TL photons σ(th) S(tr) 14.7 MeV TL photons σ(14.7 MeV) mb tr (days) barn n/cm2 barn n/cm2 LiF:Mg,Ti — — 3.3 ± 0.3 × 10−9 65 1 CaF2:Mn 1.08 ± 0.03 × 10−5 13 b 3.9 ± 0.1 × 10−7 65 1 CaF2:Dy 2.1 ± 0.14 × 10−7 2.8 kb 1.14 ± 0.05 × 10−6 65 1 Mg2SiO4:Tb 2.0 ± 0.2 × 10−5 0.535 b 7.2 ± 0.3 × 10−7 175 3 ZnO:Tma 9.6 ± 0.8 × 10−8 1 b 1.6 ± 0.2 × 10−7 200 3 CaSO4:Dyb 9.1 ± 0.4 × 10−9 2.8 kbb 1.4 ± 0.1 × 10−8 210 14 Dosemeter S(tr) Thermal TL photons σ(th) S(tr) 14.7 MeV TL photons σ(14.7 MeV) mb tr (days) barn n/cm2 barn n/cm2 LiF:Mg,Ti — — 3.3 ± 0.3 × 10−9 65 1 CaF2:Mn 1.08 ± 0.03 × 10−5 13 b 3.9 ± 0.1 × 10−7 65 1 CaF2:Dy 2.1 ± 0.14 × 10−7 2.8 kb 1.14 ± 0.05 × 10−6 65 1 Mg2SiO4:Tb 2.0 ± 0.2 × 10−5 0.535 b 7.2 ± 0.3 × 10−7 175 3 ZnO:Tma 9.6 ± 0.8 × 10−8 1 b 1.6 ± 0.2 × 10−7 200 3 CaSO4:Dyb 9.1 ± 0.4 × 10−9 2.8 kbb 1.4 ± 0.1 × 10−8 210 14 aThese are per mg values. bThe readout time for the thermal sensitivity is 1 day. Figure 52. View largeDownload slide Response functions for the four detectors used to determine neutron spectra at well-shielded locations(87). Figure 52. View largeDownload slide Response functions for the four detectors used to determine neutron spectra at well-shielded locations(87). Figure 53. View largeDownload slide Response functions for additional detectors used to determine neutron spectra in high radiation levels(87). Figure 53. View largeDownload slide Response functions for additional detectors used to determine neutron spectra in high radiation levels(87). The major fast-neutron induced reaction in CaSO4:Dy TLD is 32S(n,P)32P (threshold neutron energy is >2 MeV). If the TL due to 32P (T1/2 = 14.3 d emitting beta rays with maximum energy of 1.7 MeV (the range of such beta is ~0.3 mm in CaSO4)) alone was to be integrated, the post-irradiation annealing treatment should be given after a waiting period of at least 2 days. This annealing treatment is meant to remove the TL produced by gamma rays if present in the neutron beam as well as by radiations produced by short lived activation products such as 165Dy. Six half-lives for 32P (3 months) is generally too long to wait for the data. Hence, 2 weeks, one half-life, is arbitrarily chosen in the trade off between maximum TL signal and patience. Bhatt et al.(88) irradiated CaSO4:Dy powder and Teflon discs in a mixed field of neutrons and gamma rays in a swimming pool reactor. The fast-neutron and gamma ray doses were measured by sulphur pellets and CaSO4:Dy TLDs, respectively. The fast-neutron dose was 6.35 × 102 Gy while the accompanying gamma ray dose was 103 Gy. After a post-irradiation interval of 2 days, the TLD discs and powder samples were separately annealed at 400 and 700°C for 1 h, respectively and stored at RT. The TL signal was measured after various storage intervals. The 60Co gamma ray sensitivity of the CaSO4:Dy Teflon discs was found to be enhanced by 60% as compared to that of virgin discs. This enhancement was obtained after subtracting the background TL signal induced by 32P activity. This enhancement in TL sensitivity is due to the sensitization effect produced by the intense gamma rays which accompanied the neutrons. However, CaSO4:Dy powder samples annealed at 700°C did not show any such ‘memory’ effect since high-temperature anneal is known to remove the competing deep traps causing the sensitization effects in the dosimetry traps(7). Figure 54 shows the sensitivity functions Sσ(En) for the dosemeters. These curves are essentially the effective activation cross-sections normalized to the dosemeter sensitivities at 14.7 MeV and thermal neutron energies(87). Figure 55 shows the build-up of TL signal in CaSO4:Dy Teflon discs with post-irradiation time interval. This is plotted after taking into account the sensitization effect. The self-irradiation rate obtained from this figure was found to decrease corresponding to the 14.3 days half-life of 32P. It shows that for a fast-neutron dose of 6.35 × 102 Gy, the accumulated TL signal, after a post-irradiation interval of 20 days, is equal to the TL produced by 8 R of 60Co gamma rays. The minimum detectable gamma dose with these discs is ~1 mR of 60Co gamma rays and this corresponds to a fast-neutron dose of ~8 rad. Figure 54. View largeDownload slide Sensitivity functions Sσ(En) for the dosemeters. These curves are essentially the effective activation cross-sections normalized to the dosemeter sensitivities at 14.7 MeV and thermal neutron energies(87). Figure 54. View largeDownload slide Sensitivity functions Sσ(En) for the dosemeters. These curves are essentially the effective activation cross-sections normalized to the dosemeter sensitivities at 14.7 MeV and thermal neutron energies(87). Figure 55. View largeDownload slide Self-induced TL from 32P radioactivity in CaSO4:Dy Teflon discs as a function of post-irradiation time interval, for a fast-neutron dose of 6.35 × 104 rad(88). Figure 55. View largeDownload slide Self-induced TL from 32P radioactivity in CaSO4:Dy Teflon discs as a function of post-irradiation time interval, for a fast-neutron dose of 6.35 × 104 rad(88). Figure 56 shows the calibration plot between the fast-neutron dose and the accumulated TL in 60Co gamma equivalent roentgen for CaSO4:Dy powder after a post-irradiation interval of 20 days. It can be seen from Figure 56 that the plot is linear and a fast-neutron dose of 6.35 × 102 Gy gives a TL signal equivalent to 4.515 mC kg−1 (17.5 R) of 60Co gamma-rays. This shows that the neutron response of TLD Teflon disc is lower by a factor of ~2 as compared to that of powder sample. This is possibly due to the attenuation of beta particles by the Teflon matrix in the disc. CaSO4:Dy sintered pellets would be ideal for neutron activation since the binder concentration in pellets is much smaller when compared to Teflon discs. Figure 56. View largeDownload slide Fast neutron calibration curve for CaSO4:Dy TL powder for which TL measurements were made after a post-irradiation interval of 20 days(88). Figure 56. View largeDownload slide Fast neutron calibration curve for CaSO4:Dy TL powder for which TL measurements were made after a post-irradiation interval of 20 days(88). Fast-neutron sensitivity of CaSO4:Dy mixed sulphur pellets At the conclusion of their comparative study of fast-neutron dosimetry with several TLDs Pearson and Moran(87) comment that only the sulphur activation is of any real interest. Its (32S) cross-section is reasonably flat with energy from 3 to 15 MeV. The half-life of 32P of 14 days allows a working week—5 days—to be monitored with a worst case loss of activity of only 22% if all the neutron exposure were received on the first day. The delayed dose integration time of 2 weeks is not unreasonable for low exposures and could be shortened if a large exposure were received as from an accident. The problem of adequate sensitivity, however, remains since personal exposures of the order of μ Gy (millirad), 105 n cm−2, must be measurable. This roughly corresponds to about one activation per 3 mg of CaSO4, and that some method of increasing the activity is required. The conventional sulphur activation technique burns the sulphur to concentrate the 32P from a large sulphur mass. Therefore, Pearson and Moran(87) came up with the brilliant suggestion that by including a small amount, 100–200 mg, of CaSO4:Dy in a 100 g sulphur pellet when burned, the 32P can be concentrated on the CaSO4 grains. The delayed dose measurement of this 32P activity proceeds as described above. Although no detailed investigation of the quantitative yields has been made, their preliminary experiments showed that about half of the 32P activity in the sulphur pellet was deposited on the CaSO4 grains when burned. Both the TL and the residual activity were measured by them. Using this technique sulphur pellets of 100 g would give adequate sensitivity for measuring fluences of the order of 105 n cm−2 equivalent to ~10−2 mGy (1 mrad). The above attractive suggestion to develop a fast-neutron personnel dosemeter based on sulphur activation was seriously pursued by Pradhan et al.(89) who made pellets of CaSO4:Dy powder mixed with suplhur powder in different proportions for this purpose and then exposed them to fast neutrons, subsequently burnt in the aluminium planchettes and then stored for 14 days for TL accumulation. The increase in TL with the number of CaSO4:Dy-mixed-sulphur pellets for different percentages of phosphor powder is shown in Figure 57. For determining fast-neutron efficiency, unirradiated pellets were subjected to the same heat treatment in aluminium planchettes as that of the pellets irradiated with fast neutrons. These planchettes containing the residual powder were exposed to beta rays from a 90Sr–90Y source calibrated with the help of extrapolation chamber. An additional annealing treatment at 550°C for 3 h in a furnace was found to erase the sensitization effect induced by the gamma rays at higher dose levels (if >10 Gy) without loosing the 32P activity. Figure 57. View largeDownload slide Accumulated TL output of different percentages of CaSO4:Dy-mixed-sulphur pellets plotted against the number of pellets exposed to 50 Gy of fast neutrons and burnt in an aluminium planchette(89). Figure 57. View largeDownload slide Accumulated TL output of different percentages of CaSO4:Dy-mixed-sulphur pellets plotted against the number of pellets exposed to 50 Gy of fast neutrons and burnt in an aluminium planchette(89). The fast-neutron TL accumulation efficiency, εn/εβ (defined as the ratio of accumulated TL per rad for fast neutrons to TL per rad for 90Sr–90Y beta rays) for different percentages of CaSO4:Dy in sulphur pellets was compared with that of loose powder and CaSO4:Dy embedded Teflon discs for the same post-irradiation annealing and accumulation conditions. It was seen that the fast-neutron detection efficiency for 10 pellets with 0.1% CaSO4:Dy is 104 and 205 times that of CaSO4:Dy powder and CaSO4:Dy embedded Teflon discs, respectively. Even though TL increases, the TL per mg of the TL powder in 2 g of sulphur decreases with an increasing percentage of CaSO4:Dy. The self-absorption for the TL from CaSO4:Dy for the area of the planchette used is not significant up to a 100 mg sample. For higher amounts of CaSO4:Dy powder, the TL per mg goes down perhaps due to the dilution of the activity sticking to the phosphor grains. The linear rise of the accumulated TL with increasing amounts of sulphur suggests that the sensitivity increases with the amount of sulphur burnt. Hence, it is desirable to mix lower percentage of CaSO4:Dy with sulphur. As the uniform mixing of CaSO4:Dy <0.1% by weight in sulphur was found difficult, percentages lower than 0.1 were not studied by Pradhan et al.(89) Burning a larger number of pellets containing 0.1% CaSO4:Dy were limited by the brown colour deposit left after burning and heat treatment procedures. Burning 20 pellets each weighing 1 g and containing 0.1% of CaSO4:Dy has resulted in a dark brown residue causing saturation of TL output. Hence, for dose measurements the burning of 10 pellets with 0.1 and 1% of CaSO4:Dy were studied. Table 14 shows that the efficiency of 10 pellets with 0.1 and 1% of CaSO4:Dy are 2.6 and 0.82%, respectively. The TL sensitivity of 1% CaSO4:Dy sulphur pellets (after burning) for beta rays is 11.5 times that of the 0.1% pellets showing that 1% pellets are preferable for low fast-neutron dose measurements, even though their fast-neutron efficiency is relatively low. For example, if the minimum measurable beta dose by the CaSO4:Dy left after the burning of 10 pellets containing 0.1 and 1% of CaSO4:Dy is 0.1 and 0.01 μGy (1 and 0.1 mrad), respectively, the minimum measurable fast-neutron dose will be ~0.1 and 0.01 μGy (1 mrad and 0.1 mrad), respectively. This is very much lower than the minimum measurable fast-neutron dose which is ~0.01 Gy (1 rad) achieved by conventional activation detection methods where the beta activity from 32P isotope produced on sulphur activation is counted with a Geiger Muller counter. Table 14. TL accumulation efficiencya of the CaSO4:Dy-mixed-sulphur pellets, CaSO4:Dy loose powder and CaSO4:Dy embedded Teflon discs(89). Type of dosimeter TL per 0.01 Gy (rad) for 90Sr –90Y beta rays,εβ Accumulated TL per 0.01 Gy (rad) for fast neutronsεn Efficiencya, εn/εβ (%) 10 Pellets (each 1 g) with 0.1% of CaSO4:Dy 1.00 0.026 2.6 10 Pellets (each 1 g) with 1% of CaSO4:Dy 11.50 0.094 0.82 CaSO4:Dy loose powderb __ __ 0.025 CaSO4:Dy embedded Teflon discsb __ __ 0.013 Type of dosimeter TL per 0.01 Gy (rad) for 90Sr –90Y beta rays,εβ Accumulated TL per 0.01 Gy (rad) for fast neutronsεn Efficiencya, εn/εβ (%) 10 Pellets (each 1 g) with 0.1% of CaSO4:Dy 1.00 0.026 2.6 10 Pellets (each 1 g) with 1% of CaSO4:Dy 11.50 0.094 0.82 CaSO4:Dy loose powderb __ __ 0.025 CaSO4:Dy embedded Teflon discsb __ __ 0.013 aEfficiency is the ratio of accumulated TL per 0.01 Gy (rad) of fast neutrons and TL per rad of 90Sr–90Y beta rays. bBhatt et al.(88). Table 14. TL accumulation efficiencya of the CaSO4:Dy-mixed-sulphur pellets, CaSO4:Dy loose powder and CaSO4:Dy embedded Teflon discs(89). Type of dosimeter TL per 0.01 Gy (rad) for 90Sr –90Y beta rays,εβ Accumulated TL per 0.01 Gy (rad) for fast neutronsεn Efficiencya, εn/εβ (%) 10 Pellets (each 1 g) with 0.1% of CaSO4:Dy 1.00 0.026 2.6 10 Pellets (each 1 g) with 1% of CaSO4:Dy 11.50 0.094 0.82 CaSO4:Dy loose powderb __ __ 0.025 CaSO4:Dy embedded Teflon discsb __ __ 0.013 Type of dosimeter TL per 0.01 Gy (rad) for 90Sr –90Y beta rays,εβ Accumulated TL per 0.01 Gy (rad) for fast neutronsεn Efficiencya, εn/εβ (%) 10 Pellets (each 1 g) with 0.1% of CaSO4:Dy 1.00 0.026 2.6 10 Pellets (each 1 g) with 1% of CaSO4:Dy 11.50 0.094 0.82 CaSO4:Dy loose powderb __ __ 0.025 CaSO4:Dy embedded Teflon discsb __ __ 0.013 aEfficiency is the ratio of accumulated TL per 0.01 Gy (rad) of fast neutrons and TL per rad of 90Sr–90Y beta rays. bBhatt et al.(88). Despite the attractive features (sensitivity and simplicity) of the above technique of sulphur-mixed fast-neutron TLD activation, it suffers from the following two setbacks: (1) since sulphur activation is an active and not a passive detection technique, a knowledge of the exact time of the neutron exposure is essential for dose estimation, which is not possible in routine personnel dosimetry and (2) more importantly the inherent drawback of the sulphur activation is its high activation threshold (~2.5 MeV) which renders it insensitive in reactor environments (fission neutron spectrum) as well as the photo neutrons (1–3 MeV) produced by high-energy linear accelerators. However, in high-energy neutron environments this technique is very useful. Albedo neutron dosimetry with 6LiF-mixed CaSO4:Dy Teflon discs Albedo neutron dosemeters based on LiF TLD are popular because they are fairly sensitive. When fast neutrons are incident on the human body, they get moderated and backscattered. Since the backscattered neutrons are thermalized, they can be detected more easily than the fast neutrons themselves. So TLD materials such as LiF TLD-600 or 6Li210B4O7:Cu or 6LiF-mixed CaSO4:Dy Teflon discs are tightly held onto the body to detect these albedo neutrons. But the single most important disadvantage of these dosemeters is that they are extremely energy-dependent and over-respond to low energy neutrons because of the severe dependence of thermal neutron albedo factor (ratio of number of backscattered neutrons to number of incident neutrons) on the fast-neutron energy. The albedo neutron response of 6LiF-mixed CaSO4:Dy Teflon discs was tested by Pradhan et al.(90) by using the Am–Be fast-neutron source. The results are shown in Table 15. The experimentally measured albedo response was found to be equivalent to 0.061 Gy of 60Co gamma-rays per Sv of fast-neutron dose. Table 15. Response of 6LiF + CaSO4:Dy Teflon discs for Am–Be neutrons(90). Irradiation condition Relative TL CaSO4:Dy Teflon discs 6LiF + CaSO4:Dy Teflon discs Free air 1.00 1.31 On water phantom 1.10 4.16 Irradiation condition Relative TL CaSO4:Dy Teflon discs 6LiF + CaSO4:Dy Teflon discs Free air 1.00 1.31 On water phantom 1.10 4.16 Table 15. Response of 6LiF + CaSO4:Dy Teflon discs for Am–Be neutrons(90). Irradiation condition Relative TL CaSO4:Dy Teflon discs 6LiF + CaSO4:Dy Teflon discs Free air 1.00 1.31 On water phantom 1.10 4.16 Irradiation condition Relative TL CaSO4:Dy Teflon discs 6LiF + CaSO4:Dy Teflon discs Free air 1.00 1.31 On water phantom 1.10 4.16 CaSO4:Dy Teflon dosemeters for photon-beta exposures and CaSO4:Dy (6LiF)-Teflon dosemeters (Radi-Guard Teledyne Isotopes) for neutron exposure were used by Fontaine and Distenfeld(91) to answer certain shielding questions in the control rod drive room of a nuclear power station. The dosemeters were exposed to 50–2000 Gy with the help of a 7500 Ci 60Co source. The exposure range was selected to cover the estimated reactor photon exposure of 500 Gy and provide conservative range for the less well known neutron exposure. The neutron component could be assessed by ratios of neutron to gamma ray exposure, followed by scaling, or by direct measurement with TLDs. The results indicated that CaSO4:Dy Teflon dosemeters can be used to monitor radiation doses to at least 2000 Gy and are linear to 1000 Gy which is 2–10 times larger than the values quoted in the literature. For reactor dosimetryCaSO4:Dy Teflon and CaSO4:Dy (6LiF)-Teflon dosemeters mounted on the body of four litre water-filled phantoms, were placed in appropriate locations. Neutron activation of the aluminium, copper and cadmium filters caused an excess response in areas 3 and 4. For this reason, the photon exposure was taken to be the average of areas 1 and 2. Neutron interpretation was based on the ratio of net responses of the neutron sensitive dosemeter in area 4 to that of area 3. This ratio was termed BCD to Tlp for back cadmium to unshielded TLD pair. The conversion of DE to the more useful absorbed dose was inferred by independent neutron spectrometry measurements. Ion chamber rate measurements suggested that the 102-day photon exposure should be 500 Gy at a power level of ~70 MW electrical. The TLD photon measurement results ranged from 66 to 530 Gy. This was considered to be a reasonable agreement. The neutron exposure component was nearly constant (19–28 Gy) suggesting that a neutron cavity effect may be responsible. Fast neutron dosimetry using CaSO4:Dy embedded PE discs The material having the highest hydrogen content (14.3%) is PE. However, the sensitivity of TLDs in contact with polyethylene to fission neutrons is not sufficiently high. The best contact between the TLD material and PE is attained by embedding. However, it is not possible to raise the temperature of PE embedded TLDs to the desired temperature (usually 250–300°C) to readout the glow peaks as the melting point of the PE is ~115°C. The work of Becker et al.(92) was therefore directed towards an organic material of high melting point in which fine TL powder could be embedded. The most promising material was p-sexiphenyl, which melts around 450°C. The relative efficiency to recoil protons of CaSO4:Dy phosphor mixed with p-sexiphenyl was found to be ~40% for both fission neutrons and 14 MeV-neutrons. A part of this response was attributed to indirect response of the phosphor to the light which is produced by the protons traversing the highly luminescent sexiphenyl. Further, this material is expensive, requires purification to remove colour-producing and volatile impurities. The problem of thermal instability of PE has been overcome by Facey et al.(93) by using cross-linked PE. This can be carried out chemically (XPE) or by irradiation (IPE). IPE was obtained by subjecting 5 μm spheres of PE to 1 MGy of 60Co irradiation. This raised the melting point of PE from 130 to >350°C and also enhanced its resistance to oxidation. The recoil proton can have any value of energy from that of the incident neutron down to zero. Ideally, the size of the plastic grains should not exceed the range of weakest recoil protons. Thus, for example, phosphor grains of diameters of the order of 1–10 μm are required for protons in the energy range of 100–600 keV. TL phosphor of this grain-range was obtained by separating such grains by suspension and settling in water. Mixtures of IPE and CaSO4:Dy powders were hot pressed into pellets under vacuum at 300°C at 7900 kg cm−2 pressure for 15 min. However, these dosemeters showed unsatisfactory phototransferred TL (PTTL) on exposure to daylight or fluorescent light. This was attributed to deeply trapped electrons from the very large dose of radiation required for the crosslinking of the IPE. The effect of PTTL is to raise the reading of unirradiated IPE samples to unacceptably high levels. The PTTL extends through the visible spectrum from the orange to the blue, but is maximum in the green region. This led to the idea of using a UV-emitting phosphor, and reading the detectors through a UV-transmitting filter. CaSO4:Tm was chosen for this purpose. But even then, the values of readings from unirradiated blanks of the IPE mixed TLDs were usually in the range 10–30 mGy. Such a high background would be quite unacceptable in a health-physics dosemeter for radiation protection. However, for military or criticality monitoring purposes, there may be no need for greater accuracy of measured dose. In addition, for acute radiation exposures, the RBE = 1 for fast neutrons, and hence dosemeters having equal TL sensitivity to neutrons and gamma rays on a Gy-for-Gy basis will suffice for the measurement of DE in mixed fields. However, ICRP Publication 92 (2003) reviewed the relevant RBE values based on substantial developments in biological and dosimetric knowledge since 1990 and concluded that RBE values for deterministic effects is an issue that will demand further investigation(94). The relative n/γ response ratio of the two-phase (CaSO4:Tm + IPE) dosemeter was found to decrease with decreasing neutron energy. During these irradiations, the two-phase dosemeters were sandwiched between 1 mm thick layers of PE. Light was excluded from the samples by wrapping them with Al foil. The increasing divergence, for the two-phase dosemeter, between the calculated and measured responses as the neutron energy is decreased has been attributed by Facey et al.(93) to the higher grain-size of the phosphor used. A further reduction in the grain size of the phosphor (i.e. <1–10 μm) should result in increased response at low neutron energies. However, the procedure of large-scale production of TL grains in this grain range is yet to be standardised.Even if the grain size is reduced, it will be difficult to detect the DE of fast neutrons having energy <1 MeV in the presence of a large percentage of gamma rays in routine personnel monitoring since the inherent problem of reduced TL efficiency at higher LET would result in reduced efficiency to fast neutrons <1 MeV in relation to gamma-rays on a Gy-for-Gy basis. Optically stimulated luminescence in CaSO4:Dy embedded PE discs Optically stimulated luminescence (OSL) apart from its capability of providing repeated readouts from a single dosemeter, also permits readout without heating the dosemeter. The application of OSL in CaSO4:Dy for gamma ray dosimetry is known(95). Its usefulness in fast-neutron dosimetry has been pointed out by Pradhan et al.(96). The phosphorescence is a temperature-dependent phenomenon and the life time τ of a charge carrier in a metastable state is given by the expression 1/τ=constant×e−E/kT where E is the trap depth, K is the Boltzmann constant and T is the absolute temperature. The OSL is also a phenomenon of phosphorescence in which the trapped charge carriers on photostimulation are pumped into the traps having a short mean life and the charge carriers reach luminescence centers via these traps to cause the luminescence. Thus, it is expected that by elevating the temperature, the phosphorescence will increase. For fast-neutron dosimetry, Pradhan et al.(96) prepared CaSO4:Dy embedded PE discs. For this application, PE granules were initially ground in a specially made electrically operated stainless steel mortar and pestle kept at 77 K. The fine powder so obtained was mixed with CaSO4:Dy powder and then pelletized (cold pressed) by a hydraulic press. These discs were treated at ~110°C for 8 h to impart flexibility and strength. PE discs containing CaSO4:Dy phosphor from 1 to 50% by weight and having thicknesses in the range 0.2–0.8 mm were studied using a low pressure Hg lamp (mainly 254 nm) as the stimulating source. In view of the constraints in the instrument, luminescence measurements could not be carried out during optical stimulation. The light collection started 1 min after the photostimulation. The light collection duration was also kept at 1 min. The OSL output of CaSO4:Dy PE discs was found to increase with disc thickness in the range 0.2–0.8 mm. It was also found that repeated OSL readouts of the disc is possible though the OSL sensitivity decreases with the number of readouts. Tables 16 and 17 show the effect of sample temperature on the optical-stimulation efficiency of gamma-ray irradiated 0.8 mm thick CaSO4:Dy embedded Teflon discs and 0.6 mm thick CaSO4:Dy embedded PE discs respectively. Results clearly indicate a sharp increase in the OSL output subsequent to UV illumination. In the case of the former since the sample could be raised to 200°C, an increase in light emission was seen up to 200°C. In fact the light collected is quite high at 200°C because at that temperature the TL signal from the main dosimetric peak is collected. However, in order to avoid thermal damage to PE, PE mixed TLD discs were heated to a maximum temperature of 92°C only, during optical stimulation as well as during light collection. At 92°C, the luminescence signal is 33 times that of the signal at RT (25°C). The PE embedded discs were not affected when kept at 92°C for several hours. The minimum measurable gamma-ray dose with CaSO4:Dy embedded PE discs was estimated to be 3.5 mGy. No experimental data of the fast-neutron sensitivity of these discs is available but by considering the published value of proton radiator sensitivity, S for 14.7 MeV monoenergetic neutrons, the neutron sensitivity on a Gy basis was worked out to be = 0.45 of 60Co gamma-ray sensitivity. S is defined as the ratio of the neutron sensitivity, mn, and the gamma ray sensitivity, mγ. The mn and mγ are given by mn = Readout/φ and mγ = Readout/D, where φ is the fast-neutron fluence and D is the 60Co gamma-ray dose in Gy = 4.4 × 10−11 and taking neutron fluence/Gy = 1.08 × 10−10 n cm−2 for PE. This means that by using CaSO4:Dy PE discs, fast-neutron doses above 8 m Gy (3.5/0.45 = 8) can be easily measured. This is still more than two orders of magnitude higher than the doses to be detected in personnel dosimetry. But for accidental dosimetry this system may be useful. The reasons for the poor fast-neutron sensitivity include the poor cross-section of the (n,p) reaction with hydrogen and the reduced relative TL efficiency of CaSO4:Dy for radiations (such as recoil protons) of high stopping power(97). The drawback of poor TL efficiency to high LET radiations can be overcome by using a TLD such as 7LiF. The 200°C peak in 7LiF behaves similar to that of the dosimetric peak in CaSO4:Dy, but the high-temperature peak at 250°C in 7LiF shows an increased TL with stopping power of the radiation. The reason for this is attributed to the 2-hit nature of its traps i.e the traps giving rise to the 250°C peak in LiF TLD-700 trap two electrons in a single site whereas the traps giving rise to the 200°C peak in LiF TLD -700 or CaSO4:Dy trap only one charge carrier in a single site and are known as 1-hit traps. Table 16. OSL output of 0.8 mm thick CaSO4:Dy Teflon TLD discs as a function of temperature maintained during stimulation and readout(96). Stimulation temperature (°C) Outputa (arbitrary units) Relative output RT (25) 0.013 ± 0.002 1.0 50 0.04 ± 0.0038 3.10 100 0.44 ± 0.020 34.0 150 0.98 ± 0.040 77.0 200 4.38 ± 0.220 337.0 Stimulation temperature (°C) Outputa (arbitrary units) Relative output RT (25) 0.013 ± 0.002 1.0 50 0.04 ± 0.0038 3.10 100 0.44 ± 0.020 34.0 150 0.98 ± 0.040 77.0 200 4.38 ± 0.220 337.0 aError shown 1σ. Table 16. OSL output of 0.8 mm thick CaSO4:Dy Teflon TLD discs as a function of temperature maintained during stimulation and readout(96). Stimulation temperature (°C) Outputa (arbitrary units) Relative output RT (25) 0.013 ± 0.002 1.0 50 0.04 ± 0.0038 3.10 100 0.44 ± 0.020 34.0 150 0.98 ± 0.040 77.0 200 4.38 ± 0.220 337.0 Stimulation temperature (°C) Outputa (arbitrary units) Relative output RT (25) 0.013 ± 0.002 1.0 50 0.04 ± 0.0038 3.10 100 0.44 ± 0.020 34.0 150 0.98 ± 0.040 77.0 200 4.38 ± 0.220 337.0 aError shown 1σ. Table 17. OSL output of 0.6 mm thick CaSO4:Dy embedded polyethylene discs (50% phosphor by weight) at elevated temperature and at RT(96). Temperature of stimulation and readout OSL output (arbitrary units)a Irradiated to 5 Gy of 60Co gamma-rays Unirradiated samples 92°C 2.8 ± 0.25 (6.3 ± 0.57) × 10−3 RT (25°C) 0.09 ± 0.0074 (4.6 ± 0.41) × 10−3 Temperature of stimulation and readout OSL output (arbitrary units)a Irradiated to 5 Gy of 60Co gamma-rays Unirradiated samples 92°C 2.8 ± 0.25 (6.3 ± 0.57) × 10−3 RT (25°C) 0.09 ± 0.0074 (4.6 ± 0.41) × 10−3 aError shown 1σ. Table 17. OSL output of 0.6 mm thick CaSO4:Dy embedded polyethylene discs (50% phosphor by weight) at elevated temperature and at RT(96). Temperature of stimulation and readout OSL output (arbitrary units)a Irradiated to 5 Gy of 60Co gamma-rays Unirradiated samples 92°C 2.8 ± 0.25 (6.3 ± 0.57) × 10−3 RT (25°C) 0.09 ± 0.0074 (4.6 ± 0.41) × 10−3 Temperature of stimulation and readout OSL output (arbitrary units)a Irradiated to 5 Gy of 60Co gamma-rays Unirradiated samples 92°C 2.8 ± 0.25 (6.3 ± 0.57) × 10−3 RT (25°C) 0.09 ± 0.0074 (4.6 ± 0.41) × 10−3 aError shown 1σ. RESPONSE OF CaSO4:Dy TEFLON DISCS TO HIGH-ENERGY PHOTONS In pressurised heavy water reactors, high-energy (>6 MeV) photons could contribute to radiation dose more than 50% at some sites in the nuclear power plant. Such high-energy photons are produced in the nuclear reactor during the beta decay of 16N (t1/2 = 7.13 s) produced via the neutron induced reaction 16O(n,p)16N in the CO2 cooling loops. The energy dependence of CaSO4:Dy and LiF TLDs to high-energy photons has been compared by Pradhan and Bakshi(98). Radiotherapy beams from medical linear accelerators (6 and 15 MV from clinac-2100C/D with mean photon energies of ~2.2 and 6 MeV, respectively) were chosen as radiation sources for high-energy photons. Absorbed dose and depth-dose profile in a water phantom were carried out using a radiation field analyser having a calibrated tiny ion chamber. The source to dosemeter distance was 80 cm for 60Co teletherapy machine and 1 m for high-energy photons. Table 18 shows the response of CaSO4:Dy Teflon discs and LiF (TLD-700) ribbons with and without metal filtration irradiated at depths related to Dmax in a polystyrene phantom. The depth for Dmax in water phantom occur at 5 mm for 60Co, 15 mm for 6 MV, 25 mm for 10 MV and 30 mm for 15 MV photons. Without any metal filter, the former does not exhibit any significant energy dependence while LiF under-responds by 4–6% to high-energy photons. However, the response of both the TLDS when sandwiched between metal filters (1 mm Cu + 1 mm Al) increase with photon energy. The over-response varies from 19 to 35% for CaSO4:Dy and from 10 to 25% for LiF. This was attributed to the enhanced pair production and scattering of the secondary electrons produced by the metal filters. Table 18. Relative responsea of CaSO4:Dy Teflon TLD discs and LiF (TLD 700) ribbons (value without filter for gamma rays = 1) irradiated for 500 mGy at depths related to Dmax in a polystyrene phantom(98). Photon beam (mean energy) (MeV) Relative response at Dmax in phantoma No filter Under metal filter (1 mm Cu + 1 mm Al) CaSO4:Dy discs LiF ribbons CaSO4:Dy discs LiF ribbons 60Co (1.25) 1.00 ± 0.02 1.00 ± 0.01 0.99 ± 0.07 0.97 ± 0.02 6 MV (~2.2) 1.02 ± 0.03 0.96 ± 0.03 1.19 ± 0.07 1.10 ± 0.05 10 MV (~4) 0.96 ± 0.03 0.94 ± 0.04 1.25 ± 0.06 1.15 ± 0.03 15 MV (~6) 0.99 ± 0.03 0.95 ± 0.03 1.35 ± 0.06 1.25 ± 0.03 Photon beam (mean energy) (MeV) Relative response at Dmax in phantoma No filter Under metal filter (1 mm Cu + 1 mm Al) CaSO4:Dy discs LiF ribbons CaSO4:Dy discs LiF ribbons 60Co (1.25) 1.00 ± 0.02 1.00 ± 0.01 0.99 ± 0.07 0.97 ± 0.02 6 MV (~2.2) 1.02 ± 0.03 0.96 ± 0.03 1.19 ± 0.07 1.10 ± 0.05 10 MV (~4) 0.96 ± 0.03 0.94 ± 0.04 1.25 ± 0.06 1.15 ± 0.03 15 MV (~6) 0.99 ± 0.03 0.95 ± 0.03 1.35 ± 0.06 1.25 ± 0.03 aMTL/Dmax relative to MTL/Dmax for 60Co gamma rays. Table 18. Relative responsea of CaSO4:Dy Teflon TLD discs and LiF (TLD 700) ribbons (value without filter for gamma rays = 1) irradiated for 500 mGy at depths related to Dmax in a polystyrene phantom(98). Photon beam (mean energy) (MeV) Relative response at Dmax in phantoma No filter Under metal filter (1 mm Cu + 1 mm Al) CaSO4:Dy discs LiF ribbons CaSO4:Dy discs LiF ribbons 60Co (1.25) 1.00 ± 0.02 1.00 ± 0.01 0.99 ± 0.07 0.97 ± 0.02 6 MV (~2.2) 1.02 ± 0.03 0.96 ± 0.03 1.19 ± 0.07 1.10 ± 0.05 10 MV (~4) 0.96 ± 0.03 0.94 ± 0.04 1.25 ± 0.06 1.15 ± 0.03 15 MV (~6) 0.99 ± 0.03 0.95 ± 0.03 1.35 ± 0.06 1.25 ± 0.03 Photon beam (mean energy) (MeV) Relative response at Dmax in phantoma No filter Under metal filter (1 mm Cu + 1 mm Al) CaSO4:Dy discs LiF ribbons CaSO4:Dy discs LiF ribbons 60Co (1.25) 1.00 ± 0.02 1.00 ± 0.01 0.99 ± 0.07 0.97 ± 0.02 6 MV (~2.2) 1.02 ± 0.03 0.96 ± 0.03 1.19 ± 0.07 1.10 ± 0.05 10 MV (~4) 0.96 ± 0.03 0.94 ± 0.04 1.25 ± 0.06 1.15 ± 0.03 15 MV (~6) 0.99 ± 0.03 0.95 ± 0.03 1.35 ± 0.06 1.25 ± 0.03 aMTL/Dmax relative to MTL/Dmax for 60Co gamma rays. Table 19 shows the relative response of CaSO4:Dy TLD badge irradiated on the surface of the phantom. While the response with metal filtration increases with photon energy, an opposite trend is seen with plastic filtration and at open window position. The severe under-response (by a maximum of a factor of 2 at 15 MV in the open window case) in the last two cases is due to the lack of build-up needed for high-energy photons. This under-response would cause uncertainties in the estimation of beta radiation dose which might accompany the high-energy photons. Table 19. Response (MTL/D10) of TLD card based on CaSO4:Dy Teflon TLD discs exposed to 60Co gamma rays from a telecobalt machine and photons from medical linear accelerators. TLD cards were placed at the surface of the phantom (25 × 25 × 15 cm3) and the dose measured at 10 mm depth in the phantom for photon of field size 25 × 25 cm2(98). Photon beam (mean energy) (MeV) Relative responsea of TLD discs under different filters in the TLD badge Metal filter (1 mm Cu + 1 mm Al) Plastic filter (1.6 mm) Open window 60Co (1.25) 1.00 ± 0.02 1.18 ± 0.02 1.03 ± 0.03 6 MV (~2.2) 1.08 ± 0.03 0.93 ± 0.03 0.67 ± 0.04 10 MV (~4) 1.23 ± 0.05 0.78 ± 0.06 0.56 ± 0.05 15 MV (~6) 1.27 ± 0.01 0.72 ± 0.03 0.52 ± 0.04 Photon beam (mean energy) (MeV) Relative responsea of TLD discs under different filters in the TLD badge Metal filter (1 mm Cu + 1 mm Al) Plastic filter (1.6 mm) Open window 60Co (1.25) 1.00 ± 0.02 1.18 ± 0.02 1.03 ± 0.03 6 MV (~2.2) 1.08 ± 0.03 0.93 ± 0.03 0.67 ± 0.04 10 MV (~4) 1.23 ± 0.05 0.78 ± 0.06 0.56 ± 0.05 15 MV (~6) 1.27 ± 0.01 0.72 ± 0.03 0.52 ± 0.04 aMTL/D10 relative to MTL/D10 for 60Co gamma rays. Table 19. Response (MTL/D10) of TLD card based on CaSO4:Dy Teflon TLD discs exposed to 60Co gamma rays from a telecobalt machine and photons from medical linear accelerators. TLD cards were placed at the surface of the phantom (25 × 25 × 15 cm3) and the dose measured at 10 mm depth in the phantom for photon of field size 25 × 25 cm2(98). Photon beam (mean energy) (MeV) Relative responsea of TLD discs under different filters in the TLD badge Metal filter (1 mm Cu + 1 mm Al) Plastic filter (1.6 mm) Open window 60Co (1.25) 1.00 ± 0.02 1.18 ± 0.02 1.03 ± 0.03 6 MV (~2.2) 1.08 ± 0.03 0.93 ± 0.03 0.67 ± 0.04 10 MV (~4) 1.23 ± 0.05 0.78 ± 0.06 0.56 ± 0.05 15 MV (~6) 1.27 ± 0.01 0.72 ± 0.03 0.52 ± 0.04 Photon beam (mean energy) (MeV) Relative responsea of TLD discs under different filters in the TLD badge Metal filter (1 mm Cu + 1 mm Al) Plastic filter (1.6 mm) Open window 60Co (1.25) 1.00 ± 0.02 1.18 ± 0.02 1.03 ± 0.03 6 MV (~2.2) 1.08 ± 0.03 0.93 ± 0.03 0.67 ± 0.04 10 MV (~4) 1.23 ± 0.05 0.78 ± 0.06 0.56 ± 0.05 15 MV (~6) 1.27 ± 0.01 0.72 ± 0.03 0.52 ± 0.04 aMTL/D10 relative to MTL/D10 for 60Co gamma rays. An earlier study(99) has, however, shown that TLDs without adequate build-up severely over-respond to high-energy (6 MeV) photons. A trend towards stabilisation was reached only when the build-up thickness was ≥20 mm of PMMA. The reason for this behaviour was attributed to the presence of high-energy electron contamination in the primary photon beam. Values of the stabilised (i.e. after proper build-up) TL response to 6 MeV photons relative to 137Cs gamma-rays were 0.95 ± 0.04 for LiF:Mg,Ti; 0.90 ± 0.05 for Li2B4O7:Cu; and 1.00 ± 0.03 for CaSO4:Tm. CONCLUSION Solid forms of CaSO4:Dy TLD especially in Teflon embedded form in various shapes and configurations have been found to be well suited for the large-scale routine measurements of ICRU operational quantities, Hp(10), Hs(0.07) and H*(10) as well for measurement of the effective dose, HE. They are also capable of measuring high-energy photons encountered in nuclear reactors and medical accelerators. Thin Teflon wafers with a suitable top support appears to be an ideal TL device for routine beta dosimetric applications. Their production techniques are well standardised. They have been found to be amenable to contact ohmic heating as well as hot-air/hot nitrogen heating for TL measurements. Other solid forms of CaSO4:Dy TLDs useful for specific applications include graphite mixed ones for beta-dosimetry, sulphur-mixed ones for fast-neutron dosimetry using neutron activation, 6LiF-mixed ones for thermal and moderated neutron measurements and aluminium mixed ones for high-level gamma-dosimetry. Further development of sintered pellets of CaSO4:Dy with minimal binder is, however, needed as it has potential applications. ACKNOWLEDGEMENTS The author is thankful to the Board of Research in Nuclear Sciences (BRNS), Department of Atomic Energy, Govt. of India, for providing financial assistance to carry out a part of this work under the research project on High Level Radiation Dosimetry sanctioned via letter No. 2013/36/47-BRNS/2422. He is grateful to Dr A.S. Pradhan from BARC for useful suggestions. The author is extremely grateful to Dr. G. Napappan from Computer Science Department, SEC for the extraordinary efforts made by him in improving the quality of the figures included in this paper. REFERENCES 1 UNSCEAR . Sources and effects of ionizing radiation. United Nations Scientific Committee on the Effect of Atomic Radiation, Report to the General Assembly with Scientific Annexes, New York ( 1983 ). 2 Yamashita , T. , Nada , N. , Onishi , H. and Kitamura , S. Calcium sulfate activated by thulium or dysprosium for thermoluminescent dosimetry . Health Phys. 21 , 295 – 300 ( 1971 ). 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TI - DEVELOPMENT AND APPLICATION OF SOLID FORMS OF CaSO4:Dy THERMOLUMINESCENT DOSEMETERS IN RADIATION PROTECTION DOSIMETRY—A REVIEW
JF - Radiation Protection Dosimetry
DO - 10.1093/rpd/ncx287
DA - 2018-10-01
UR - https://www.deepdyve.com/lp/oxford-university-press/development-and-application-of-solid-forms-of-caso4-dy-0p0Y5frF6y
SP - 57
VL - 181
IS - 2
DP - DeepDyve
ER -