DEVELOPMENT OF AN IONIZATION CHAMBER FOR LOW AND MEDIUM ENERGY PHOTON DOSIMETRY

DEVELOPMENT OF AN IONIZATION CHAMBER FOR LOW AND MEDIUM ENERGY PHOTON DOSIMETRY Abstract Measurement of dose due to low and medium energy X-ray using an ionization chamber needs special considerations as the wall thickness of the detector plays an important role in signal generation. A thin and plane wall ionization chamber having 900 cc volume was developed to study its calibration coefficient with air kerma (Kair) rate and ambient dose equivalent (H*(10)) rate at various X-ray beam qualities generated from a dosimetry grade X-ray machine. Optimized ionization chamber wall thickness was determined through measurements where a flat energy response of the ionization chamber could be established. The measurement shows that the air kerma rate based calibration coefficient for ionization chamber varies up to 45% and ambient dose equivalent rate based calibration coefficient varies up to 20% for wall thickness 1.2–1200 mg/cm2 in the energy range 17.6–213 keV. INTRODUCTION In external radiation dosimetry, the protection quantity like effective dose is not directly measureable and thus cannot be used for radiation protection purpose. Therefore, operational quantities(1) have been defined by ICRU for the assessment of these protection quantities. The operational quantities provide a conservative estimate of the protection quantities. The operational quantity, ambient dose equivalent (H*(10)) is used for the monitoring of strongly penetrating radiation. Measurement of dose due to low and medium energy X-ray (particularly generated from synchrotron radiation, diagnostic and radiographic facilities) needs special considerations as the wall thickness of the detector plays an important role in signal generation. Nearly all detectors show a small to very high energy dependent response for low and medium energy photon fields. Monitoring of dose due to these low and medium energy photon fields is difficult as the recommended energy response acceptance criteria for air kerma rate based radiation monitoring instruments is ±30% (in 50 keV to 1.5 MeV energy range)(2) and −29% to +67% (in 20–150 keV energy range) for ambient dose equivalent rate based instruments(3). However, the monitoring/dosimetry of these radiation fields are essential for the radiation protection of the experimenters, operators and other personnel’s associated with above mentioned facilities. The ionization chamber is the simplest of all the gas-filled radiation detectors and is widely used for the detection and measurement of X-rays, gamma rays and beta particles(4). It is considered to be one of the most reliable detectors and is less affected (depending on wall thickness) by incident energy as compared to other detectors. Therefore, the selection of an optimized wall thickness of ionization chamber is very important to achieve this uniform energy response. Studies with spherical and cylindrical wall ion chambers at 15 kV X-ray beams reveals that the chamber sensitivity at low energies strongly depends on thickness and curvature of the chamber(5). It also shows that a thin and plane walled ionization chamber suits best for the radiation monitoring from such type of radiation fields. The aim of the current study is to develop a plane wall ionization chamber having energy independent response over a wide range of energies so that it can be used for the dosimetry of low and medium energy strongly penetrating radiation (photon energies above 15 keV(6)) generated in accelerators and X-ray facilities. Plane wall ionization chamber is designed by using very thin Mylar films, to address problems related to curvature effect and excess photon attenuation in the chamber wall(5). Four PMMA build-up caps of different thickness were fabricated and used to study the effect of wall thickness at various low and medium energy photon fields and arrive at an optimized wall thickness. The H*(10) conversion factors for strongly penetrating radiation are available from 15 keV onwards in the literature(7–9). The effect of wall thickness on the response of the chamber, for the monitoring quantity air kerma and ambient dose equivalent, are experimentally evaluated and discussed from 17.6 keV onwards. The response time of the ion chamber is generally of the order of ms. However, in the present study, it is not measured for the developed ionization chamber, as all the measurement were made for continuous X-ray beams. Design of the ionization chamber For the measurement of low dose rate radiation field, a large volume (~1000 cc) and high sensitivity ionization chamber design was thought to be suitable. Thus a plane wall ionization chamber having dimensions 8 cm × 8 cm × 15 cm with nominal volume ~900 cc was exclusively designed and fabricated. The ionization chamber consists of four 10 mm × 10 mm thick aluminum rods (length: 15 cm) mounted on a 5 mm thick and 8 × 8 cm2 aluminum plate on one side. The other end of the aluminum rods is fixed with a square aluminum frame of thickness 1 mm (external dimensions 8 cm × 8 cm and internal diameter of 6 cm × 6 cm). Copper wire (diameter 1.6 mm and 10 cm length) is used as the central electrode of the ionization chamber. The wall of ionization chamber is made up of thin Mylar foil having thickness ~1.24 mg/cm2. Standard panel mount bulkhead fitting BNC connector(10) is used as connector, where the central pin is connected to charge collecting central electrode while its body works as guard. A 5 mm PMMA insulation is provided between ion chamber body and BNC connector body. The body of the ionization chamber works as ground. The schematic diagram and the photograph of the developed plane wall ionization chamber is shown in Figures 1 and 2, respectively. Four polymethyl meta acrylic (PMMA) build-up caps with internal dimension 8.1 cm × 8.1 cm × 16 cm having thickness of 1, 2, 3 and 10 mm were fabricated and used for studying effect of wall thickness on the energy response of the ion chamber (Figure 2). Figure 1. View largeDownload slide Schematic diagram of plane wall 900 cc ionization chamber. Figure 1. View largeDownload slide Schematic diagram of plane wall 900 cc ionization chamber. Figure 2. View largeDownload slide The plane wall ionization chamber and the PMMA build-up cap. Figure 2. View largeDownload slide The plane wall ionization chamber and the PMMA build-up cap. Measurement technique Dosimetric grade X-ray machine (YXLON MG325)(11) is used to generate various ISO 4037-1(12) specified narrow series and fluorescence reference X-ray beam qualities (Table 1) for the study. The output of the X-ray machine is standardized, in terms of air kerma (Kair), using parallel plate free air ionization chamber (FAIC)(13), available as an absolute standard in the laboratory, in conjunction with reference class electrometer(14). The sensitive volume of the FAIC is 7.85 cc. These beam qualities were standardized at the height of 100 cm above the ground level at a distance of 200 cm from the center of the focal spot of X-ray machine. The X-ray beams were collimated using a lead collimator (which provides a field size of 40 cm × 30 cm at 200 cm distance). A laser alignment system was used to align the center of the ionization chambers with the focal spot of the X-ray machine. Table 1. ISO 4037-1 beam qualities used for estimation of calibration coefficients for 900 cc ionization chamber. Beam quality Mean energy (keV) h*K(10;H) (Sv/Gy) Characteristic, F-Mo 17.6 0.44 Characteristic, F-Cd 22.7 0.80 Characteristic, F-Sn 24.9 0.91 Narrow, N-40 30 1.18 Narrow, N-60 46 1.59 Narrow, N-80 63 1.73 Narrow, N-100 83 1.71 Narrow, N-120 101 1.64 Narrow, N-150 120 1.58 Narrow, N-200 167 1.46 Narrow, N-250 213 1.39 Beam quality Mean energy (keV) h*K(10;H) (Sv/Gy) Characteristic, F-Mo 17.6 0.44 Characteristic, F-Cd 22.7 0.80 Characteristic, F-Sn 24.9 0.91 Narrow, N-40 30 1.18 Narrow, N-60 46 1.59 Narrow, N-80 63 1.73 Narrow, N-100 83 1.71 Narrow, N-120 101 1.64 Narrow, N-150 120 1.58 Narrow, N-200 167 1.46 Narrow, N-250 213 1.39 Table 1. ISO 4037-1 beam qualities used for estimation of calibration coefficients for 900 cc ionization chamber. Beam quality Mean energy (keV) h*K(10;H) (Sv/Gy) Characteristic, F-Mo 17.6 0.44 Characteristic, F-Cd 22.7 0.80 Characteristic, F-Sn 24.9 0.91 Narrow, N-40 30 1.18 Narrow, N-60 46 1.59 Narrow, N-80 63 1.73 Narrow, N-100 83 1.71 Narrow, N-120 101 1.64 Narrow, N-150 120 1.58 Narrow, N-200 167 1.46 Narrow, N-250 213 1.39 Beam quality Mean energy (keV) h*K(10;H) (Sv/Gy) Characteristic, F-Mo 17.6 0.44 Characteristic, F-Cd 22.7 0.80 Characteristic, F-Sn 24.9 0.91 Narrow, N-40 30 1.18 Narrow, N-60 46 1.59 Narrow, N-80 63 1.73 Narrow, N-100 83 1.71 Narrow, N-120 101 1.64 Narrow, N-150 120 1.58 Narrow, N-200 167 1.46 Narrow, N-250 213 1.39 Calibration coefficient of the fabricated ionization chamber is measured against FAIC, using substitution technique(9), by placing the ionization chamber in front of X-ray machine at a distance of 200 cm from focal spot. The ionization chamber was coupled with a reference class electrometer (PTW UNIDOS)(15) (the coupled systems shows a leakage current of ~3 fA) for its radiation response measurement. A potential difference of 400 V was applied to the electrodes of ionization chamber through built-in high voltage unit of the electrometer. The output current of the ion chamber was recorded for the generated ISO narrow series and fluorescence reference X-ray beam qualities. Thereafter the PMMA build-up cap of 1 mm was mounted on the ionization chamber and was positioned at the same place, i.e. at 100 cm height and 200 cm from focal spot. The output of the chamber with 1 mm wall build-up cap was also recorded for the above mentioned X-ray beam qualities. Same measurement was repeated with 2, 3 and 10 mm PMMA wall build-up caps. The air kerma rate based calibration coefficients, NK, of the ionization chamber (with uncertainty at coverage factor 1, as errorbar) for all the wall thicknesses and energies (beam qualities), are evaluated and plotted against mean energy of the beams, as shown in Figure 3. The air kerma rate calibration coefficients (NK) of the ionization chamber are converted to H*(10) rate based calibration coefficients (NH) using air kerma to ambient dose equivalent conversion factors (h*K(10;H)) (Table 1) and the plot of the same (with uncertainty at coverage factor 1, as errorbar) is shown in Figure 4. Figure 3. View largeDownload slide Kair rate based calibration coefficient of 900 cc ionization chamber at various energies and wall thickness. Figure 3. View largeDownload slide Kair rate based calibration coefficient of 900 cc ionization chamber at various energies and wall thickness. Figure 4. View largeDownload slide H*(10) rate based calibration coefficient of 900 cc ionization chamber at various energies and wall thickness. Figure 4. View largeDownload slide H*(10) rate based calibration coefficient of 900 cc ionization chamber at various energies and wall thickness. Evaluation technique The signal from the FAIC is measured in terms of current, I (pA). The average current (Iavg) of the FAIC measured at 200 cm distance is used to calculate conventional true value of Kair rate (K̇T)(16) as follows: K̇T(Gy/h)=Iavg.(W¯/e).(1−g).mair∏iki (1) where W¯/e = average energy required to produce one ion pair in air (33.97 eV); ∏iki = product of correction factors#; # The correction factors includes correction for temperature, pressure, saturation and photon attenuation (ka)(16). (1 − g) = mean correction for energy given to radiative photons; and mair = air mass of the FAIC sensitive volume at NTP (kg). From the conventional true value of air kerma rate (K̇T), conventional true value of ambient dose equivalent rate (ḢT) is evaluated using air kerma to ambient dose equivalent conversion factor h*K(10;H) as follows: ḢT(Sv/h)=K̇T(Gy/h).h⁎K(10;H)(Sv/Gy) (2) The response or signal of the 900 cc ionization chamber is also measured in terms of current R (pA). The average signal corrected for background (Ravg) received from the ionization chamber, at the same distance of 200 cm, is corrected for charge collection efficiency (f), temperature (kT) and pressure (kP) corrections to evaluate the corrected instrument response (Rc). f=4/3−(RV/RV/2)/3 (3) where RV is the ‘average instrument response’ at voltage V and RV/2 is ‘average instrument response’ at voltage V/2. kT=[273.15+t(°C)]/293.15 (4) where ‘t (°C)’ is the ambient temperature at the time of measurement in °C. kP=1013.25/P(mbar) (5) where ‘P’ is the ambient pressure at the time of measurement given in ‘mbar’. Rc=Ravg.ks.kT.kP (6) where ‘ks’ (= 1/f) is the saturation correction evaluated from collection efficiency. The conventional true values and corrected signal (Rc) of the 900 cc ionization chamber is used to evaluate the calibration coefficient of ionization chamber with respect to air kerma (NK) rates and ambient dose equivalent (NH) rate respectively, as follows: NK(μGy/h/pA)=K̇T(μGy/h)Rc(pA) (7) NH(μSv/h/pA)=ḢT(μSv/h)Rc(pA) (8) The air kerma rate (K̇), from the 900 cc ionization chamber, is evaluated for the corrected signal (Rc) as follows. K̇=NK.Rc=NK.Ravg.ks.kT.kP (9) To estimate the air kerma rate linearity response of the chamber, the percentage relative intrinsic error(3) ‘I’ (RIE) of the estimated air kerma rate K̇ from the conventional true value K̇T was evaluated as follows: I(%)=(K̇−K̇T).100K̇T (10) RESULTS AND DISCUSSIONS Charge collection efficiency and linearity Since high air kerma rates (~1 Gy/h) cannot be generated for narrow series and fluorescence reference X-ray beams(9, 12), the direct X-ray beam of 60 kV (2 mm Al filtration) was standardized by FAIC and is used for measuring charge collection efficiency (equation 3) of the chamber (two voltage method(13)). The calibration coefficient of the developed IC was evaluated (at NTP) for the standardized direct X-ray beam of 60 kV (2 mm Al filtration) at Kair rate of 84 mGy/h to measure the collection efficiency of the chamber with increasing air kerma rates. The measured charge collection efficiency is found to be better than 95.0% for Kair rates up to 1.5 Gy/h. The plot of collection efficiency with increasing Kair rate is shown in Figure 5, which clearly shows decreasing collection efficiency with increasing Kair rate. The air kerma rate linearity of the chamber (in terms of RIE) was also evaluated for air kerma rate up to ~ 1.5 Gy/h as shown in Table 2, for an applied electrode potential of 400 V. Figure 5. View largeDownload slide Variation of collection efficiency with air kerma rate for 900 cc ionization chamber. Figure 5. View largeDownload slide Variation of collection efficiency with air kerma rate for 900 cc ionization chamber. Table 2. Collection efficiency and air kerma rate linearity response of 900 cc ionization chamber. Air kerma rate ( K̇T) (mGy/h) Collection efficiency (f) (%) Measured air kerma rate ( K̇) (mGy/h) Relative intrinsic error (I) (%) 4.22 100 4.23 0.24 16.8 100 16.8 0.00 50.4 99.8 50.4 0.00 83.9 99.6 84.0 0.12 167.9 99.2 167.8 −0.06 293.8 98.7 293.8 −0.00 419.7 98.3 418.6 −0.26 587.6 97.8 585.0 −0.44 839.5 97.1 833.3 −0.74 1259 96.0 1247 −0.95 1511 95.0 1494 −1.13 Air kerma rate ( K̇T) (mGy/h) Collection efficiency (f) (%) Measured air kerma rate ( K̇) (mGy/h) Relative intrinsic error (I) (%) 4.22 100 4.23 0.24 16.8 100 16.8 0.00 50.4 99.8 50.4 0.00 83.9 99.6 84.0 0.12 167.9 99.2 167.8 −0.06 293.8 98.7 293.8 −0.00 419.7 98.3 418.6 −0.26 587.6 97.8 585.0 −0.44 839.5 97.1 833.3 −0.74 1259 96.0 1247 −0.95 1511 95.0 1494 −1.13 Table 2. Collection efficiency and air kerma rate linearity response of 900 cc ionization chamber. Air kerma rate ( K̇T) (mGy/h) Collection efficiency (f) (%) Measured air kerma rate ( K̇) (mGy/h) Relative intrinsic error (I) (%) 4.22 100 4.23 0.24 16.8 100 16.8 0.00 50.4 99.8 50.4 0.00 83.9 99.6 84.0 0.12 167.9 99.2 167.8 −0.06 293.8 98.7 293.8 −0.00 419.7 98.3 418.6 −0.26 587.6 97.8 585.0 −0.44 839.5 97.1 833.3 −0.74 1259 96.0 1247 −0.95 1511 95.0 1494 −1.13 Air kerma rate ( K̇T) (mGy/h) Collection efficiency (f) (%) Measured air kerma rate ( K̇) (mGy/h) Relative intrinsic error (I) (%) 4.22 100 4.23 0.24 16.8 100 16.8 0.00 50.4 99.8 50.4 0.00 83.9 99.6 84.0 0.12 167.9 99.2 167.8 −0.06 293.8 98.7 293.8 −0.00 419.7 98.3 418.6 −0.26 587.6 97.8 585.0 −0.44 839.5 97.1 833.3 −0.74 1259 96.0 1247 −0.95 1511 95.0 1494 −1.13 Energy response with wall thickness The air kerma rate calibration coefficient, NK, (equation 7) of the ionization chamber is evaluated for the used ISO 4037-1 reference X-ray beams (Table 1) ranging from 17.6 to 213 keV against FAIC. The variation in calibration coefficient ‘NK’ of the ionization chamber with varying wall thickness and energy was recorded and plotted for narrow and fluorescence (ISO 4037-1) X-ray beam qualities as shown in Figure 3. The h*K(10;H) (Sv/Gy) [IAEA Safety Series-16(9)] conversion factors were used to convert air kerma rate to ambient dose equivalent rate. This ambient dose equivalent rate is used to calculate the calibration coefficient ‘NH’ for the ionization chamber. Figure 4 shows the variation of NH with wall thickness and energy for the used ISO beam qualities. From Figure 4, it can be seen that, this ionization chamber (with 10 mm PMMA build-up cap) has a better energy response for H*(10) rate based monitoring of radiation fields. The H*(10) rate based average calibration coefficient (ionization chamber with 10 mm PMMA build-up cap) is found to be 148.9 μSv/h/pA (maximum deviation of 18.8% at 17.6 keV) for mean energy range of 17.6–213 keV while for energy range 22.7–213 keV the average calibration coefficient is found to be 151.7 μSv/h/pA (maximum deviation of 4.3% at 63 keV) as shown in Table 3. Table 3. Calibration coefficients for 900 cc ionization chamber for 10 mm PMMA build-up cap. Beam quality Mean energy (keV) Calibration coefficient, (NH) (μSv/h/pA) Deviation from average, NH (all energies considered) (%) Deviation from average, NH (all, except F-Mo, energies considered) (%) F-Mo 17.6 120.8 −18.9 — F-Cd 22.7 151.6 1.8 −0.1 F-Sn 24.9 155.1 4.2 2.2 N-40 30 147.8 −0.7 −2.6 N-60 46 151.0 1.4 −0.5 N-80 63 145.1 −2.6 −4.3 N-100 83 148.2 −0.5 −2.3 N-120 101 150.7 1.2 −0.7 N-150 120 157.5 5.8 3.8 N-200 167 152.1 2.2 0.3 N-250 213 157.6 5.8 3.9 Average NH = 148.9 μSv/h/pA; Standard deviation = 6.8% Average NH (all, except F-Mo energy) = 151.7 μSv/h/pA; Standard deviation = 2.7% Beam quality Mean energy (keV) Calibration coefficient, (NH) (μSv/h/pA) Deviation from average, NH (all energies considered) (%) Deviation from average, NH (all, except F-Mo, energies considered) (%) F-Mo 17.6 120.8 −18.9 — F-Cd 22.7 151.6 1.8 −0.1 F-Sn 24.9 155.1 4.2 2.2 N-40 30 147.8 −0.7 −2.6 N-60 46 151.0 1.4 −0.5 N-80 63 145.1 −2.6 −4.3 N-100 83 148.2 −0.5 −2.3 N-120 101 150.7 1.2 −0.7 N-150 120 157.5 5.8 3.8 N-200 167 152.1 2.2 0.3 N-250 213 157.6 5.8 3.9 Average NH = 148.9 μSv/h/pA; Standard deviation = 6.8% Average NH (all, except F-Mo energy) = 151.7 μSv/h/pA; Standard deviation = 2.7% Table 3. Calibration coefficients for 900 cc ionization chamber for 10 mm PMMA build-up cap. Beam quality Mean energy (keV) Calibration coefficient, (NH) (μSv/h/pA) Deviation from average, NH (all energies considered) (%) Deviation from average, NH (all, except F-Mo, energies considered) (%) F-Mo 17.6 120.8 −18.9 — F-Cd 22.7 151.6 1.8 −0.1 F-Sn 24.9 155.1 4.2 2.2 N-40 30 147.8 −0.7 −2.6 N-60 46 151.0 1.4 −0.5 N-80 63 145.1 −2.6 −4.3 N-100 83 148.2 −0.5 −2.3 N-120 101 150.7 1.2 −0.7 N-150 120 157.5 5.8 3.8 N-200 167 152.1 2.2 0.3 N-250 213 157.6 5.8 3.9 Average NH = 148.9 μSv/h/pA; Standard deviation = 6.8% Average NH (all, except F-Mo energy) = 151.7 μSv/h/pA; Standard deviation = 2.7% Beam quality Mean energy (keV) Calibration coefficient, (NH) (μSv/h/pA) Deviation from average, NH (all energies considered) (%) Deviation from average, NH (all, except F-Mo, energies considered) (%) F-Mo 17.6 120.8 −18.9 — F-Cd 22.7 151.6 1.8 −0.1 F-Sn 24.9 155.1 4.2 2.2 N-40 30 147.8 −0.7 −2.6 N-60 46 151.0 1.4 −0.5 N-80 63 145.1 −2.6 −4.3 N-100 83 148.2 −0.5 −2.3 N-120 101 150.7 1.2 −0.7 N-150 120 157.5 5.8 3.8 N-200 167 152.1 2.2 0.3 N-250 213 157.6 5.8 3.9 Average NH = 148.9 μSv/h/pA; Standard deviation = 6.8% Average NH (all, except F-Mo energy) = 151.7 μSv/h/pA; Standard deviation = 2.7% CONCLUSION The developed ionization chamber can be used as a detector for measuring H*(10) rate of up to ~1 Sv/h in protection level radiation monitoring instruments like survey meters and area gamma monitors. In such instruments, generally, automatic correction for temperature, pressure and charge collection efficiency is not applied. To avoid the correction for temperature and pressure, the ionization chamber can be sealed and used. Thus only saturation correction will lead to deviation in ambient dose equivalent rate measurement. The sealed ionization chamber may show an overall relative intrinsic error of ~8–10% (i.e. ~4–5% due to saturation correction and 4–5% due to energy response) in the energy range 20–200 keV. The unsealed ionization chamber can also be used as secondary or working standard for standardization of known low and medium energy photon beams by applying required corrections. ACKNOWLEDGEMENTS Authors are thankful to Dr Pradeepkumar K.S., Associate Director, Health Safety & Environment Group, Bhabha Atomic Research Center for his constant encouragement and support during the course of this work and Radiation Safety Systems Division workshop for timely fabrication of the ionization chamber and build-up caps. REFERENCES 1 ICRU Report 51 ( 1993 ). Quantities and units in radiation protection dosimetry. 2 IEC 61584 First edition ( 2001 –06). Radiation protection instrumentation—installed, portable or transportable assemblies—measurement of air kerma direction and air kerma rate. 3 IEC 60846-1 Edition 1.0 ( 2009 –04). Radiation protection instrumentation—ambient and/or directional dose equivalent (rate) meters and/or monitors for beta, X and gamma radiation—Part 1: Portable workplace and environmental meters and monitors. 4 Knoll , G. F. Radiation Detection and Measurement , 3rd edn . ( New York : John Wiley & Sons ) ( 2000 ). 5 Mahant , A. K. , Singh , S. K. and Vinatha , S. P. Development of ion chambers for the measurement of low energy synchrotron radiation . Nucl. Instrum. Methods Phys. Res. A 601 , 354 – 357 ( 2009 ). Google Scholar CrossRef Search ADS 6 Podgorsak , E. B. Radiation Oncology Physics: A Handbook for Teachers and Students ( Vienna : IAEA ) ( 2005 ). 7 ISO Standard 4037-3 ( 1999 ). X and gamma reference radiation for calibrating dosemeters and doserate meters and for determining their response as a function of photon energy—calibration of area and personal dosemeters and the measurement of their response as a function of energy and angle of incidence, first edition. 8 ICRP Publication 74 ( 1996 ). Conversion coefficients for use in radiological protection against external radiation. 9 IAEA, Safety Report Series No. 16 . Calibration of Radiation Protection Monitoring Instruments ( Vienna : IAEA ) ( 2000 ) 2000. 10 BNC Connector: https://uk.rs-online.com/web/p/bnc-connectors/5121174/ (23 March 2018, date last accessed). 11 X-ray Machine Model YXLON MG325 (Y.MG X-Ray Systems). Available on https://www.yxlon.com/Yxlon/media/Content/Products/X-ray%20modules/Y.MG/MG_Brochure_eng_2015_01(A4-OP).pdf (23 March 2018, date last accessed). 12 ISO Standard 4037-1 ( 1996 ). X and gamma reference radiation for calibrating dosemeters and doserate meters and for determining their response as a function of photon energy—radiation characteristics and production methods, first edition, Case Postale 56, CH-1211, Geneva 20, Switzerland. 13 Attix , F. H. Introduction to Radiological Physics and Radiation Dosimetry ( Strauss GmbH, Morlenbach : Wiley-VCH ) ( 1986 ). Google Scholar CrossRef Search ADS 14 IEC 60731 Edition 3.1 ( 2016 ). Medical electrical equipment—dosimeters with ionization chambers as used in radiotherapy. 15 Manual/Specification of PTW UNIDOS E Electrometer. Available on http://www.ptw.de/unidos_e_dosemeter_ad0.html (23 March 2018, date last accessed). 16 Burns , D. T. and Buermann , L. Free-air ionization chambers . Metrologia 46 , S9 – S23 ( 2009 ). Google Scholar CrossRef Search ADS © The Author(s) 2018. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Radiation Protection Dosimetry Oxford University Press

DEVELOPMENT OF AN IONIZATION CHAMBER FOR LOW AND MEDIUM ENERGY PHOTON DOSIMETRY

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Abstract

Abstract Measurement of dose due to low and medium energy X-ray using an ionization chamber needs special considerations as the wall thickness of the detector plays an important role in signal generation. A thin and plane wall ionization chamber having 900 cc volume was developed to study its calibration coefficient with air kerma (Kair) rate and ambient dose equivalent (H*(10)) rate at various X-ray beam qualities generated from a dosimetry grade X-ray machine. Optimized ionization chamber wall thickness was determined through measurements where a flat energy response of the ionization chamber could be established. The measurement shows that the air kerma rate based calibration coefficient for ionization chamber varies up to 45% and ambient dose equivalent rate based calibration coefficient varies up to 20% for wall thickness 1.2–1200 mg/cm2 in the energy range 17.6–213 keV. INTRODUCTION In external radiation dosimetry, the protection quantity like effective dose is not directly measureable and thus cannot be used for radiation protection purpose. Therefore, operational quantities(1) have been defined by ICRU for the assessment of these protection quantities. The operational quantities provide a conservative estimate of the protection quantities. The operational quantity, ambient dose equivalent (H*(10)) is used for the monitoring of strongly penetrating radiation. Measurement of dose due to low and medium energy X-ray (particularly generated from synchrotron radiation, diagnostic and radiographic facilities) needs special considerations as the wall thickness of the detector plays an important role in signal generation. Nearly all detectors show a small to very high energy dependent response for low and medium energy photon fields. Monitoring of dose due to these low and medium energy photon fields is difficult as the recommended energy response acceptance criteria for air kerma rate based radiation monitoring instruments is ±30% (in 50 keV to 1.5 MeV energy range)(2) and −29% to +67% (in 20–150 keV energy range) for ambient dose equivalent rate based instruments(3). However, the monitoring/dosimetry of these radiation fields are essential for the radiation protection of the experimenters, operators and other personnel’s associated with above mentioned facilities. The ionization chamber is the simplest of all the gas-filled radiation detectors and is widely used for the detection and measurement of X-rays, gamma rays and beta particles(4). It is considered to be one of the most reliable detectors and is less affected (depending on wall thickness) by incident energy as compared to other detectors. Therefore, the selection of an optimized wall thickness of ionization chamber is very important to achieve this uniform energy response. Studies with spherical and cylindrical wall ion chambers at 15 kV X-ray beams reveals that the chamber sensitivity at low energies strongly depends on thickness and curvature of the chamber(5). It also shows that a thin and plane walled ionization chamber suits best for the radiation monitoring from such type of radiation fields. The aim of the current study is to develop a plane wall ionization chamber having energy independent response over a wide range of energies so that it can be used for the dosimetry of low and medium energy strongly penetrating radiation (photon energies above 15 keV(6)) generated in accelerators and X-ray facilities. Plane wall ionization chamber is designed by using very thin Mylar films, to address problems related to curvature effect and excess photon attenuation in the chamber wall(5). Four PMMA build-up caps of different thickness were fabricated and used to study the effect of wall thickness at various low and medium energy photon fields and arrive at an optimized wall thickness. The H*(10) conversion factors for strongly penetrating radiation are available from 15 keV onwards in the literature(7–9). The effect of wall thickness on the response of the chamber, for the monitoring quantity air kerma and ambient dose equivalent, are experimentally evaluated and discussed from 17.6 keV onwards. The response time of the ion chamber is generally of the order of ms. However, in the present study, it is not measured for the developed ionization chamber, as all the measurement were made for continuous X-ray beams. Design of the ionization chamber For the measurement of low dose rate radiation field, a large volume (~1000 cc) and high sensitivity ionization chamber design was thought to be suitable. Thus a plane wall ionization chamber having dimensions 8 cm × 8 cm × 15 cm with nominal volume ~900 cc was exclusively designed and fabricated. The ionization chamber consists of four 10 mm × 10 mm thick aluminum rods (length: 15 cm) mounted on a 5 mm thick and 8 × 8 cm2 aluminum plate on one side. The other end of the aluminum rods is fixed with a square aluminum frame of thickness 1 mm (external dimensions 8 cm × 8 cm and internal diameter of 6 cm × 6 cm). Copper wire (diameter 1.6 mm and 10 cm length) is used as the central electrode of the ionization chamber. The wall of ionization chamber is made up of thin Mylar foil having thickness ~1.24 mg/cm2. Standard panel mount bulkhead fitting BNC connector(10) is used as connector, where the central pin is connected to charge collecting central electrode while its body works as guard. A 5 mm PMMA insulation is provided between ion chamber body and BNC connector body. The body of the ionization chamber works as ground. The schematic diagram and the photograph of the developed plane wall ionization chamber is shown in Figures 1 and 2, respectively. Four polymethyl meta acrylic (PMMA) build-up caps with internal dimension 8.1 cm × 8.1 cm × 16 cm having thickness of 1, 2, 3 and 10 mm were fabricated and used for studying effect of wall thickness on the energy response of the ion chamber (Figure 2). Figure 1. View largeDownload slide Schematic diagram of plane wall 900 cc ionization chamber. Figure 1. View largeDownload slide Schematic diagram of plane wall 900 cc ionization chamber. Figure 2. View largeDownload slide The plane wall ionization chamber and the PMMA build-up cap. Figure 2. View largeDownload slide The plane wall ionization chamber and the PMMA build-up cap. Measurement technique Dosimetric grade X-ray machine (YXLON MG325)(11) is used to generate various ISO 4037-1(12) specified narrow series and fluorescence reference X-ray beam qualities (Table 1) for the study. The output of the X-ray machine is standardized, in terms of air kerma (Kair), using parallel plate free air ionization chamber (FAIC)(13), available as an absolute standard in the laboratory, in conjunction with reference class electrometer(14). The sensitive volume of the FAIC is 7.85 cc. These beam qualities were standardized at the height of 100 cm above the ground level at a distance of 200 cm from the center of the focal spot of X-ray machine. The X-ray beams were collimated using a lead collimator (which provides a field size of 40 cm × 30 cm at 200 cm distance). A laser alignment system was used to align the center of the ionization chambers with the focal spot of the X-ray machine. Table 1. ISO 4037-1 beam qualities used for estimation of calibration coefficients for 900 cc ionization chamber. Beam quality Mean energy (keV) h*K(10;H) (Sv/Gy) Characteristic, F-Mo 17.6 0.44 Characteristic, F-Cd 22.7 0.80 Characteristic, F-Sn 24.9 0.91 Narrow, N-40 30 1.18 Narrow, N-60 46 1.59 Narrow, N-80 63 1.73 Narrow, N-100 83 1.71 Narrow, N-120 101 1.64 Narrow, N-150 120 1.58 Narrow, N-200 167 1.46 Narrow, N-250 213 1.39 Beam quality Mean energy (keV) h*K(10;H) (Sv/Gy) Characteristic, F-Mo 17.6 0.44 Characteristic, F-Cd 22.7 0.80 Characteristic, F-Sn 24.9 0.91 Narrow, N-40 30 1.18 Narrow, N-60 46 1.59 Narrow, N-80 63 1.73 Narrow, N-100 83 1.71 Narrow, N-120 101 1.64 Narrow, N-150 120 1.58 Narrow, N-200 167 1.46 Narrow, N-250 213 1.39 Table 1. ISO 4037-1 beam qualities used for estimation of calibration coefficients for 900 cc ionization chamber. Beam quality Mean energy (keV) h*K(10;H) (Sv/Gy) Characteristic, F-Mo 17.6 0.44 Characteristic, F-Cd 22.7 0.80 Characteristic, F-Sn 24.9 0.91 Narrow, N-40 30 1.18 Narrow, N-60 46 1.59 Narrow, N-80 63 1.73 Narrow, N-100 83 1.71 Narrow, N-120 101 1.64 Narrow, N-150 120 1.58 Narrow, N-200 167 1.46 Narrow, N-250 213 1.39 Beam quality Mean energy (keV) h*K(10;H) (Sv/Gy) Characteristic, F-Mo 17.6 0.44 Characteristic, F-Cd 22.7 0.80 Characteristic, F-Sn 24.9 0.91 Narrow, N-40 30 1.18 Narrow, N-60 46 1.59 Narrow, N-80 63 1.73 Narrow, N-100 83 1.71 Narrow, N-120 101 1.64 Narrow, N-150 120 1.58 Narrow, N-200 167 1.46 Narrow, N-250 213 1.39 Calibration coefficient of the fabricated ionization chamber is measured against FAIC, using substitution technique(9), by placing the ionization chamber in front of X-ray machine at a distance of 200 cm from focal spot. The ionization chamber was coupled with a reference class electrometer (PTW UNIDOS)(15) (the coupled systems shows a leakage current of ~3 fA) for its radiation response measurement. A potential difference of 400 V was applied to the electrodes of ionization chamber through built-in high voltage unit of the electrometer. The output current of the ion chamber was recorded for the generated ISO narrow series and fluorescence reference X-ray beam qualities. Thereafter the PMMA build-up cap of 1 mm was mounted on the ionization chamber and was positioned at the same place, i.e. at 100 cm height and 200 cm from focal spot. The output of the chamber with 1 mm wall build-up cap was also recorded for the above mentioned X-ray beam qualities. Same measurement was repeated with 2, 3 and 10 mm PMMA wall build-up caps. The air kerma rate based calibration coefficients, NK, of the ionization chamber (with uncertainty at coverage factor 1, as errorbar) for all the wall thicknesses and energies (beam qualities), are evaluated and plotted against mean energy of the beams, as shown in Figure 3. The air kerma rate calibration coefficients (NK) of the ionization chamber are converted to H*(10) rate based calibration coefficients (NH) using air kerma to ambient dose equivalent conversion factors (h*K(10;H)) (Table 1) and the plot of the same (with uncertainty at coverage factor 1, as errorbar) is shown in Figure 4. Figure 3. View largeDownload slide Kair rate based calibration coefficient of 900 cc ionization chamber at various energies and wall thickness. Figure 3. View largeDownload slide Kair rate based calibration coefficient of 900 cc ionization chamber at various energies and wall thickness. Figure 4. View largeDownload slide H*(10) rate based calibration coefficient of 900 cc ionization chamber at various energies and wall thickness. Figure 4. View largeDownload slide H*(10) rate based calibration coefficient of 900 cc ionization chamber at various energies and wall thickness. Evaluation technique The signal from the FAIC is measured in terms of current, I (pA). The average current (Iavg) of the FAIC measured at 200 cm distance is used to calculate conventional true value of Kair rate (K̇T)(16) as follows: K̇T(Gy/h)=Iavg.(W¯/e).(1−g).mair∏iki (1) where W¯/e = average energy required to produce one ion pair in air (33.97 eV); ∏iki = product of correction factors#; # The correction factors includes correction for temperature, pressure, saturation and photon attenuation (ka)(16). (1 − g) = mean correction for energy given to radiative photons; and mair = air mass of the FAIC sensitive volume at NTP (kg). From the conventional true value of air kerma rate (K̇T), conventional true value of ambient dose equivalent rate (ḢT) is evaluated using air kerma to ambient dose equivalent conversion factor h*K(10;H) as follows: ḢT(Sv/h)=K̇T(Gy/h).h⁎K(10;H)(Sv/Gy) (2) The response or signal of the 900 cc ionization chamber is also measured in terms of current R (pA). The average signal corrected for background (Ravg) received from the ionization chamber, at the same distance of 200 cm, is corrected for charge collection efficiency (f), temperature (kT) and pressure (kP) corrections to evaluate the corrected instrument response (Rc). f=4/3−(RV/RV/2)/3 (3) where RV is the ‘average instrument response’ at voltage V and RV/2 is ‘average instrument response’ at voltage V/2. kT=[273.15+t(°C)]/293.15 (4) where ‘t (°C)’ is the ambient temperature at the time of measurement in °C. kP=1013.25/P(mbar) (5) where ‘P’ is the ambient pressure at the time of measurement given in ‘mbar’. Rc=Ravg.ks.kT.kP (6) where ‘ks’ (= 1/f) is the saturation correction evaluated from collection efficiency. The conventional true values and corrected signal (Rc) of the 900 cc ionization chamber is used to evaluate the calibration coefficient of ionization chamber with respect to air kerma (NK) rates and ambient dose equivalent (NH) rate respectively, as follows: NK(μGy/h/pA)=K̇T(μGy/h)Rc(pA) (7) NH(μSv/h/pA)=ḢT(μSv/h)Rc(pA) (8) The air kerma rate (K̇), from the 900 cc ionization chamber, is evaluated for the corrected signal (Rc) as follows. K̇=NK.Rc=NK.Ravg.ks.kT.kP (9) To estimate the air kerma rate linearity response of the chamber, the percentage relative intrinsic error(3) ‘I’ (RIE) of the estimated air kerma rate K̇ from the conventional true value K̇T was evaluated as follows: I(%)=(K̇−K̇T).100K̇T (10) RESULTS AND DISCUSSIONS Charge collection efficiency and linearity Since high air kerma rates (~1 Gy/h) cannot be generated for narrow series and fluorescence reference X-ray beams(9, 12), the direct X-ray beam of 60 kV (2 mm Al filtration) was standardized by FAIC and is used for measuring charge collection efficiency (equation 3) of the chamber (two voltage method(13)). The calibration coefficient of the developed IC was evaluated (at NTP) for the standardized direct X-ray beam of 60 kV (2 mm Al filtration) at Kair rate of 84 mGy/h to measure the collection efficiency of the chamber with increasing air kerma rates. The measured charge collection efficiency is found to be better than 95.0% for Kair rates up to 1.5 Gy/h. The plot of collection efficiency with increasing Kair rate is shown in Figure 5, which clearly shows decreasing collection efficiency with increasing Kair rate. The air kerma rate linearity of the chamber (in terms of RIE) was also evaluated for air kerma rate up to ~ 1.5 Gy/h as shown in Table 2, for an applied electrode potential of 400 V. Figure 5. View largeDownload slide Variation of collection efficiency with air kerma rate for 900 cc ionization chamber. Figure 5. View largeDownload slide Variation of collection efficiency with air kerma rate for 900 cc ionization chamber. Table 2. Collection efficiency and air kerma rate linearity response of 900 cc ionization chamber. Air kerma rate ( K̇T) (mGy/h) Collection efficiency (f) (%) Measured air kerma rate ( K̇) (mGy/h) Relative intrinsic error (I) (%) 4.22 100 4.23 0.24 16.8 100 16.8 0.00 50.4 99.8 50.4 0.00 83.9 99.6 84.0 0.12 167.9 99.2 167.8 −0.06 293.8 98.7 293.8 −0.00 419.7 98.3 418.6 −0.26 587.6 97.8 585.0 −0.44 839.5 97.1 833.3 −0.74 1259 96.0 1247 −0.95 1511 95.0 1494 −1.13 Air kerma rate ( K̇T) (mGy/h) Collection efficiency (f) (%) Measured air kerma rate ( K̇) (mGy/h) Relative intrinsic error (I) (%) 4.22 100 4.23 0.24 16.8 100 16.8 0.00 50.4 99.8 50.4 0.00 83.9 99.6 84.0 0.12 167.9 99.2 167.8 −0.06 293.8 98.7 293.8 −0.00 419.7 98.3 418.6 −0.26 587.6 97.8 585.0 −0.44 839.5 97.1 833.3 −0.74 1259 96.0 1247 −0.95 1511 95.0 1494 −1.13 Table 2. Collection efficiency and air kerma rate linearity response of 900 cc ionization chamber. Air kerma rate ( K̇T) (mGy/h) Collection efficiency (f) (%) Measured air kerma rate ( K̇) (mGy/h) Relative intrinsic error (I) (%) 4.22 100 4.23 0.24 16.8 100 16.8 0.00 50.4 99.8 50.4 0.00 83.9 99.6 84.0 0.12 167.9 99.2 167.8 −0.06 293.8 98.7 293.8 −0.00 419.7 98.3 418.6 −0.26 587.6 97.8 585.0 −0.44 839.5 97.1 833.3 −0.74 1259 96.0 1247 −0.95 1511 95.0 1494 −1.13 Air kerma rate ( K̇T) (mGy/h) Collection efficiency (f) (%) Measured air kerma rate ( K̇) (mGy/h) Relative intrinsic error (I) (%) 4.22 100 4.23 0.24 16.8 100 16.8 0.00 50.4 99.8 50.4 0.00 83.9 99.6 84.0 0.12 167.9 99.2 167.8 −0.06 293.8 98.7 293.8 −0.00 419.7 98.3 418.6 −0.26 587.6 97.8 585.0 −0.44 839.5 97.1 833.3 −0.74 1259 96.0 1247 −0.95 1511 95.0 1494 −1.13 Energy response with wall thickness The air kerma rate calibration coefficient, NK, (equation 7) of the ionization chamber is evaluated for the used ISO 4037-1 reference X-ray beams (Table 1) ranging from 17.6 to 213 keV against FAIC. The variation in calibration coefficient ‘NK’ of the ionization chamber with varying wall thickness and energy was recorded and plotted for narrow and fluorescence (ISO 4037-1) X-ray beam qualities as shown in Figure 3. The h*K(10;H) (Sv/Gy) [IAEA Safety Series-16(9)] conversion factors were used to convert air kerma rate to ambient dose equivalent rate. This ambient dose equivalent rate is used to calculate the calibration coefficient ‘NH’ for the ionization chamber. Figure 4 shows the variation of NH with wall thickness and energy for the used ISO beam qualities. From Figure 4, it can be seen that, this ionization chamber (with 10 mm PMMA build-up cap) has a better energy response for H*(10) rate based monitoring of radiation fields. The H*(10) rate based average calibration coefficient (ionization chamber with 10 mm PMMA build-up cap) is found to be 148.9 μSv/h/pA (maximum deviation of 18.8% at 17.6 keV) for mean energy range of 17.6–213 keV while for energy range 22.7–213 keV the average calibration coefficient is found to be 151.7 μSv/h/pA (maximum deviation of 4.3% at 63 keV) as shown in Table 3. Table 3. Calibration coefficients for 900 cc ionization chamber for 10 mm PMMA build-up cap. Beam quality Mean energy (keV) Calibration coefficient, (NH) (μSv/h/pA) Deviation from average, NH (all energies considered) (%) Deviation from average, NH (all, except F-Mo, energies considered) (%) F-Mo 17.6 120.8 −18.9 — F-Cd 22.7 151.6 1.8 −0.1 F-Sn 24.9 155.1 4.2 2.2 N-40 30 147.8 −0.7 −2.6 N-60 46 151.0 1.4 −0.5 N-80 63 145.1 −2.6 −4.3 N-100 83 148.2 −0.5 −2.3 N-120 101 150.7 1.2 −0.7 N-150 120 157.5 5.8 3.8 N-200 167 152.1 2.2 0.3 N-250 213 157.6 5.8 3.9 Average NH = 148.9 μSv/h/pA; Standard deviation = 6.8% Average NH (all, except F-Mo energy) = 151.7 μSv/h/pA; Standard deviation = 2.7% Beam quality Mean energy (keV) Calibration coefficient, (NH) (μSv/h/pA) Deviation from average, NH (all energies considered) (%) Deviation from average, NH (all, except F-Mo, energies considered) (%) F-Mo 17.6 120.8 −18.9 — F-Cd 22.7 151.6 1.8 −0.1 F-Sn 24.9 155.1 4.2 2.2 N-40 30 147.8 −0.7 −2.6 N-60 46 151.0 1.4 −0.5 N-80 63 145.1 −2.6 −4.3 N-100 83 148.2 −0.5 −2.3 N-120 101 150.7 1.2 −0.7 N-150 120 157.5 5.8 3.8 N-200 167 152.1 2.2 0.3 N-250 213 157.6 5.8 3.9 Average NH = 148.9 μSv/h/pA; Standard deviation = 6.8% Average NH (all, except F-Mo energy) = 151.7 μSv/h/pA; Standard deviation = 2.7% Table 3. Calibration coefficients for 900 cc ionization chamber for 10 mm PMMA build-up cap. Beam quality Mean energy (keV) Calibration coefficient, (NH) (μSv/h/pA) Deviation from average, NH (all energies considered) (%) Deviation from average, NH (all, except F-Mo, energies considered) (%) F-Mo 17.6 120.8 −18.9 — F-Cd 22.7 151.6 1.8 −0.1 F-Sn 24.9 155.1 4.2 2.2 N-40 30 147.8 −0.7 −2.6 N-60 46 151.0 1.4 −0.5 N-80 63 145.1 −2.6 −4.3 N-100 83 148.2 −0.5 −2.3 N-120 101 150.7 1.2 −0.7 N-150 120 157.5 5.8 3.8 N-200 167 152.1 2.2 0.3 N-250 213 157.6 5.8 3.9 Average NH = 148.9 μSv/h/pA; Standard deviation = 6.8% Average NH (all, except F-Mo energy) = 151.7 μSv/h/pA; Standard deviation = 2.7% Beam quality Mean energy (keV) Calibration coefficient, (NH) (μSv/h/pA) Deviation from average, NH (all energies considered) (%) Deviation from average, NH (all, except F-Mo, energies considered) (%) F-Mo 17.6 120.8 −18.9 — F-Cd 22.7 151.6 1.8 −0.1 F-Sn 24.9 155.1 4.2 2.2 N-40 30 147.8 −0.7 −2.6 N-60 46 151.0 1.4 −0.5 N-80 63 145.1 −2.6 −4.3 N-100 83 148.2 −0.5 −2.3 N-120 101 150.7 1.2 −0.7 N-150 120 157.5 5.8 3.8 N-200 167 152.1 2.2 0.3 N-250 213 157.6 5.8 3.9 Average NH = 148.9 μSv/h/pA; Standard deviation = 6.8% Average NH (all, except F-Mo energy) = 151.7 μSv/h/pA; Standard deviation = 2.7% CONCLUSION The developed ionization chamber can be used as a detector for measuring H*(10) rate of up to ~1 Sv/h in protection level radiation monitoring instruments like survey meters and area gamma monitors. In such instruments, generally, automatic correction for temperature, pressure and charge collection efficiency is not applied. To avoid the correction for temperature and pressure, the ionization chamber can be sealed and used. Thus only saturation correction will lead to deviation in ambient dose equivalent rate measurement. The sealed ionization chamber may show an overall relative intrinsic error of ~8–10% (i.e. ~4–5% due to saturation correction and 4–5% due to energy response) in the energy range 20–200 keV. The unsealed ionization chamber can also be used as secondary or working standard for standardization of known low and medium energy photon beams by applying required corrections. ACKNOWLEDGEMENTS Authors are thankful to Dr Pradeepkumar K.S., Associate Director, Health Safety & Environment Group, Bhabha Atomic Research Center for his constant encouragement and support during the course of this work and Radiation Safety Systems Division workshop for timely fabrication of the ionization chamber and build-up caps. REFERENCES 1 ICRU Report 51 ( 1993 ). Quantities and units in radiation protection dosimetry. 2 IEC 61584 First edition ( 2001 –06). Radiation protection instrumentation—installed, portable or transportable assemblies—measurement of air kerma direction and air kerma rate. 3 IEC 60846-1 Edition 1.0 ( 2009 –04). Radiation protection instrumentation—ambient and/or directional dose equivalent (rate) meters and/or monitors for beta, X and gamma radiation—Part 1: Portable workplace and environmental meters and monitors. 4 Knoll , G. F. Radiation Detection and Measurement , 3rd edn . ( New York : John Wiley & Sons ) ( 2000 ). 5 Mahant , A. K. , Singh , S. K. and Vinatha , S. P. Development of ion chambers for the measurement of low energy synchrotron radiation . Nucl. Instrum. Methods Phys. Res. A 601 , 354 – 357 ( 2009 ). Google Scholar CrossRef Search ADS 6 Podgorsak , E. B. Radiation Oncology Physics: A Handbook for Teachers and Students ( Vienna : IAEA ) ( 2005 ). 7 ISO Standard 4037-3 ( 1999 ). X and gamma reference radiation for calibrating dosemeters and doserate meters and for determining their response as a function of photon energy—calibration of area and personal dosemeters and the measurement of their response as a function of energy and angle of incidence, first edition. 8 ICRP Publication 74 ( 1996 ). Conversion coefficients for use in radiological protection against external radiation. 9 IAEA, Safety Report Series No. 16 . Calibration of Radiation Protection Monitoring Instruments ( Vienna : IAEA ) ( 2000 ) 2000. 10 BNC Connector: https://uk.rs-online.com/web/p/bnc-connectors/5121174/ (23 March 2018, date last accessed). 11 X-ray Machine Model YXLON MG325 (Y.MG X-Ray Systems). Available on https://www.yxlon.com/Yxlon/media/Content/Products/X-ray%20modules/Y.MG/MG_Brochure_eng_2015_01(A4-OP).pdf (23 March 2018, date last accessed). 12 ISO Standard 4037-1 ( 1996 ). X and gamma reference radiation for calibrating dosemeters and doserate meters and for determining their response as a function of photon energy—radiation characteristics and production methods, first edition, Case Postale 56, CH-1211, Geneva 20, Switzerland. 13 Attix , F. H. Introduction to Radiological Physics and Radiation Dosimetry ( Strauss GmbH, Morlenbach : Wiley-VCH ) ( 1986 ). Google Scholar CrossRef Search ADS 14 IEC 60731 Edition 3.1 ( 2016 ). Medical electrical equipment—dosimeters with ionization chambers as used in radiotherapy. 15 Manual/Specification of PTW UNIDOS E Electrometer. Available on http://www.ptw.de/unidos_e_dosemeter_ad0.html (23 March 2018, date last accessed). 16 Burns , D. T. and Buermann , L. Free-air ionization chambers . Metrologia 46 , S9 – S23 ( 2009 ). Google Scholar CrossRef Search ADS © The Author(s) 2018. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices)

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Radiation Protection DosimetryOxford University Press

Published: Apr 5, 2018

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