CHARACTERIZATION AND PERFORMANCE TESTS OF A NEW OSL/TL PERSONAL DOSEMETER FOR INDIVIDUAL MONITORING

CHARACTERIZATION AND PERFORMANCE TESTS OF A NEW OSL/TL PERSONAL DOSEMETER FOR INDIVIDUAL MONITORING Abstract We propose a personal dosemeter based on the combination of thermoluminescence (TL) and optically stimulated luminescence (OSL) detectors employing the advantages of both techniques. The new OSL/TL dosemeter using a badge manufactured in a 3D printer was tested for assessment of photon doses in simulated and actual situations of exposure. Additionally, Brazilian national performance tests adapted to the new dosemeter were run as well as the performance tests of international standards on the passive dosimetry systems. The results showed the advantages of combined OSL and TL techniques when using the three different configurations of detector combination, Al2O3/BeO, BeO/CaSO4 and Al2O3/LiF. The dosemeter allowed corrections for radiation energy without the necessity of attenuation filters, the evaluation of single and accumulated doses and the triple check of the dose values. Further, the performance tests were consistent with national and international requirements, showing the viability of application of the new dosemeter to the assessment of equivalent doses. INTRODUCTION Thermoluminescence (TL) and optically stimulated luminescence (OSL) techniques are extensively used in individual monitoring for assessment of occupational doses. TL technique is used in several personal dosimetry systems with a variety of materials largely studied. Besides, there are several commercial readers available. OSL technique, although more recent, has several advantageous characteristics, as the possibility of reassessing the dose information and faster readouts due to the optical nature of the process. The Brazilian accreditation body responsible for certification process in personal dosimetry (CASEC/IRD/CNEN) established performance tests and criteria for dosimetric systems based on TL and photographic techniques (IRD—RT 002.01/95)(1). Those are similar to performance tests and criteria in international standards for passive dosimetric systems, such as the International Electrotechnical Commission IEC 62387-1(2), but it is restricted to these two techniques. In this study, we propose a new personal dosemeter based on three different configurations of OSL/TL detectors (Al2O3/BeO, BeO/CaSO4 and Al2O3/LiF), employing the advantages of TL and OSL techniques combined and the intrinsic properties of each material(3, 4). The new OSL/TL dosemeter was submitted to national and international performance tests for TL dosemeters (IRD—RT 002.01/95) and passive dosemeters (IEC 62387-1) to verify if the requirements and criteria of both standards were fulfilled. In addition, the three configurations of the new dosemeter were tested in actual routine and the results were compared to individual monitoring of an accredited individual monitoring service (IMS) in Brazil. This study also had the aim of contributing to the development of the OSL dosimetry in Brazil and in other countries, showing that published national performance tests and criteria for TL dosemeters can be adapted and applied to the OSL dosemeters. EXPERIMENTAL Detectors and readouts The detectors used in the developed individual monitor were BeO (Thermalox 995), Al2O3:C (Landauer Luxel Tape), CaSO4:Dy (produced at IPEN/Brazil) and LiF:Mg,Ti (TLD100—Bicron) (Table 1). The combination of materials with high and low atomic numbers, such as OSL or TL detector, allowed the development of the new OSL/TL dosemeter without the use of attenuation filters(5). Table 1. Dimensions and effective atomic number (Zeff) of the detectors applied with TL and/or OSL techniques. Detectors Technique Zeff Dimensions LiF:Mg,Ti (TLD100—Bicron) TL 8.2 3.1 mm2 side and 0.9 mm thickness CaSO4:Dy (IPEN) 15.3 6 mm diameter and 0.5 mm thickness BeO:Si,Mg,Al (Thermalox 995—Materion) TL/OSL 7.2 4.5 mm2 side and 0.5 mm thickness Al2O3:C (Luxel—Landauer) OSL 11.3 7 mm diameter and 0.1 mm thickness Detectors Technique Zeff Dimensions LiF:Mg,Ti (TLD100—Bicron) TL 8.2 3.1 mm2 side and 0.9 mm thickness CaSO4:Dy (IPEN) 15.3 6 mm diameter and 0.5 mm thickness BeO:Si,Mg,Al (Thermalox 995—Materion) TL/OSL 7.2 4.5 mm2 side and 0.5 mm thickness Al2O3:C (Luxel—Landauer) OSL 11.3 7 mm diameter and 0.1 mm thickness Table 1. Dimensions and effective atomic number (Zeff) of the detectors applied with TL and/or OSL techniques. Detectors Technique Zeff Dimensions LiF:Mg,Ti (TLD100—Bicron) TL 8.2 3.1 mm2 side and 0.9 mm thickness CaSO4:Dy (IPEN) 15.3 6 mm diameter and 0.5 mm thickness BeO:Si,Mg,Al (Thermalox 995—Materion) TL/OSL 7.2 4.5 mm2 side and 0.5 mm thickness Al2O3:C (Luxel—Landauer) OSL 11.3 7 mm diameter and 0.1 mm thickness Detectors Technique Zeff Dimensions LiF:Mg,Ti (TLD100—Bicron) TL 8.2 3.1 mm2 side and 0.9 mm thickness CaSO4:Dy (IPEN) 15.3 6 mm diameter and 0.5 mm thickness BeO:Si,Mg,Al (Thermalox 995—Materion) TL/OSL 7.2 4.5 mm2 side and 0.5 mm thickness Al2O3:C (Luxel—Landauer) OSL 11.3 7 mm diameter and 0.1 mm thickness Before any measurement, the detectors were previously selected using a sensitivity selection process, which consists of sorting each detector batch in sets with similar sensitivity (differences lower than 5%). When needed, the detectors were submitted to optical (bleaching) or thermal (annealing) treatments, to erase stored residual signals. The bleaching process in Al2O3 was performed with white light (4 Digilight ATEC lamps, each one with power of 55 W) for at least 5 h. According to manufacturer’s recommendation, CaSO4 detectors underwent annealing at 300°C for 3 h. LiF detectors were submitted to two annealing steps, the first at 400°C for 1 h and the second at 100°C for 2 h. BeO detectors underwent annealing at 400°C for 30 min, as previously reported(6). The OSL and TL readouts were carried out in a Risø TL/OSL reader with readout parameters of the equipment optimized to each technique (TL and OSL) and material. For this purpose, appropriate light transmission filters were placed in front of the photomultiplier tube fed with 1225 V. OSL readouts were run using 90% of blue LED power and Hoya U-340 transmission filter. TL readouts of BeO, CaSO4 and LiF detectors were run using heating rate of 5°C/s up to temperatures of 250, 450 and 400°C, respectively, with the combination of CN7-59 and BG39 transmission filters. To explore the possibility of running repeated OSL measurements in the same detector with low degradation of signal (<1%)(6, 7), and thus making use of the advantage of this technique, OSL readouts of BeO and Al2O3 detectors were done with blue LED continuous stimulation (continuous wave—OSL mode) for 0.2 and 0.1 s (initial OSL signal), respectively. For BeO detectors, the TL readout was performed before the OSL readout in the same detector. For this material, it was previously reported that TL readouts up to 250°C do not affect the OSL signal(8) and that, using this intrinsic advantage of BeO, the assessment of single and accumulated doses over time is possible, in the same BeO detector(6). Badge project Figure 1 shows the badge project with four positions to accommodate two pairs of OSL/TL detectors. The badge was symmetrically designed with respect to the plane through the center of the detector and perpendicular to the reference direction, according to the IEC standard(2). It considers also the compatibility with available OSL and TL commercial readers, including portable readers, such as MicroStar (Landauer Inc.). Figure 1. View largeDownload slide Badge project: three configurations of the internal holder for the pairs of OSL/TL detectors Al2O3/LiF, Al2O3/BeO and BeO/CaSO4, respectively, and the cover of the badge (right). Figure 1. View largeDownload slide Badge project: three configurations of the internal holder for the pairs of OSL/TL detectors Al2O3/LiF, Al2O3/BeO and BeO/CaSO4, respectively, and the cover of the badge (right). The badge was made of a low-density material (RGB 520—Veroblack, with density at ~1 g/cm3 after sintering) in a 3D printer (Stratasys Objet Connex 350). Corrections with respect to photon energy and evaluation of operational quantities The criterion used to select the detector pairs was the combination of materials with different energy responses evaluated with TL and/or OSL. The energy or energy range can be estimated by the ratio between the different energy responses (RTL/OSL, ROSL/TL, ROSL/OSL and RTL/TL), as a function of energy, without using the commonly employed attenuation filters(5). The corrections of detector response according to the photon energy were carried out using an algorithm that employs the ratio between relative energy responses to 60Co photons from OSL/TL detectors as a function of the energy. The input data of this algorithm are the net OSL and TL intensities (background signal from non-irradiated detectors subtracted) from each pair of detectors ( STLmaterial and SOSLmaterial). The values of STLmaterial and SOSLmaterial are converted in relative doses using calibration factors evaluated to 60Co photon energy ( FCTLmaterial and FCOSLmaterial), in terms of air kerma, for each technique (TL and OSL) and material (Al2O3, BeO, CaSO4 and LiF). The next step of the algorithm consists of calculating the ratio between relative doses to estimate the effective energy or range of photon energy to which the dosemeter was exposed. For example, in the case of Al2O3/BeO configuration, we obtained, for the same irradiation, three values of relative dose, evaluated, respectively, through OSL intensities from BeO and Al2O3, and through TL intensity from BeO. The first ratio of relative doses, evaluated from TL and OSL intensities from BeO, respectively, will give the value of RTL/OSL. The second ratio of relative doses, evaluated using OSL intensities from BeO and Al2O3, will give the value of ROSL/OSL. Finally, the third ratio between the relative doses, evaluated with OSL intensity from Al2O3 and TL intensity from BeO respectively, will give the ROSL/TL. Analogously, CaSO4/BeO configuration has three values of ratio: ROSL/TL using OSL and TL intensities from BeO; RTL/OSL evaluated through TL and OSL intensities from CaSO4 and BeO, respectively; and RTL/TL obtained with TL intensities from CaSO4 and BeO. The configuration Al2O3/LiF has one value of ratio, ROSL/TL, calculated from OSL and TL intensities from Al2O3 and LiF, respectively. The energy can be estimated by interpolation of ratio values in the already obtained curves (or spreadsheets) of each specific ratio in function of energy. After determining the energy and relative energy response of the different materials, the last step of the algorithm is to evaluate the correction factors with respect to energy ( f(E)signalmaterial). These correction factors can then be applied to the relative dose obtained with each detector and technique, resulting in the absorbed dose to air in the position of the dosemeter, which is given by the product between f(E)signalmaterial and relative dose ( FCsignalmaterial∗Ssignalmaterial) for each detector and each technique. The operational quantities of personal dose equivalent (Hp(10)) and individual dose (Hx), adopted respectively by IEC and Brazilian standards for individual monitoring, were evaluated through the averages for absorbed doses to air estimated by each detector, at the dosemeter position. For example, using the configuration Al2O3/BeO with two pairs of detectors from each material (index of summation i from 1 to 2), those operational quantities are derived by the following equations: Hp(10)=hpk(E)∗16∗[∑i=12(FCOSLAl2O3∗f(E)OSLAl2O3∗SOSLiAl2O3+FCOSLBeO∗f(E)OSLBeO∗SOSLiBeO+FCTLBeO∗f(E)TLBeO∗STLiBeO)] (1) Hx=1.14∗16∗[∑i=12(FCOSLAl2O3∗f(E)OSLAl2O3∗SOSLiAl2O3+FCOSLBeO∗f(E)OSLBeO∗SOSLiBeO+FCTLBeO∗f(E)TLBeO∗STLiBeO)] (2) where, hpk(E) is the conversion factor of air-kerma to personal dose equivalent(9), FCsignalmaterial and f(E)signalmaterial are the calibration and energy correction factors, respectively, for each type of detector and technique, considering the material (BeO or Al2O3) and signal (TL or OSL intensities), and the factor 1.14 Sv/Gy is the conversion coefficient air-kerma to individual dose adopted by CASEC for the individual monitoring in photon fields(10). Performance tests and criteria The three configurations of the new OSL/TL dosemeter were submitted to performance tests and criteria according to the Brazilian standard IRD—RT 002.01/95 (Performance of Individual Monitoring Systems—Criteria and Conditions) and international standard IEC 62387-1. It is noteworthy that, although Brazilian tests and criteria are part of the requirements that must be fulfilled for IMS accreditation, which employs TL and film techniques in personal dosimetry, those tests were adapted to the developed dosemeter and the criteria were totally applied to performance test results. The standard conditions to carry out the tests were in accordance with both Brazilian (IRD) and international (IEC) standards. In addition, adequate equipment was used, such as ISO water slab phantom, calibrated ionization chamber, and beams with radiation qualities (ISO narrow beams) validated and implemented in the Medical Physics and Radiation Dosimetry Laboratory of Institute of Physics of University of São Paulo(11, 12). Blind test and intercomparison in the routine of an IMS The developed OSL/TL dosemeter, in Al2O3/BeO configuration, had its performance evaluated in a controlled blind test, which simulated the conditions of irradiation for evaluation of single and accumulated doses. The set of Al2O3/BeO dosemeters was submitted to three successive irradiations with photons of low and high energies (N60 and N300, respectively) during one month and the quantity Hp(10) was evaluated considering a single exposure and the accumulated dose due to the sequence of irradiations. Using OSL intensities of BeO and Al2O3, it was possible to evaluate Hp(10) accumulated over one month, as the detectors did not undergo any bleaching or thermal treatment between the successive irradiations. Through OSL intensities, Hp(10) related to a single irradiation was assessed subtracting the previously accumulated OSL intensity from the current evaluated value. The same BeO detector was also employed as TL detector, which allowed evaluating Hp(10), due to a single irradiation, directly using TL intensity from BeO through TL readouts up to 250°C before OSL readouts. To verify the performance of the three OSL/TL dosemeter configurations, but now in an actual situation of exposure, the three configurations were employed in a routine use together with a certified badge using TL detectors of an accredited IMS. Two sets of OSL/TL dosemeters were sent to the IMS, one set was used as personal dosemeter by two radiation workers in a department of diagnostic imaging for 1 month, and the second set was used as control dosemeter (background radiation). The control dosemeter was kept, all the time, in a place reserved to store the dosemeters when those were not in use by the workers. The Hx quantity adopted by CASEC to individual monitoring was evaluated, and the results obtained with the three configurations were compared to the Hx values estimated by the IMS. RESULTS AND DISCUSSION The Brazilian and IEC standards specify performance requirements for the complete dosimetry systems, including readers, detectors, dosemeters and additional equipment used in the tests. Alternatively, we show here the most important results of the performance tests regarding the dosimetric properties of the developed OSL/TL dosemeters using the badges designed and manufactured in a 3D printer (Figure 2). Figure 2. View largeDownload slide OSL/TL badges for personal dosimetry developed in three different configurations based on combination of TL and OSL detectors from the left to the right side: Al2O3:C/LiF:Mg,Ti (A-11), Al2O3:C/BeO (B-40) and CaSO4:Dy/BeO (C-33). Figure 2. View largeDownload slide OSL/TL badges for personal dosimetry developed in three different configurations based on combination of TL and OSL detectors from the left to the right side: Al2O3:C/LiF:Mg,Ti (A-11), Al2O3:C/BeO (B-40) and CaSO4:Dy/BeO (C-33). Performance test results according to the Brazilian and IEC standards The variation coefficient and linearity tests should be performed together as described in IEC standard. In those tests, a set of five dosemeters were irradiated giving five values of an indicated value; in our case, as allowed in the IEC standard, the value of absorbed dose at air. According to IEC requirements, for each dose value, the variation coefficient (ratio between standard deviation and average of indicated dose value) multiplied by 56.5% must fulfill the criteria shown in Figure 3a. As the three configurations had variation coefficient smaller than the criteria for all dose range, this requirement was fulfilled. Figure 3. View largeDownload slide (a) Variation coefficient and (b) linearity for dose range from 0.2 up to 100 mGy. Figure 3. View largeDownload slide (a) Variation coefficient and (b) linearity for dose range from 0.2 up to 100 mGy. The linearity criteria for both IEC and Brazilian (CASEC) standards require that relative response (ratio between average of the indicated values and the reference value) agrees within 10%, with differences between −9 and 11%, in the case of IEC. As shown in Figure 3b, the criteria were achieved for the dose range evaluated, except for one dose value in one configuration. The coverage factor, given by student’s t-value(2), decrease with the number of dosemeters used in the test. Because of this, an associated uncertainty, sharply high, can be observed in the case of low dose range (<1 mGy). Different from international standards, the Brazilian performance tests of angular and energy dependences should be performed without the slab phantom. Therefore, these tests were held in two separated runs. To take into account the scattering conditions, the national standard had a particular test, the phantom influence test, which verifies the influence of slab phantom (Table 2). Table 2. Summary of performance test results according to Brazilian (CASEC) and International (IEC) standards and criteria applied to the three OSL/TL dosemeter configurations. Al2O3/LiF Al2O3/BeO BeO/CaSO4 Document/test CASEC IEC CASEC IEC CASEC IEC Low limit detection <0.2 mGy (CASEC) and Hlow<0.1 mSv (IEC) Self- irradiation Changes were not observed in the dose zero value and in the sensitivity of the dosemeters Residual signal, after effects and reusability Light exposure Mechanical performance (drop) Phantom influence <7% n/aa <11% n/aa <2% n/aa Radiation incidence from the side of the dosemeter <2% <5% <8% Al2O3/LiF Al2O3/BeO BeO/CaSO4 Document/test CASEC IEC CASEC IEC CASEC IEC Low limit detection <0.2 mGy (CASEC) and Hlow<0.1 mSv (IEC) Self- irradiation Changes were not observed in the dose zero value and in the sensitivity of the dosemeters Residual signal, after effects and reusability Light exposure Mechanical performance (drop) Phantom influence <7% n/aa <11% n/aa <2% n/aa Radiation incidence from the side of the dosemeter <2% <5% <8% aN/a, not applicable. Table 2. Summary of performance test results according to Brazilian (CASEC) and International (IEC) standards and criteria applied to the three OSL/TL dosemeter configurations. Al2O3/LiF Al2O3/BeO BeO/CaSO4 Document/test CASEC IEC CASEC IEC CASEC IEC Low limit detection <0.2 mGy (CASEC) and Hlow<0.1 mSv (IEC) Self- irradiation Changes were not observed in the dose zero value and in the sensitivity of the dosemeters Residual signal, after effects and reusability Light exposure Mechanical performance (drop) Phantom influence <7% n/aa <11% n/aa <2% n/aa Radiation incidence from the side of the dosemeter <2% <5% <8% Al2O3/LiF Al2O3/BeO BeO/CaSO4 Document/test CASEC IEC CASEC IEC CASEC IEC Low limit detection <0.2 mGy (CASEC) and Hlow<0.1 mSv (IEC) Self- irradiation Changes were not observed in the dose zero value and in the sensitivity of the dosemeters Residual signal, after effects and reusability Light exposure Mechanical performance (drop) Phantom influence <7% n/aa <11% n/aa <2% n/aa Radiation incidence from the side of the dosemeter <2% <5% <8% aN/a, not applicable. Figure 4 shows the relative response according to changes in radiation energy and incidence angle, evaluated by the CASEC requirement (Brazilian standard). The differences in relative response should not exceed 30% due to changes in energy and 15% due to changes in angle. Therefore, the three configurations of the developed dosemeter satisfied both CASEC criteria. Figure 4. View largeDownload slide (a) Energy and (b) angular response performance tests results according to CASEC criteria applied to the three dosemeter configurations. Figure 4. View largeDownload slide (a) Energy and (b) angular response performance tests results according to CASEC criteria applied to the three dosemeter configurations. The performance of the dosemeters with respect to changes in energy, according IEC criteria, was tested evaluating both Hp(10) and Hx quantities (Figure 5). Figure 5. View largeDownload slide Energy response performance test results according IEC standard considering the (a) Hx and (b) Hp(10) quantities. Figure 5. View largeDownload slide Energy response performance test results according IEC standard considering the (a) Hx and (b) Hp(10) quantities. The relative responses obtained from Hx showed smaller differences, at low energies (<100 keV), when compared with the results obtained with Hp(10). Those differences are expected because, according to CNEN regulatory standards, Hx quantity uses only one value of conversion coefficient from air kerma to personal dose equivalent, for the whole energy range. In IEC 62387 and ISO 4037(9), it is recommended to use conversion coefficients according to energy and angle of incidence in the estimative of Hp(10). In the case of Al2O3/LiF configuration, these different concepts in Hx and Hp(10) explain the increase in the relative response for the lowest energy range, considering Hp(10). It is more difficult to determine the energy and conversion factors, f(E) and hpk(E), respectively, with this configuration (just one ratio value—ROSL/TL) than using the configurations with BeO detectors (three ratio values used to determine the energy). Despite this, the relative responses evaluated through the two quantities, for the three OSL/TL detector configurations, fulfill the IEC criteria for this test. The relative responses with respect to energy for two different angles, according to the IEC recommendation for angular dependence test, were evaluated for four energies (from a few keV up to 1250 MeV). The performance test results are in agreement with the IEC criteria (Figure 6). In addition, the results in Figure 4b showed the low angular dependence, as we have a symmetrical badge, and reinforced the advantage of TL and OSL intensities combined, instead of the use of attenuation filters. Figure 6. View largeDownload slide Angular response according to IEC for the three configurations of detectors evaluated for energies between 15 keV and 1.25 MeV. Figure 6. View largeDownload slide Angular response according to IEC for the three configurations of detectors evaluated for energies between 15 keV and 1.25 MeV. Table 2 compares the results of other performance tests according to both IEC and Brazilian (CASEC) standards. Additionally, all criteria of both standards, applied to the new dosemeter, were fulfilled for the three configurations. Single and accumulated doses in a blind test The results obtained in the blind test with the configuration Al2O3/BeO were submitted to trumpet curve criteria (Figure 7), which are criteria widely employed in quality assessment of IMS(13). Accumulated and single doses evaluated in terms of Hp(10) for two radiation qualities agreed with the nominal values, being the differences in single dose values smaller than 10% and 20% for high and low energies, respectively. Figure 7. View largeDownload slide Trumpet curve criteria using the reference level (H0 = 0.2 mSv) established by CNEN for individual monitoring(10) applied to single and accumulated doses resulting from exposure to narrow beams (N60 and N300) over 1 month. Figure 7. View largeDownload slide Trumpet curve criteria using the reference level (H0 = 0.2 mSv) established by CNEN for individual monitoring(10) applied to single and accumulated doses resulting from exposure to narrow beams (N60 and N300) over 1 month. Intercomparison of individual doses in the routine of an IMS Table 3 shows the results of intercomparison between the three configurations of the new OSL/TL dosemeters and the TL dosemeters of an accredited IMS in Brazil. Considering the results of the IMS dosemeters as a standard (true value), the best results were obtained with the Al2O3/BeO configuration. Regarding the uncertainties associated to personal dosimetry, all three configurations had results similar to Hx values estimated by IMS dosemeters. Table 3. Operational quantity evaluated through the new OSL/TL and estimated by TL dosemeters from an accredited IMS. Dosemeter designation Hx (mSv) Developed OSL/TL dosemeters IMS dosemeters Al2O3/LiF Al2O3/BeO CaSO4/BeO BG (control) 0.29 ± 0.03 0.31 ± 0.02 0.28 ± 0.06 0.27 User #1 0.24 ± 0.06 0.31 ± 0.08 0.25 ± 0.07 0.32 User #2 0.13 ± 0.05 0.23 ± 0.09 0.18 ± 0.07 0.24 Dosemeter designation Hx (mSv) Developed OSL/TL dosemeters IMS dosemeters Al2O3/LiF Al2O3/BeO CaSO4/BeO BG (control) 0.29 ± 0.03 0.31 ± 0.02 0.28 ± 0.06 0.27 User #1 0.24 ± 0.06 0.31 ± 0.08 0.25 ± 0.07 0.32 User #2 0.13 ± 0.05 0.23 ± 0.09 0.18 ± 0.07 0.24 Table 3. Operational quantity evaluated through the new OSL/TL and estimated by TL dosemeters from an accredited IMS. Dosemeter designation Hx (mSv) Developed OSL/TL dosemeters IMS dosemeters Al2O3/LiF Al2O3/BeO CaSO4/BeO BG (control) 0.29 ± 0.03 0.31 ± 0.02 0.28 ± 0.06 0.27 User #1 0.24 ± 0.06 0.31 ± 0.08 0.25 ± 0.07 0.32 User #2 0.13 ± 0.05 0.23 ± 0.09 0.18 ± 0.07 0.24 Dosemeter designation Hx (mSv) Developed OSL/TL dosemeters IMS dosemeters Al2O3/LiF Al2O3/BeO CaSO4/BeO BG (control) 0.29 ± 0.03 0.31 ± 0.02 0.28 ± 0.06 0.27 User #1 0.24 ± 0.06 0.31 ± 0.08 0.25 ± 0.07 0.32 User #2 0.13 ± 0.05 0.23 ± 0.09 0.18 ± 0.07 0.24 Moreover, the energy range evaluated using the developed algorithm was <100 keV, which was in agreement with the information provided by the IMS that the dosemeters were exposed to radiation from medical x-ray equipment at a department of diagnostic imaging. CONCLUSION The three configurations of the developed OSL/TL dosemeter satisfied all requirements and criteria of national and international standards to which they were submitted, showing the possibility of hybrid dosemeter use in personal dosimetry for photon. Also, we were able to use the Brazilian national performance test, already published for film and TL dosemeters, with the criteria totally applied for the OSL detector responses, suggesting that those tests can be adapted and used to OSL dosemeters as part of requirements in the regulation process of IMS. The combination of both TL and OSL techniques, associated with intrinsic dosimetric characteristics of the detector materials used, improves the evaluation process of the operational quantities, regarding the corrections with respect to energy and angle of incidence. First, because attenuation filters were not employed and, second, due to the symmetric project of the badge. The configurations using BeO detectors, which were used simultaneously as TL and OSL detector, had the best evaluation of the correction factors, with respect to energy and triple check of dose values. In addition, the blind test, in a simulated situation of exposure, demonstrated the feasibility of assessment of operational quantities in two modes (single doses and doses accumulated over time), as the Al2O3 and BeO detectors, combined with OSL technique, incorporate the advantage of reassessment of doses. Also, the intercomparison with a badge using TL detectors, already in use by an accredited IMS, pointed to the potential applicability of the OSL/TL dosemeters as individual monitors and reinforced the Al2O3/BeO combination as the best configuration among the developed OSL/TL dosemeters. FUNDING The authors thank to FAPESP (State of Sao Paulo Research Foundation—Process 2010/16437-0), CAPES (Coordination for the Improvement of Higher Education Personnel) for financial support and fellowships, National Council of Scientific and Technological Development of The Ministry of Science, Technology, Innovation and Communication of Brazil (CNPq-MTIC/Brazil), CTI/MTIC (Center of Information Technology Renato Archer of MTIC—project PROEXP), IPEN/CNEN (Nuclear and Energy Research Institute of National Commission of Nuclear Energy) for CaSO4 detectors used in this work and to the individual monitoring service that allowed the intercomparison. REFERENCES 1 IRD . IRD—RT n. 002.01/95—Desempenho de Sistemas de Monitoração Individual: Critérios e Condições. Instituto de Radioproteção e Dosimetria (IRD/CNEN) ( 1995 ). 2 IEC . BS EN IEC 62387-1:2012—Radiation protection instrumentation. Passive integrating dosimetry systems for environmental and personal monitoring. 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C. , Freitas , M. B. , Yoshimura , E. M. and Button , V. L. S. N. Application of optically stimulated luminescence technique to evaluate simultaneously accumulated and single doses with the same dosimeter . Radiat. Phys. Chem. 95 , 134 – 136 ( 2014 ). 8 Yukihara , E. Luminescence properties of BeO optically stimulated luminescence (OSL) detectors . Radiat. Meas. 46 , 580 – 587 ( 2011 ). 9 ISO . ISO 4037-3—X and Gamma Reference Radiation for Calibrating Dosemeters and Doserate Meters and for Determining Their Response as a Function of Photon Energy—Part 3: Calibration of Area and Personal Dosemeters ( Geneva : International Organization for Standardization ) ( 1999 ). 10 CNEN . POSIÇÃO REGULATÓRIA 3.01/002:2011: Fatores de Ponderação para as Grandezas de Proteção Radiológica. Comissão Nacional de Energia Nuclear ( 2011 ). 11 Guimarães , C. C. Implementação de Grandezas Operacionais na Monitoração Individual e de Área. Universidade de São Paulo. São Paulo. Dissertação de Mestrado ( 2000 ). 12 Guimarães , C. C. Monitoração individual externa: experimentos e simulações com o método de Monte Carlo. Universidade de São Paulo. São Paulo. Tese de Doutorado ( 2005 ). 13 IAEA . Assessment of occupational exposures due to external sources of radiation—safety standards series No. RS-G-1.3. International Atomic Energy Agency ( 1999 ). © 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

CHARACTERIZATION AND PERFORMANCE TESTS OF A NEW OSL/TL PERSONAL DOSEMETER FOR INDIVIDUAL MONITORING

Radiation Protection Dosimetry , Volume Advance Article – Apr 19, 2018

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0144-8420
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1742-3406
D.O.I.
10.1093/rpd/ncy058
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Abstract

Abstract We propose a personal dosemeter based on the combination of thermoluminescence (TL) and optically stimulated luminescence (OSL) detectors employing the advantages of both techniques. The new OSL/TL dosemeter using a badge manufactured in a 3D printer was tested for assessment of photon doses in simulated and actual situations of exposure. Additionally, Brazilian national performance tests adapted to the new dosemeter were run as well as the performance tests of international standards on the passive dosimetry systems. The results showed the advantages of combined OSL and TL techniques when using the three different configurations of detector combination, Al2O3/BeO, BeO/CaSO4 and Al2O3/LiF. The dosemeter allowed corrections for radiation energy without the necessity of attenuation filters, the evaluation of single and accumulated doses and the triple check of the dose values. Further, the performance tests were consistent with national and international requirements, showing the viability of application of the new dosemeter to the assessment of equivalent doses. INTRODUCTION Thermoluminescence (TL) and optically stimulated luminescence (OSL) techniques are extensively used in individual monitoring for assessment of occupational doses. TL technique is used in several personal dosimetry systems with a variety of materials largely studied. Besides, there are several commercial readers available. OSL technique, although more recent, has several advantageous characteristics, as the possibility of reassessing the dose information and faster readouts due to the optical nature of the process. The Brazilian accreditation body responsible for certification process in personal dosimetry (CASEC/IRD/CNEN) established performance tests and criteria for dosimetric systems based on TL and photographic techniques (IRD—RT 002.01/95)(1). Those are similar to performance tests and criteria in international standards for passive dosimetric systems, such as the International Electrotechnical Commission IEC 62387-1(2), but it is restricted to these two techniques. In this study, we propose a new personal dosemeter based on three different configurations of OSL/TL detectors (Al2O3/BeO, BeO/CaSO4 and Al2O3/LiF), employing the advantages of TL and OSL techniques combined and the intrinsic properties of each material(3, 4). The new OSL/TL dosemeter was submitted to national and international performance tests for TL dosemeters (IRD—RT 002.01/95) and passive dosemeters (IEC 62387-1) to verify if the requirements and criteria of both standards were fulfilled. In addition, the three configurations of the new dosemeter were tested in actual routine and the results were compared to individual monitoring of an accredited individual monitoring service (IMS) in Brazil. This study also had the aim of contributing to the development of the OSL dosimetry in Brazil and in other countries, showing that published national performance tests and criteria for TL dosemeters can be adapted and applied to the OSL dosemeters. EXPERIMENTAL Detectors and readouts The detectors used in the developed individual monitor were BeO (Thermalox 995), Al2O3:C (Landauer Luxel Tape), CaSO4:Dy (produced at IPEN/Brazil) and LiF:Mg,Ti (TLD100—Bicron) (Table 1). The combination of materials with high and low atomic numbers, such as OSL or TL detector, allowed the development of the new OSL/TL dosemeter without the use of attenuation filters(5). Table 1. Dimensions and effective atomic number (Zeff) of the detectors applied with TL and/or OSL techniques. Detectors Technique Zeff Dimensions LiF:Mg,Ti (TLD100—Bicron) TL 8.2 3.1 mm2 side and 0.9 mm thickness CaSO4:Dy (IPEN) 15.3 6 mm diameter and 0.5 mm thickness BeO:Si,Mg,Al (Thermalox 995—Materion) TL/OSL 7.2 4.5 mm2 side and 0.5 mm thickness Al2O3:C (Luxel—Landauer) OSL 11.3 7 mm diameter and 0.1 mm thickness Detectors Technique Zeff Dimensions LiF:Mg,Ti (TLD100—Bicron) TL 8.2 3.1 mm2 side and 0.9 mm thickness CaSO4:Dy (IPEN) 15.3 6 mm diameter and 0.5 mm thickness BeO:Si,Mg,Al (Thermalox 995—Materion) TL/OSL 7.2 4.5 mm2 side and 0.5 mm thickness Al2O3:C (Luxel—Landauer) OSL 11.3 7 mm diameter and 0.1 mm thickness Table 1. Dimensions and effective atomic number (Zeff) of the detectors applied with TL and/or OSL techniques. Detectors Technique Zeff Dimensions LiF:Mg,Ti (TLD100—Bicron) TL 8.2 3.1 mm2 side and 0.9 mm thickness CaSO4:Dy (IPEN) 15.3 6 mm diameter and 0.5 mm thickness BeO:Si,Mg,Al (Thermalox 995—Materion) TL/OSL 7.2 4.5 mm2 side and 0.5 mm thickness Al2O3:C (Luxel—Landauer) OSL 11.3 7 mm diameter and 0.1 mm thickness Detectors Technique Zeff Dimensions LiF:Mg,Ti (TLD100—Bicron) TL 8.2 3.1 mm2 side and 0.9 mm thickness CaSO4:Dy (IPEN) 15.3 6 mm diameter and 0.5 mm thickness BeO:Si,Mg,Al (Thermalox 995—Materion) TL/OSL 7.2 4.5 mm2 side and 0.5 mm thickness Al2O3:C (Luxel—Landauer) OSL 11.3 7 mm diameter and 0.1 mm thickness Before any measurement, the detectors were previously selected using a sensitivity selection process, which consists of sorting each detector batch in sets with similar sensitivity (differences lower than 5%). When needed, the detectors were submitted to optical (bleaching) or thermal (annealing) treatments, to erase stored residual signals. The bleaching process in Al2O3 was performed with white light (4 Digilight ATEC lamps, each one with power of 55 W) for at least 5 h. According to manufacturer’s recommendation, CaSO4 detectors underwent annealing at 300°C for 3 h. LiF detectors were submitted to two annealing steps, the first at 400°C for 1 h and the second at 100°C for 2 h. BeO detectors underwent annealing at 400°C for 30 min, as previously reported(6). The OSL and TL readouts were carried out in a Risø TL/OSL reader with readout parameters of the equipment optimized to each technique (TL and OSL) and material. For this purpose, appropriate light transmission filters were placed in front of the photomultiplier tube fed with 1225 V. OSL readouts were run using 90% of blue LED power and Hoya U-340 transmission filter. TL readouts of BeO, CaSO4 and LiF detectors were run using heating rate of 5°C/s up to temperatures of 250, 450 and 400°C, respectively, with the combination of CN7-59 and BG39 transmission filters. To explore the possibility of running repeated OSL measurements in the same detector with low degradation of signal (<1%)(6, 7), and thus making use of the advantage of this technique, OSL readouts of BeO and Al2O3 detectors were done with blue LED continuous stimulation (continuous wave—OSL mode) for 0.2 and 0.1 s (initial OSL signal), respectively. For BeO detectors, the TL readout was performed before the OSL readout in the same detector. For this material, it was previously reported that TL readouts up to 250°C do not affect the OSL signal(8) and that, using this intrinsic advantage of BeO, the assessment of single and accumulated doses over time is possible, in the same BeO detector(6). Badge project Figure 1 shows the badge project with four positions to accommodate two pairs of OSL/TL detectors. The badge was symmetrically designed with respect to the plane through the center of the detector and perpendicular to the reference direction, according to the IEC standard(2). It considers also the compatibility with available OSL and TL commercial readers, including portable readers, such as MicroStar (Landauer Inc.). Figure 1. View largeDownload slide Badge project: three configurations of the internal holder for the pairs of OSL/TL detectors Al2O3/LiF, Al2O3/BeO and BeO/CaSO4, respectively, and the cover of the badge (right). Figure 1. View largeDownload slide Badge project: three configurations of the internal holder for the pairs of OSL/TL detectors Al2O3/LiF, Al2O3/BeO and BeO/CaSO4, respectively, and the cover of the badge (right). The badge was made of a low-density material (RGB 520—Veroblack, with density at ~1 g/cm3 after sintering) in a 3D printer (Stratasys Objet Connex 350). Corrections with respect to photon energy and evaluation of operational quantities The criterion used to select the detector pairs was the combination of materials with different energy responses evaluated with TL and/or OSL. The energy or energy range can be estimated by the ratio between the different energy responses (RTL/OSL, ROSL/TL, ROSL/OSL and RTL/TL), as a function of energy, without using the commonly employed attenuation filters(5). The corrections of detector response according to the photon energy were carried out using an algorithm that employs the ratio between relative energy responses to 60Co photons from OSL/TL detectors as a function of the energy. The input data of this algorithm are the net OSL and TL intensities (background signal from non-irradiated detectors subtracted) from each pair of detectors ( STLmaterial and SOSLmaterial). The values of STLmaterial and SOSLmaterial are converted in relative doses using calibration factors evaluated to 60Co photon energy ( FCTLmaterial and FCOSLmaterial), in terms of air kerma, for each technique (TL and OSL) and material (Al2O3, BeO, CaSO4 and LiF). The next step of the algorithm consists of calculating the ratio between relative doses to estimate the effective energy or range of photon energy to which the dosemeter was exposed. For example, in the case of Al2O3/BeO configuration, we obtained, for the same irradiation, three values of relative dose, evaluated, respectively, through OSL intensities from BeO and Al2O3, and through TL intensity from BeO. The first ratio of relative doses, evaluated from TL and OSL intensities from BeO, respectively, will give the value of RTL/OSL. The second ratio of relative doses, evaluated using OSL intensities from BeO and Al2O3, will give the value of ROSL/OSL. Finally, the third ratio between the relative doses, evaluated with OSL intensity from Al2O3 and TL intensity from BeO respectively, will give the ROSL/TL. Analogously, CaSO4/BeO configuration has three values of ratio: ROSL/TL using OSL and TL intensities from BeO; RTL/OSL evaluated through TL and OSL intensities from CaSO4 and BeO, respectively; and RTL/TL obtained with TL intensities from CaSO4 and BeO. The configuration Al2O3/LiF has one value of ratio, ROSL/TL, calculated from OSL and TL intensities from Al2O3 and LiF, respectively. The energy can be estimated by interpolation of ratio values in the already obtained curves (or spreadsheets) of each specific ratio in function of energy. After determining the energy and relative energy response of the different materials, the last step of the algorithm is to evaluate the correction factors with respect to energy ( f(E)signalmaterial). These correction factors can then be applied to the relative dose obtained with each detector and technique, resulting in the absorbed dose to air in the position of the dosemeter, which is given by the product between f(E)signalmaterial and relative dose ( FCsignalmaterial∗Ssignalmaterial) for each detector and each technique. The operational quantities of personal dose equivalent (Hp(10)) and individual dose (Hx), adopted respectively by IEC and Brazilian standards for individual monitoring, were evaluated through the averages for absorbed doses to air estimated by each detector, at the dosemeter position. For example, using the configuration Al2O3/BeO with two pairs of detectors from each material (index of summation i from 1 to 2), those operational quantities are derived by the following equations: Hp(10)=hpk(E)∗16∗[∑i=12(FCOSLAl2O3∗f(E)OSLAl2O3∗SOSLiAl2O3+FCOSLBeO∗f(E)OSLBeO∗SOSLiBeO+FCTLBeO∗f(E)TLBeO∗STLiBeO)] (1) Hx=1.14∗16∗[∑i=12(FCOSLAl2O3∗f(E)OSLAl2O3∗SOSLiAl2O3+FCOSLBeO∗f(E)OSLBeO∗SOSLiBeO+FCTLBeO∗f(E)TLBeO∗STLiBeO)] (2) where, hpk(E) is the conversion factor of air-kerma to personal dose equivalent(9), FCsignalmaterial and f(E)signalmaterial are the calibration and energy correction factors, respectively, for each type of detector and technique, considering the material (BeO or Al2O3) and signal (TL or OSL intensities), and the factor 1.14 Sv/Gy is the conversion coefficient air-kerma to individual dose adopted by CASEC for the individual monitoring in photon fields(10). Performance tests and criteria The three configurations of the new OSL/TL dosemeter were submitted to performance tests and criteria according to the Brazilian standard IRD—RT 002.01/95 (Performance of Individual Monitoring Systems—Criteria and Conditions) and international standard IEC 62387-1. It is noteworthy that, although Brazilian tests and criteria are part of the requirements that must be fulfilled for IMS accreditation, which employs TL and film techniques in personal dosimetry, those tests were adapted to the developed dosemeter and the criteria were totally applied to performance test results. The standard conditions to carry out the tests were in accordance with both Brazilian (IRD) and international (IEC) standards. In addition, adequate equipment was used, such as ISO water slab phantom, calibrated ionization chamber, and beams with radiation qualities (ISO narrow beams) validated and implemented in the Medical Physics and Radiation Dosimetry Laboratory of Institute of Physics of University of São Paulo(11, 12). Blind test and intercomparison in the routine of an IMS The developed OSL/TL dosemeter, in Al2O3/BeO configuration, had its performance evaluated in a controlled blind test, which simulated the conditions of irradiation for evaluation of single and accumulated doses. The set of Al2O3/BeO dosemeters was submitted to three successive irradiations with photons of low and high energies (N60 and N300, respectively) during one month and the quantity Hp(10) was evaluated considering a single exposure and the accumulated dose due to the sequence of irradiations. Using OSL intensities of BeO and Al2O3, it was possible to evaluate Hp(10) accumulated over one month, as the detectors did not undergo any bleaching or thermal treatment between the successive irradiations. Through OSL intensities, Hp(10) related to a single irradiation was assessed subtracting the previously accumulated OSL intensity from the current evaluated value. The same BeO detector was also employed as TL detector, which allowed evaluating Hp(10), due to a single irradiation, directly using TL intensity from BeO through TL readouts up to 250°C before OSL readouts. To verify the performance of the three OSL/TL dosemeter configurations, but now in an actual situation of exposure, the three configurations were employed in a routine use together with a certified badge using TL detectors of an accredited IMS. Two sets of OSL/TL dosemeters were sent to the IMS, one set was used as personal dosemeter by two radiation workers in a department of diagnostic imaging for 1 month, and the second set was used as control dosemeter (background radiation). The control dosemeter was kept, all the time, in a place reserved to store the dosemeters when those were not in use by the workers. The Hx quantity adopted by CASEC to individual monitoring was evaluated, and the results obtained with the three configurations were compared to the Hx values estimated by the IMS. RESULTS AND DISCUSSION The Brazilian and IEC standards specify performance requirements for the complete dosimetry systems, including readers, detectors, dosemeters and additional equipment used in the tests. Alternatively, we show here the most important results of the performance tests regarding the dosimetric properties of the developed OSL/TL dosemeters using the badges designed and manufactured in a 3D printer (Figure 2). Figure 2. View largeDownload slide OSL/TL badges for personal dosimetry developed in three different configurations based on combination of TL and OSL detectors from the left to the right side: Al2O3:C/LiF:Mg,Ti (A-11), Al2O3:C/BeO (B-40) and CaSO4:Dy/BeO (C-33). Figure 2. View largeDownload slide OSL/TL badges for personal dosimetry developed in three different configurations based on combination of TL and OSL detectors from the left to the right side: Al2O3:C/LiF:Mg,Ti (A-11), Al2O3:C/BeO (B-40) and CaSO4:Dy/BeO (C-33). Performance test results according to the Brazilian and IEC standards The variation coefficient and linearity tests should be performed together as described in IEC standard. In those tests, a set of five dosemeters were irradiated giving five values of an indicated value; in our case, as allowed in the IEC standard, the value of absorbed dose at air. According to IEC requirements, for each dose value, the variation coefficient (ratio between standard deviation and average of indicated dose value) multiplied by 56.5% must fulfill the criteria shown in Figure 3a. As the three configurations had variation coefficient smaller than the criteria for all dose range, this requirement was fulfilled. Figure 3. View largeDownload slide (a) Variation coefficient and (b) linearity for dose range from 0.2 up to 100 mGy. Figure 3. View largeDownload slide (a) Variation coefficient and (b) linearity for dose range from 0.2 up to 100 mGy. The linearity criteria for both IEC and Brazilian (CASEC) standards require that relative response (ratio between average of the indicated values and the reference value) agrees within 10%, with differences between −9 and 11%, in the case of IEC. As shown in Figure 3b, the criteria were achieved for the dose range evaluated, except for one dose value in one configuration. The coverage factor, given by student’s t-value(2), decrease with the number of dosemeters used in the test. Because of this, an associated uncertainty, sharply high, can be observed in the case of low dose range (<1 mGy). Different from international standards, the Brazilian performance tests of angular and energy dependences should be performed without the slab phantom. Therefore, these tests were held in two separated runs. To take into account the scattering conditions, the national standard had a particular test, the phantom influence test, which verifies the influence of slab phantom (Table 2). Table 2. Summary of performance test results according to Brazilian (CASEC) and International (IEC) standards and criteria applied to the three OSL/TL dosemeter configurations. Al2O3/LiF Al2O3/BeO BeO/CaSO4 Document/test CASEC IEC CASEC IEC CASEC IEC Low limit detection <0.2 mGy (CASEC) and Hlow<0.1 mSv (IEC) Self- irradiation Changes were not observed in the dose zero value and in the sensitivity of the dosemeters Residual signal, after effects and reusability Light exposure Mechanical performance (drop) Phantom influence <7% n/aa <11% n/aa <2% n/aa Radiation incidence from the side of the dosemeter <2% <5% <8% Al2O3/LiF Al2O3/BeO BeO/CaSO4 Document/test CASEC IEC CASEC IEC CASEC IEC Low limit detection <0.2 mGy (CASEC) and Hlow<0.1 mSv (IEC) Self- irradiation Changes were not observed in the dose zero value and in the sensitivity of the dosemeters Residual signal, after effects and reusability Light exposure Mechanical performance (drop) Phantom influence <7% n/aa <11% n/aa <2% n/aa Radiation incidence from the side of the dosemeter <2% <5% <8% aN/a, not applicable. Table 2. Summary of performance test results according to Brazilian (CASEC) and International (IEC) standards and criteria applied to the three OSL/TL dosemeter configurations. Al2O3/LiF Al2O3/BeO BeO/CaSO4 Document/test CASEC IEC CASEC IEC CASEC IEC Low limit detection <0.2 mGy (CASEC) and Hlow<0.1 mSv (IEC) Self- irradiation Changes were not observed in the dose zero value and in the sensitivity of the dosemeters Residual signal, after effects and reusability Light exposure Mechanical performance (drop) Phantom influence <7% n/aa <11% n/aa <2% n/aa Radiation incidence from the side of the dosemeter <2% <5% <8% Al2O3/LiF Al2O3/BeO BeO/CaSO4 Document/test CASEC IEC CASEC IEC CASEC IEC Low limit detection <0.2 mGy (CASEC) and Hlow<0.1 mSv (IEC) Self- irradiation Changes were not observed in the dose zero value and in the sensitivity of the dosemeters Residual signal, after effects and reusability Light exposure Mechanical performance (drop) Phantom influence <7% n/aa <11% n/aa <2% n/aa Radiation incidence from the side of the dosemeter <2% <5% <8% aN/a, not applicable. Figure 4 shows the relative response according to changes in radiation energy and incidence angle, evaluated by the CASEC requirement (Brazilian standard). The differences in relative response should not exceed 30% due to changes in energy and 15% due to changes in angle. Therefore, the three configurations of the developed dosemeter satisfied both CASEC criteria. Figure 4. View largeDownload slide (a) Energy and (b) angular response performance tests results according to CASEC criteria applied to the three dosemeter configurations. Figure 4. View largeDownload slide (a) Energy and (b) angular response performance tests results according to CASEC criteria applied to the three dosemeter configurations. The performance of the dosemeters with respect to changes in energy, according IEC criteria, was tested evaluating both Hp(10) and Hx quantities (Figure 5). Figure 5. View largeDownload slide Energy response performance test results according IEC standard considering the (a) Hx and (b) Hp(10) quantities. Figure 5. View largeDownload slide Energy response performance test results according IEC standard considering the (a) Hx and (b) Hp(10) quantities. The relative responses obtained from Hx showed smaller differences, at low energies (<100 keV), when compared with the results obtained with Hp(10). Those differences are expected because, according to CNEN regulatory standards, Hx quantity uses only one value of conversion coefficient from air kerma to personal dose equivalent, for the whole energy range. In IEC 62387 and ISO 4037(9), it is recommended to use conversion coefficients according to energy and angle of incidence in the estimative of Hp(10). In the case of Al2O3/LiF configuration, these different concepts in Hx and Hp(10) explain the increase in the relative response for the lowest energy range, considering Hp(10). It is more difficult to determine the energy and conversion factors, f(E) and hpk(E), respectively, with this configuration (just one ratio value—ROSL/TL) than using the configurations with BeO detectors (three ratio values used to determine the energy). Despite this, the relative responses evaluated through the two quantities, for the three OSL/TL detector configurations, fulfill the IEC criteria for this test. The relative responses with respect to energy for two different angles, according to the IEC recommendation for angular dependence test, were evaluated for four energies (from a few keV up to 1250 MeV). The performance test results are in agreement with the IEC criteria (Figure 6). In addition, the results in Figure 4b showed the low angular dependence, as we have a symmetrical badge, and reinforced the advantage of TL and OSL intensities combined, instead of the use of attenuation filters. Figure 6. View largeDownload slide Angular response according to IEC for the three configurations of detectors evaluated for energies between 15 keV and 1.25 MeV. Figure 6. View largeDownload slide Angular response according to IEC for the three configurations of detectors evaluated for energies between 15 keV and 1.25 MeV. Table 2 compares the results of other performance tests according to both IEC and Brazilian (CASEC) standards. Additionally, all criteria of both standards, applied to the new dosemeter, were fulfilled for the three configurations. Single and accumulated doses in a blind test The results obtained in the blind test with the configuration Al2O3/BeO were submitted to trumpet curve criteria (Figure 7), which are criteria widely employed in quality assessment of IMS(13). Accumulated and single doses evaluated in terms of Hp(10) for two radiation qualities agreed with the nominal values, being the differences in single dose values smaller than 10% and 20% for high and low energies, respectively. Figure 7. View largeDownload slide Trumpet curve criteria using the reference level (H0 = 0.2 mSv) established by CNEN for individual monitoring(10) applied to single and accumulated doses resulting from exposure to narrow beams (N60 and N300) over 1 month. Figure 7. View largeDownload slide Trumpet curve criteria using the reference level (H0 = 0.2 mSv) established by CNEN for individual monitoring(10) applied to single and accumulated doses resulting from exposure to narrow beams (N60 and N300) over 1 month. Intercomparison of individual doses in the routine of an IMS Table 3 shows the results of intercomparison between the three configurations of the new OSL/TL dosemeters and the TL dosemeters of an accredited IMS in Brazil. Considering the results of the IMS dosemeters as a standard (true value), the best results were obtained with the Al2O3/BeO configuration. Regarding the uncertainties associated to personal dosimetry, all three configurations had results similar to Hx values estimated by IMS dosemeters. Table 3. Operational quantity evaluated through the new OSL/TL and estimated by TL dosemeters from an accredited IMS. Dosemeter designation Hx (mSv) Developed OSL/TL dosemeters IMS dosemeters Al2O3/LiF Al2O3/BeO CaSO4/BeO BG (control) 0.29 ± 0.03 0.31 ± 0.02 0.28 ± 0.06 0.27 User #1 0.24 ± 0.06 0.31 ± 0.08 0.25 ± 0.07 0.32 User #2 0.13 ± 0.05 0.23 ± 0.09 0.18 ± 0.07 0.24 Dosemeter designation Hx (mSv) Developed OSL/TL dosemeters IMS dosemeters Al2O3/LiF Al2O3/BeO CaSO4/BeO BG (control) 0.29 ± 0.03 0.31 ± 0.02 0.28 ± 0.06 0.27 User #1 0.24 ± 0.06 0.31 ± 0.08 0.25 ± 0.07 0.32 User #2 0.13 ± 0.05 0.23 ± 0.09 0.18 ± 0.07 0.24 Table 3. Operational quantity evaluated through the new OSL/TL and estimated by TL dosemeters from an accredited IMS. Dosemeter designation Hx (mSv) Developed OSL/TL dosemeters IMS dosemeters Al2O3/LiF Al2O3/BeO CaSO4/BeO BG (control) 0.29 ± 0.03 0.31 ± 0.02 0.28 ± 0.06 0.27 User #1 0.24 ± 0.06 0.31 ± 0.08 0.25 ± 0.07 0.32 User #2 0.13 ± 0.05 0.23 ± 0.09 0.18 ± 0.07 0.24 Dosemeter designation Hx (mSv) Developed OSL/TL dosemeters IMS dosemeters Al2O3/LiF Al2O3/BeO CaSO4/BeO BG (control) 0.29 ± 0.03 0.31 ± 0.02 0.28 ± 0.06 0.27 User #1 0.24 ± 0.06 0.31 ± 0.08 0.25 ± 0.07 0.32 User #2 0.13 ± 0.05 0.23 ± 0.09 0.18 ± 0.07 0.24 Moreover, the energy range evaluated using the developed algorithm was <100 keV, which was in agreement with the information provided by the IMS that the dosemeters were exposed to radiation from medical x-ray equipment at a department of diagnostic imaging. CONCLUSION The three configurations of the developed OSL/TL dosemeter satisfied all requirements and criteria of national and international standards to which they were submitted, showing the possibility of hybrid dosemeter use in personal dosimetry for photon. Also, we were able to use the Brazilian national performance test, already published for film and TL dosemeters, with the criteria totally applied for the OSL detector responses, suggesting that those tests can be adapted and used to OSL dosemeters as part of requirements in the regulation process of IMS. The combination of both TL and OSL techniques, associated with intrinsic dosimetric characteristics of the detector materials used, improves the evaluation process of the operational quantities, regarding the corrections with respect to energy and angle of incidence. First, because attenuation filters were not employed and, second, due to the symmetric project of the badge. The configurations using BeO detectors, which were used simultaneously as TL and OSL detector, had the best evaluation of the correction factors, with respect to energy and triple check of dose values. In addition, the blind test, in a simulated situation of exposure, demonstrated the feasibility of assessment of operational quantities in two modes (single doses and doses accumulated over time), as the Al2O3 and BeO detectors, combined with OSL technique, incorporate the advantage of reassessment of doses. Also, the intercomparison with a badge using TL detectors, already in use by an accredited IMS, pointed to the potential applicability of the OSL/TL dosemeters as individual monitors and reinforced the Al2O3/BeO combination as the best configuration among the developed OSL/TL dosemeters. FUNDING The authors thank to FAPESP (State of Sao Paulo Research Foundation—Process 2010/16437-0), CAPES (Coordination for the Improvement of Higher Education Personnel) for financial support and fellowships, National Council of Scientific and Technological Development of The Ministry of Science, Technology, Innovation and Communication of Brazil (CNPq-MTIC/Brazil), CTI/MTIC (Center of Information Technology Renato Archer of MTIC—project PROEXP), IPEN/CNEN (Nuclear and Energy Research Institute of National Commission of Nuclear Energy) for CaSO4 detectors used in this work and to the individual monitoring service that allowed the intercomparison. REFERENCES 1 IRD . IRD—RT n. 002.01/95—Desempenho de Sistemas de Monitoração Individual: Critérios e Condições. Instituto de Radioproteção e Dosimetria (IRD/CNEN) ( 1995 ). 2 IEC . BS EN IEC 62387-1:2012—Radiation protection instrumentation. Passive integrating dosimetry systems for environmental and personal monitoring. General characteristics and performance requirements ( 2012 ). 3 McKeever , S. W. S. and Moscovitch , M. Topics under debate—on the advantages and disadvantages of optically stimulated luminescence dosimetry and thermoluminescence dosimetry . Radiat. Prot. Dosim. 104 ( 3 ), 263 – 270 ( 2003 ). 4 Olko , P. Advantages and disadvantages of luminescence dosimetry . Radiat. Meas. 45 , 506 – 511 ( 2010 ). 5 Malthez , A. L. M. C. , Freitas , M. B. , Yoshimura , E. M. and Button , V. L. S. N. Experimental photon energy response of different dosimetric materials for a dual detector system combining thermoluminescence and optically stimulated luminescence . Radiat. Meas. 71 , 133 – 138 ( 2014 ). 6 Malthez , A. L. M. C. , Freitas , M. B. , Yoshimura , E. M. , Umisedo , N. K. and Button , V. L. S. N. OSL and TL techniques combined in a Beryllium Oxide detector to evaluate simultaneously accumulated and single doses . Appl. Radiat. Isotop. 110 , 155 – 159 ( 2016 ). 7 Malthez , A. L. M. C. , Freitas , M. B. , Yoshimura , E. M. and Button , V. L. S. N. Application of optically stimulated luminescence technique to evaluate simultaneously accumulated and single doses with the same dosimeter . Radiat. Phys. Chem. 95 , 134 – 136 ( 2014 ). 8 Yukihara , E. Luminescence properties of BeO optically stimulated luminescence (OSL) detectors . Radiat. Meas. 46 , 580 – 587 ( 2011 ). 9 ISO . ISO 4037-3—X and Gamma Reference Radiation for Calibrating Dosemeters and Doserate Meters and for Determining Their Response as a Function of Photon Energy—Part 3: Calibration of Area and Personal Dosemeters ( Geneva : International Organization for Standardization ) ( 1999 ). 10 CNEN . POSIÇÃO REGULATÓRIA 3.01/002:2011: Fatores de Ponderação para as Grandezas de Proteção Radiológica. Comissão Nacional de Energia Nuclear ( 2011 ). 11 Guimarães , C. C. Implementação de Grandezas Operacionais na Monitoração Individual e de Área. Universidade de São Paulo. São Paulo. Dissertação de Mestrado ( 2000 ). 12 Guimarães , C. C. Monitoração individual externa: experimentos e simulações com o método de Monte Carlo. Universidade de São Paulo. São Paulo. Tese de Doutorado ( 2005 ). 13 IAEA . Assessment of occupational exposures due to external sources of radiation—safety standards series No. RS-G-1.3. International Atomic Energy Agency ( 1999 ). © 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 19, 2018

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