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Comparison of the dosimetric response of 4-elements OSL and TL passive personal dosimeters
Accepted Manuscript Comparison of the dosimetric response of 4-elements OSL and TL passive personal dosimeters L. Oliver, C. Candela-Juan, J.D. Palma, M.C. Pujades, A. Soriano, J. Vilar, J. Martínez, V. Mestre, J.C. Ruiz-Rodríguez, N. Llorca PII:
S1350-4487(17)30140-3
DOI:
10.1016/j.radmeas.2017.09.001
Reference:
RM 5833
To appear in:
Radiation Measurements
Received Date: 4 March 2017 Revised Date:
21 August 2017
Accepted Date: 7 September 2017
Please cite this article as: Oliver, L., Candela-Juan, C., Palma, J.D., Pujades, M.C., Soriano, A., Vilar, J., Martínez, J., Mestre, V., Ruiz-Rodríguez, J.C., Llorca, N., Comparison of the dosimetric response of 4-elements OSL and TL passive personal dosimeters, Radiation Measurements (2017), doi: 10.1016/ j.radmeas.2017.09.001. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Comparison of the dosimetric response of 4-elements OSL and TL passive personal dosimeters
L. Oliver,1,2 C. Candela-Juan,1* J. D. Palma,1 M. C. Pujades,1 A. Soriano,1 J. Vilar,1
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J. Martínez,1 V. Mestre,1 J. C. Ruiz-Rodríguez,1 and N. Llorca1
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Centro Nacional de Dosimetría (CND), Instituto Nacional de Gestión Sanitaria, Valencia, Spain
2
Fundación Instituto Valenciano de Oncología (IVO), Valencia, Spain
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*Corresponding author: Name: C. Candela-Juan Address: Centro Nacional de Dosimetría (CND). Av. Campanar, 21, 46009, Valencia, Spain E-mail: [email protected]
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E-mails of all the other authors: • L. Oliver: [email protected] • C. Candela-Juan: [email protected] • J. D. Palma: [email protected] • M. C. Pujades: [email protected] • A. Soriano: [email protected] • J. Vilar: [email protected] • J. Martínez: [email protected] • V. Mestre: [email protected] • J. C. Ruiz-Rodríguez: [email protected] • N. Llorca: [email protected]
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Highlights • This study shows a dosimetric comparison between a LiF-TL and a BeO-OSL dosimetry system.
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• The irradiations and dosimetric tests were performed following the IEC 62387 (2012-12). • Both systems satisfy the IEC 62387 requirements for the tests performed.
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• Despite this, differences are faced and discussed.
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Abstract Introduction Thermoluminescent dosimeters (TLD) made from LiF:Mg,Ti (manufacturer: Thermo Fisher
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Scientific, USA, brand name: TLD-100) and optically stimulated luminescence detectors (OSLD) made from BeO (manufacturer: Dosimetrics GmbH, Germany, brand name: BeOSL) are used as passive personal detectors. Although differences exist between them, mainly in terms of handling, reading, material response and dose algorithms, both systems have to satisfy some
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specific requirements. The aim of this study is to verify that the 4-element version of both dosimeters match the IEC 62387 requirements for photon fields, and to compare them.
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Materials and Methods
The LiF-TLDs tested were those provided by the Spanish National Dosimetry Centre (CND), which makes use of an in-house dose calculation algorithm. The BeO-OSLDs were those distributed by Dosimetrics. The following features were evaluated for photon radiation: coefficient of variation, non-linearity, photon angular and energy dependence, reusability, dose build-up, fading, self-irradiation and response to natural radiation, as well as accuracy of energy
determined. Results
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estimation. For the BeO-OSLDs the consistency between consecutive re-readings was also
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Both systems satisfy the IEC 62387 requirements for the tests described above. One of the main advantages of the BeO-OSLDs over the LiF-TLDs is their lower energy dependency and higher
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reusability. They are also easier to manipulate, read and prepare. On the other hand, advantages of the LiF-TLD system over the BeO-OSLD system are the more accurate energy estimations provided by the CND dose algorithm, which are used to correct the higher energy dependency, and the glow curves provided by the TL readers. Conclusion
The LiF-TLDs and BeO-OSLDs tested satisfy the dosimetric requirements for their use as passive personal dosimeters in photon fields. For this reason, a study of the simplicity, reusability, amount of dosimeters read each month or automation in the handling and reading
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process can be helpful to decide the best option. The choice between one of them from a dosimetric point of view might also depend on the importance given to the different criteria here studied.
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Key words: OSLD; TLD; BeO; LiF; personal dosimetry; IEC 62387.
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1.
INTRODUCTION
External exposures to ionizing radiation can be measured with passive dosimeters, which should be worn on a representative site on the surface of the body (ICRP, 2007). Thermoluminescent
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dosimeters (TLD) have been widely used in recent decades for personal external dose monitoring. However, optically stimulated luminescence detectors (OSLD) have shown advances that have contributed to it being considered an effective alternative. A survey performed in 2012 by the European Radiation Dosimetry (EURADOS) group showed that 40% of dosimeters issued in Europe for whole body dosimetry are based on TL, whereas OSL only
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contributes with 3% (Gilvin, 2015). However, its use is increasing.
A disadvantage of the reading process in TL is that it involves heating the dosimeter. In some TL
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materials, the luminescence efficiency decreases as the temperature of the material is increased, and the TL sensitivity becomes dependent on the heating rate. Among others, it is observed in Al2O3:C and LiF:Mg,Cu,P as reported by Pradhan (1995). This problem is not found if light is used instead of heat to stimulate the luminescence. Another disadvantage of the TL is that the act of measuring the dose destroys most of the signal. In contrast, with optically stimulation, it is possible to control the amount of signal emitted per reading, allowing for multiple readings.
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Jursinic (2007) reported a 0.05% signal decreasing per reading with Al2O3:C OSLDs. Nevertheless, a recognized advantage of the TL over the OSL commercially available for personal dosimetry, is the capability to generate glow curves, which can be used to identify a
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defective reading of the dosimeter.
Theoretical and numerical comparisons for OSL and TL systems have been reported in the
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literature (Moscovitch, 2003; Romanyukha, 2016). However, to our knowledge, this comparison cannot be found for an OSLD system based on BeO. The Spanish National Dosimetry Centre (Centro Nacional de Dosimetría, CND) offers an inhouse whole-body personal dosimeter based on four LiF:Mg,Ti (TLD-100) detectors (Thermo Fisher Scientific, USA) and four filters (Casal, 2015). This TLD system meets the technical and dosimetric requirements established by the standard 61066 (IEC, 1991) from the International Electrotechnical Commission (IEC). That publication has been replaced by the newer IEC 62387 (IEC, 2012), which establishes the requirements needed for personal and environmental passive integrating dosimetry systems.
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Althought BeO has been used for decades (Bulur, 1998), in the last few years, the beryllium-oxid OSL dosimeters, named BeOSL, have been introduced in the market (Sommer, 2008; Sommer, 2011; Hübner, 2014). Each dosimeter is available with either two or four detectors, each of them having a different filter material. Extended information about the 2-element version of the
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dosimeter can be found elsewhere (Sommer, 2011; Jahn, 2014). However, only little data about the dosimetric response of the 4-element version have been published in the literature. This version allows energy estimations and provides a dose estimation with less energy dependency (Haninger, 2016).
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The first goal of this study was to perform the tests stated in the IEC 62387 to verify that both, the 4-element version of the BeOSL system together with the TLD system provided by the CND
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satisfy the requirements established in this publication. The second goal was to perform a comparison between both systems according to these results.
2.
MATERIAL AND METHODS
2.1. Dosimetry systems tested
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The TL dosimeter tested (named LiF-TLD in this study) is based on a detector card with four LiF:Mg,Ti (TLD-100) detectors, each one sandwiched in between two equal filter plates (Casal, 2015), as shown in Figure 1a. Each detector is 4.5 mm in diameter and 0.6 mm thick, whereas each filter is 11.8 mm in diameter. The following filter materials are used: 4 mm aluminum, 3
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mm copper in between 0.5 mm thick plastic slices, 3.9 mm plastic PTFE (Teflon), and air (open window). The card and the filter plates are wrapped with aluminized plastic, which protects the
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detectors from light and contamination. The energy is estimated from the ratios of the readings of the four detectors and the dose reading of each element is corrected to take into account the energy dependency and the attenuation of the filter. The dose readings are then used to compute the deep dose Hp(10) and the shallow dose Hp(0.07). The specific dose calculation algorithm has already been described by Casal et al. (2015).
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(c)
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Figure 1. Components and filters of the 4-element (a) LiF-TLD and (b) BeO-OSLD. (c) Radiation incident angle for the BeO-OSLD (top) and the LiF-TLD (bottom) in the photon angular dependency test.
The LiF-TLD are read with an automatic Harshaw 8800 reader (Moscovitch, 1990). Each reading cycle consists of a pre-heating of 9 seconds at 160ºC, followed by the reading period of 30 seconds at 300ºC, with a heating rate of 12ºC/s. In order to ensure that the dosimeters are
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completely erased and to recover the sensitivity of the detectors, they are annealed in an oven for 18h at 80ºC, unless doses higher than 18 mSv are accumulated, in which case the annealing is done at a temperature up to 300ºC.
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The OSL dosimeters tested (named BeO-OSLD in this study) are made of a card with four BeO OSL detectors that are also sandwiched in between two filter plates (Figure 1b). The following filter materials are used: copper, a sandwich of lead with low content of 210Pb and tin (the tin is
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between the Pb and the detector, in order to reduce self-irradiation caused by lead impurities), 2.4 mm PTFE (Teflon) and a 0.5 mm thick plastic window. Each element is a square of 4.7 mm size and 0.5 mm thick. The detector card and the filter plates are inside a black holder that protects the dosimeters from light and contamination. For Hp(0.07) the dose measured by the plastic window detector is assigned, whereas Hp(10) is computed as a linear combination of the reading of the four elements. The linear coefficients used were those that minimized the energy dependency of the dose measurement, and were provided by Dosimetrics. Independently of the dose reading, which is not corrected by energy dependency, the energy of the radiation beam is
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estimated using the relative readings of all four elements. Unlike the LiF-TLD system by the CND, in which an integer energy value is estimated by the algorithm, the BeOSL software provides a range of possible energy values in which the energy is classified.
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The OSL reader is based on a photodiode controlled blue LED and a photo sensor module, with the addition of optical filters between the LED and the BeO detector. Additional details have been published by Sommer et al. (2011). Each OSL measurement consists of five offset measurements (without stimulation, to get the actual offset signal of the photo sensor module) and five dose measurements, totaling 1 second. Offset and dose measurements are equally long
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(100 ms). These 10 measurements have to pass a consistency check, otherwise a warning message is shown by the software. The eraser bleaches all four elements during several seconds
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(10 s were used in this study) and then the dosimeter is re-read in order to evaluate the reusability of the dosimeter and if it needs to be re-erased or not.
2.2. Dosimetric assays
The following dosimetric assays for photon fields, reported by the IEC 62387 standard and
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described below, were performed for both systems: coefficient of variation, non-linearity, aftereffects, reusability, energy dependency, angular dependency, dose build-up, fading, selfirradiation, and response to natural radiation. Furthermore, although not included in that standard, the accuracy of the energy estimation was also evaluated. In addition, for the BeO-
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OSLDs, the consistency between several re-readings was scored. For each test, both Hp(10) and Hp(0.07) were evaluated.
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Most of the LiF-TL and BeO-OSL dosimeters were irradiated at the ionizing radiation metrology laboratory by the CND. These were placed on the surface of a standard ISO slab phantom, placed 250 cm away from the radiation focus. The beam qualities used conform to ISO 4037 (ISO, 1999) and IEC 61267 (IEC, 2005). For the energy dependency test, the irradiations with a 137Cs source were performed at CIEMAT (Madrid, Spain) for the LiF-TLDs, and at the Helmholts Zentrum (Muenchen, Germany) for the BeO-OSLDs.
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3.
RESULTS
3.1. Coefficient of variation and non-linearity Five groups were irradiated to different dose values, ranging from Hp(10) = 0.08 mSv to Hp(10)
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= 41.0 mSv. Six dosimeters were used for each group. The broad beam quality W-300 was used for all the irradiations. For each group, the coefficient of variation was calculated as the standard deviation divided by the mean dose, and was compared to the threshold value established by the IEC 62387, which depends on the irradiation dose.
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The same readings were used to evaluate the linearity, which is defined as the measured mean dose value of each group divided by the nominal dose value. The test value is the linearity of
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each group normalized by the linearity of the reference group, which in this case was that irradiated to 10 mSv. Additional details on the equations and parameters used can be found in the IEC 62387. Uncertainties are used with coverage factor k = 2.
For the 4-element version of LiF-TLDs and BeO-OSLDs tested in this study, Figure 2 shows the coefficient of variation as a function of the nominal dose. All the values are below the threshold
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established by the IEC 62387 for this test.
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Figure 2. Coefficient of variation for (a) Hp(0.07) and (b) Hp(10), as a function of the nominal dose for the 4-element version of LiF-TLDs and BeO-OSLDs tested in this study. The solid line shows the upper limit.
Figure 3 shows the results of the non-linearity test, compared with the limits defined by the IEC 62387. In the LiF-TLDs for dose values below 0.1 mSv the uncertainty exceeds the threshold values. However, doses below 0.1 mSv are not reported in the CND, nor in most of the European
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countries according to a recent survey (Gilvin, 2015) . It is also noticed that only six dosimeters were irradiated for this dose group. Generally, if the number of dosimeters is increased, the relative uncertainty due to reproducibility is decreased. Therefore, this result is acceptable for the
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dose range in which the dosimeters are used.
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Figure 3. Linearity test results of (a) Hp(0.07) and (b) Hp(10), as a function of the nominal dose for the 4-element version of LiF-TLDs and BeO-OSLDs tested. Solid lines show the tolerances.
3.2. After-effects and reusability
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Four groups composed of six dosimeters were used for this test. The reference group (group 1) was irradiated to 10.2 mSv. Groups 2 to 4 were irradiated to 0.08 mSv, 10.2 mSv and 41 mSv respectively; afterwards they were erased and irradiated again with the minimum detectable dose of roughly 0.05 mSv. Then, the test values were obtained by introducing the second readings in
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the same expression as in the linearity test.
The results for each group here defined are shown in Figure 4. The test values for the BeO-
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OSLDs meet the limits fixed by the IEC 62387, whereas in case of the LiF-TLDs with previous doses above 10 mSv the condition is not fulfilled. Since in the CND dosimeters with doses above 4 mSv are subject to a reusability test, these dosimeters would have been evaluated before reuse and, depending on the result, either they would have been rejected or a new sensitivity factor would have been assigned to them. Therefore, this is not a case of a wrong estimation in the dose reported by the LiF-TLD system, but rather a case of time efficiency and more reusability of the BeO-OSLDs compared to the LiF-TLDs.
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Figure 4. Reusability test for each group, i, for (a) Hp(0.07) and (b) Hp(10) for the 4-element version of LiF-TLDs and BeO-OSLDs tested in this study. Solid lines show the tolerances.
3.3. Photon energy dependency
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The variation of the photon energy response at normal incidence was tested. Groups of three dosimeters were irradiated with the following beam qualities:
137
Cs, N-300, N-200, N-150, N-
100, N-80, N-60, N-40, N-30 and RQR-M1.
The energy dependency for LiF-TL and BeO-OSL dosimeters is shown in Figure 5. Both systems met the IEC 62387 requirements. The flat energy response observed for the LiF-TLDs is
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due to the fact that the energy is estimated and the reading is corrected by the dose calculation algorithm. Due to the low energy dependency of the BeO-OSLDs, the manufacturer decided not
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to correct the dose reading depending on the energy.
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Figure 5. Photon energy dependency for (a) Hp(0.07) and (b) Hp(10) for the 4-element version of LiF-TLDs and BeO-OSLDs tested in this study. Solid lines show the tolerances, which are not continuous since they depend on the uncertainty of the nominal dose value.
3.4. Photon angular dependency
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The variation of the response with the radiation incident angle was studied for two beam qualities, N-40 (low energy) and N-300 (high energy). The irradiation was performed, for the angles θ and ϕ represented in Figure 1c, at θ= ± 60º in both dosimetry systems, at ϕ= ± 60º for
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the LiF-TLD and only at ϕ= + 60º for the BeO-OSLD due to the symmetry of the system. Groups of three dosimeters were irradiated with doses of Hp(10) = 3 mSv (a few groups required 6 extra dosimeters due to the dispersion of the measurements). The analysis of the dose readings was performed according to IEC 62387. The reference group is the one irradiated with normal incidence. Although the IEC 62387 requires the test to be done for the three lowest energies in
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rated energy range, in this study, for simplicity, a low beam quality was chosen. However, in order to compare the angular dose response of both dosimetry systems in a broader energy range,
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the test was also repeated for a high energy beam quality.
The irradiation conditions of the incident angles and beam qualities for each group are shown in Table 1, whereas results are shown in Figure 6. For both systems the condition established by the IEC 62387 is met.
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Table 1. Angle of incidence and beam quality in the irradiation for each group of dosimeters for the angular dependency test.
Beam quality
1 2 3 4 5 6 7 8 9 10
N-40 N-40 N-40 N-40 N-40 N-300 N-300 N-300 N-300 N-300
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Group i
Mean photon energy (keV) 33 33 33 33 33 250 250 250 250 250
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θ (°)
ϕ (°)
0 60 -60 0 0 0 60 -60 0 0
0 0 0 60 -60 0 0 0 60 -60
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Figure 6. Photon angular dependency for (a) Hp(0.07) and (b) Hp(10) for the 4-element version of LiF-TLDs and BeO-OSLDs tested in this study. Solid lines show the tolerances.
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3.5. Dose build-up, fading, self-irradiation and response to natural radiation Eight groups of dosimeters were used for this test. The number of dosimeters compounding each one is in Table 2, as well as the irradiation dose for Hp(10). Hlow is the lower limit for the measuring range, which in this case is 0.05 mSv.
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Table 2. Number of dosimeters, n, and irradiation doses for each group for the dose tests: build-up, fading, self-irradiation and response to natural radiation.
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Group 1 to 3 4 5 to 7 8
n 6 27 6 25
Nominal dose 7 x Hlow=0.35 mSv approx. Hlow=0.05 mSv approx. Unexposed Unexposed
Groups 1 and 5 were read 24 hours post irradiation, groups 2 and 6 were read 7 days post irradiation, and groups 3, 4, 7 and 8 were read 28 days post irradiation. The corresponding background accumulated in groups 5 to 8 was subtracted from each dosimeter in groups 1 to 4, obtaining groups G1’ to G4’ respectively:
,
,
,
,
,
,
,
, where
G is the mean value of the background readings in group i. According to IEC 62387 (IEC, 2012), expression 1 is the one that groups 1’ to 3’ have to meet, where rmin and rmax depend on the irradiation dose and Ucom is the uncertainty associated to ′ / ′ . Group 4’ is analyzed according to expression 2. Expression 3 is applied to group 8’, where Cnat is the theoretical background - 13 -
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accumulated until the reading date, and Um is the half-width of the confidence interval of the mean dose. For this study a natural radiation rate of 0.1 µSv/h was assumed, which leads to 0.067 mSv accumulated in 28 days.
≤
H'
(
G ±U G
7·G G
!≤r !≤r
±U
≤G ±U
C
"*
(1)
"#
"#
≤ +H'
(
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r
≤
(2)
(3)
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r
Test results for groups 1’ to 4’, described above, are between 0.93 and 1.04, which are within the
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tolerance levels 0.9 and 1.1. For group 8’, results are included in Table 3, for which the test value (± the uncertainty) must be included in the range defined by ± Hlow. It can be observed that both systems, the BeO-OSL and the LiF-TL, match the requirements established by the IEC 62387 for these dose dependencies.
Table 3. Response to natural radiation test results for the 4-element version of LiF-TLDs and BeO-OSLDs tested in this study.
± Hlow
Hp(0.07)
-0.004
0.006
±0.057
Hp(10)
-0.017
-0.007
±0.059
Hp(0.07)
-0.008
0.006
±0.057
-0.007
-0.007
±0.059
BeO-OSLD
LiF-TLD
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Dosimetric magnitude Test value - Um Test value + Um
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System
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Hp(10)
3.6. Accuracy of the energy estimation The LiF-TLD system provides a range of continuous integer values for the energy estimated, while the BeO-OSL system provides an interval in which the energy value is in between. Intervals at which the energies are classified are: “very soft” (for energies below 20 keV), “soft” (for energies between 20 and 55 keV), “medium” (for energies between 55 and 200 keV) and “hard” (for energies above 200 keV). Irradiations at different energy values and angles were performed and the nominal energies provided by the irradiation laboratory were compared with
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the estimations provided by each system. Irradiations were performed in groups of three dosimeters for the following beam qualities (the mean energy is noted in parentheses):
137
Cs (662 keV), N-300 (250 keV), N-200 (164 keV), N-
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150 (118 keV), N-100 (83 keV), N-80 (65 keV), N-60 (48 keV), N-40 (33 keV), N-30 (24 keV) and RQR-M1 (16,11 keV). The irradiations were performed at normal incidence for each beam quality, and at θ= ± 60 º , and ϕ= ± 60 º for the beam qualities N-300 and N-40.
For all dosimeters tested, the energy interval estimated by the BeO-OSLD system was correct for
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both, normal incidence and for different angles. For the LiF-TLD system, the deviation between the nominal and the estimated mean photon energy was below 4.2% in all cases, except for one dosimeter irradiated with the beam quality N-80 where it reached 7.7%. For radiation incidence
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at different angles, the deviation in the energy estimated increases up to 30%. However, this leads to a maximum deviation of only 3% in the dose correction factor.
3.7. Consistency between consecutive re-readings
Although it is not part of IEC 62387 standard, the possibility to read a dosimeter more than once
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was also tested for the 4-element OSLD based on BeO. For these detectors, only a small part (roughly 3% in the BeOSL system) is erased. The software by Dosimetrics takes this into account and corrects the re-reading signals by the loss due to the previous readings. In fact, as stated by the manufacturer, the signal loss per read is an individual figure for each detector,
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which is determined during calibration. The signal loss also depends on the number of re-read. The re-reading capability of LiF-TLDs has been published elsewhere (Moscovitch, 2011;
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McKinlay, 1980) and is not evaluated in this study. Groups of six dosimeters were irradiated with Hp(10) doses ranging from 0.08 mSv to 51.1 mSv. Each dosimeter was read six times (the first five times in 10-minute intervals and the last one four weeks later). For each dose reading the deviation to the first reading was computed. Results are shown in Figure 7.
For doses above 1 mSv, deviations are lower than 3%, increasing up to 20% for lower dose values. It should be emphasized that the re-read capability seems to be more useful in cases of high dose levels, close to or beyond the dose limits. These are clearly higher than 1 mSv, where
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the BeOSL re-reading system has shown to provide high accuracy.
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4.
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Figure 7. Deviation for consecutive dose readings for (a) Hp(0.07) and (b) Hp(10) for the 4-element version of BeO-OSLDs tested in this study. For a clearer representation, the sixth reading of a dosimeter irradiated to 0.08 mSv was omitted in both figures, which showed deviations of 120% and 75% for Hp(0.07) and Hp(10), respectively.
DISCUSSION
The dosimetric systems and dose algorithms that have been studied here satisfy the IEC 62387
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requirements for the tests described above, which makes them suitable to be used as personal dosimeters for photon fields. However, there are some important differences that should be emphasized. It should be also noted that this discussion is true for the systems here studied, but
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may be different for other TL or OSL systems.
The BeO-OSLDs show less energy dependency than the LiF-TLDs. Hence, the required dose algorithm can be simpler and, therefore, provides less options for making operational mistakes.
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In the LiF-TLDs this characteristic is compensated with the CND dose calculation algorithm by estimating the energy (with 1 keV resolution) and applying a dose correction (Casal, 2015). It provides a flatter response with energy variations than in the BeO-OSLD system. However, this correction has the disadvantage that the estimation of the dose depends on the estimation of the energy, and if a wrong energy value is obtained, it leads to a wrong dose correction factor of the LiF-TLD reading. This can be important, for example, in mixed field irradiations (i.e. irradiations of both photons and electrons). Nevertheless, this higher dependency allows for more precision in the estimation of the energy radiation, providing a continuous range of values
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for the estimated energy, instead of intervals in which it is classified. Energy dependency for Hp(10) for the 4-element version of the OSLDs based on BeO was already reported by Haninger et al. (2016). For the Hp(0.07) element of these dosimeters,
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graphics for the energy and angular response are provided by Sommer et al. (2011). The results of the current study for these specific tests are consistent with data published in the literature. For instance, a variation in the relative energy response below 0.3 for energies comprised between 20 and 300 keV is observed in both cases. In addition, in the case of Hp(10) the results in both studies show a local minimum between 40 and 50 keV and a local maximum between 100 and
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200 keV. For the Hp(0.07) estimation, a local maximum between 30 and 40 keV is observed as well as a slight increase in the dosimeter's response with energy above 90 keV.
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In this study, it is verified that, in comparison to the 2-element version of the BeOSL system reported by Haninger et al. (2016), the 4-element version flattens the response with the energy variation. Therefore, it provides more accurate dose estimations. As shown in section 3.6, it also provides an estimation of the energy, which is not possible with the 2-element version. An advantage of the BeO-OSLD system over the LiF-TLD system is that the reading and erasing
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process consume less power. In addition, as shown in section 3.2, they are more reusable, since they meet the requirements to be reused after having been irradiated to higher doses. This is an important factor taking into account the large amount of workers that are monitored by the CND every month (more than 42 000 dosimeters are read monthly).
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Another advantage of the BeOSL dosimeters is that they can be read easily more than once. This provides the possibility of repeating dose readings if a questionable value has been obtained.
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Dosimeters with high dose values can also be archived to re-read them in a future. In this study it has been proven that the tool works well above 1 mSv for the system by Dosimetrics. Notwithstanding, glow curves are obtained in the LiF-TLD system during the reading process. A pattern with evident glow peaks is a sign that the TLD signal is not spurious, but is a result of a real radiation exposure. These curves can also be used to identify a defective element in the dosimeter or a malfunctioning reader. This is not possible with the current state of the BeOSL system. However, this is partly compensated by the fact that each dose value provided by the BeOSL system is the average of 5 measurements, and a consistency check is made. Either one system or another will be more appropriate depending on which of the analyzed - 17 -
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aspects is more important for the intended purpose.
5.
CONCLUSIONS
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This study verifies that, for photon beams, the LiF-TLD system by the National Dosimetry Centre and the BeO-OSLD system by Dosimetrics meet the requirements established in the IEC 62387 to be used as passive personal dosimeters. Among the main differences between them, the BeO-OSLDs show less energy dependency, while the higher dependency of the LiF-TLDs is
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compensated by the dose calculation algorithm of the CND, which provides more accurate energy estimations. The BeOSL dosimeters are more reusable than the LiF-TLDs, they also can
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simpler, faster and consumes less energy.
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be read easily more than once with good accuracy, and their reading and erasing process is
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ACKNOWLEDGMENTS We would like to thank Dosimetrics for kindly providing the BeO-OSLD system used in this
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CONFLICTS OF INTERESTS
This research did not receive any specific grant from funding agencies in the public, commercial,
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or not-for-profit sectors. The authors declare that they have no conflicts of interests.
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REFERENCES Bulur E, Gösku HY (1998). OSL from BeO ceramics: new observations from an old material. Radiation Measurements, 29:639-650.
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Casal E, Soriano A, Alabau J, Palma J. (2015).Results obtained by the personnel dosimetry service of the Spanish health department in the EURADOS intercomparison 2012 for whole body photon dosemeters. EURADOS International Conference on Individual Monitoring of ionising radiation, Bruges -Belgium.
Gilvin PJ, Alves JG, Cherestes C, van Dijk JWE, Lehtinen M, Rossi F, Vekic B, Chevallier MA.
Survey 2012. EURADOS Report 2015-04.
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Quality assurance in individual monitoring for external radiation – Results of EURADOS
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Haninger T, Hödlmoser H, Figel M, Köning-Meier D, Sommer M, Jahn A, Ledtermann G, Eßer R. (2016). Properties of the BeOSL dosimetry system in the framework of a large-scale personal monitoring service. Radiation protection dosimetry,170: 269-273. Hübner S (2014). Introduction of a new dosimetry system based on optically stimulated luminescence (OSL) in our personal monitoring service. ISSSD. Cusco, Peru. International Commission on Radiological Protection (ICRP) 103 (2007). The 2007
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recommendations of the International Commission on Radiological Protection. International Electrotechnical Commission (IEC) 61066 (1991-12). Thermoluminescence dosimetry systems for personal and environmental monitoring. Edition 1.0.
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International Electrotechnical Commission (IEC) 61267 (2005-11). Medical diagnostic X-ray equipment - Radiation conditions for use in the determination of characteristics. Edition 2.0. International Electrotechnical Commission (IEC) 62387 (2012-12). Radiation protection
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instrumentation – Passive integrating dosimetry systems for personal and environmental monitoring of photon and beta radiation. Edition 1.0. International Organization for Standardization, ISO 4037-3. (1999). X and gamma reference radiation for calibrating dosemeters and dose rate meters and for determining their response as a function of photon energy. ISO 4037-3. Jahn A, Sommer M, Henniger J (2014). Environmental dosimetry with the BeOSL personal dosemeter – State of the Art. Radiation Measurements, 71: 438-441.
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Jursinic PA (2007). Characterization of optically stimulated luminescent dosimeters, OSLDs, for clinical dosimetric measurements. Medical Physics, 34:4594-4604. McKinlay AF, Bartlett DT, Smith PA (1980). The development of photo-transferred
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thermoluminescence (PTTL) technique and its application to the routine re-assessment of absorbed dose in the NRPB automated personal dosimetry system. Nuclear Instruments and Methods, 175: 57-59.
Moscovitch M, Szalanczy A, Bruml WW, Velbeck KJ, Tawil RA. (1990). A TLD system based on gas heating with linear time-temperature profile. Radiation ProtectionDosimetry, 34:
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361-364.
Moscovitch M, McKeever SW. (2003). Topics under Debate - On the advantages and
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disadvantages of optically stimulated luminescence dosimetry and thermoluminescence dosimetry. Radiation Protection Dosimetry, 104: 263-270.
Moscovitch M, Benevides L, Romanyukha A, Hull F, Duffy M, Voss S, Velbeck K. J, Nita I, Rotunda J. E. (2011). The Applicability of the PTTL dose re-analysis method to the harshaw Lif:Mg,Cu,P material. Radiation Protection Dosimetry, 144: 161-164. Pradhan AS (1995). Influence of Heating Rate on the TL Response of LiF TLD-700,
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LiF:Mg,Cu,P and Al2O3:C. Radiation Protection Dosimetry, 58 (3): 205-209. Romanyukha A, Grypp MD, Faifchild GR, Williams AS (2016). Performance comparison of OSLD (Al2O3:C) and TLD (LiF:Mg,Cu,P) in accreditation proficiency testing. Radiation
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Measurements, 93: 7-12.
Sommer M, Jahn A, Henniger J. (2011). A new personal dosimetry system for Hp(10) and Hp(0,07) photon dose based on OSL-dosimetry of beryllium oxide. Radiation
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Measurements, 46: 1818-1821.
Sommer M, Jahn A, Henniger J. (2008). Beryllium oxide as optically stimulated luminescence dosimeter. Radiation Measurements, 43: 353-356.
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1 2 3 4 5 6 7 8 9 10
N-40 N-40 N-40 N-40 N-40 N-300 N-300 N-300 N-300 N-300
Mean photon energy (keV) 33 33 33 33 33 250 250 250 250 250
θ (°)
ϕ (°)
0 60 -60 0 0 0 60 -60 0 0
0 0 0 60 -60 0 0 0 60 -60
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Nominal dose 7 x Hlow=0.35 mSv approx. Hlow=0.05 mSv approx. Unexposed Unexposed
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System
Dosimetric magnitude Test value - Um
Test value + Um
± Hlow
Hp(0.07)
-0.004
0.006
±0.057
Hp(10)
-0.017
-0.007
±0.059
Hp(0.07)
-0.008
0.006
±0.057
Hp(10)
-0.007
-0.007
±0.059
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LiF-TLD
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S1350-4487(17)30140-3
DOI:
10.1016/j.radmeas.2017.09.001
Reference:
RM 5833
To appear in:
Radiation Measurements
Received Date: 4 March 2017 Revised Date:
21 August 2017
Accepted Date: 7 September 2017
Please cite this article as: Oliver, L., Candela-Juan, C., Palma, J.D., Pujades, M.C., Soriano, A., Vilar, J., Martínez, J., Mestre, V., Ruiz-Rodríguez, J.C., Llorca, N., Comparison of the dosimetric response of 4-elements OSL and TL passive personal dosimeters, Radiation Measurements (2017), doi: 10.1016/ j.radmeas.2017.09.001. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Comparison of the dosimetric response of 4-elements OSL and TL passive personal dosimeters
L. Oliver,1,2 C. Candela-Juan,1* J. D. Palma,1 M. C. Pujades,1 A. Soriano,1 J. Vilar,1
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J. Martínez,1 V. Mestre,1 J. C. Ruiz-Rodríguez,1 and N. Llorca1
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Centro Nacional de Dosimetría (CND), Instituto Nacional de Gestión Sanitaria, Valencia, Spain
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Fundación Instituto Valenciano de Oncología (IVO), Valencia, Spain
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*Corresponding author: Name: C. Candela-Juan Address: Centro Nacional de Dosimetría (CND). Av. Campanar, 21, 46009, Valencia, Spain E-mail: [email protected]
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E-mails of all the other authors: • L. Oliver: [email protected] • C. Candela-Juan: [email protected] • J. D. Palma: [email protected] • M. C. Pujades: [email protected] • A. Soriano: [email protected] • J. Vilar: [email protected] • J. Martínez: [email protected] • V. Mestre: [email protected] • J. C. Ruiz-Rodríguez: [email protected] • N. Llorca: [email protected]
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Highlights • This study shows a dosimetric comparison between a LiF-TL and a BeO-OSL dosimetry system.
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• The irradiations and dosimetric tests were performed following the IEC 62387 (2012-12). • Both systems satisfy the IEC 62387 requirements for the tests performed.
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• Despite this, differences are faced and discussed.
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Abstract Introduction Thermoluminescent dosimeters (TLD) made from LiF:Mg,Ti (manufacturer: Thermo Fisher
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Scientific, USA, brand name: TLD-100) and optically stimulated luminescence detectors (OSLD) made from BeO (manufacturer: Dosimetrics GmbH, Germany, brand name: BeOSL) are used as passive personal detectors. Although differences exist between them, mainly in terms of handling, reading, material response and dose algorithms, both systems have to satisfy some
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specific requirements. The aim of this study is to verify that the 4-element version of both dosimeters match the IEC 62387 requirements for photon fields, and to compare them.
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Materials and Methods
The LiF-TLDs tested were those provided by the Spanish National Dosimetry Centre (CND), which makes use of an in-house dose calculation algorithm. The BeO-OSLDs were those distributed by Dosimetrics. The following features were evaluated for photon radiation: coefficient of variation, non-linearity, photon angular and energy dependence, reusability, dose build-up, fading, self-irradiation and response to natural radiation, as well as accuracy of energy
determined. Results
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estimation. For the BeO-OSLDs the consistency between consecutive re-readings was also
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Both systems satisfy the IEC 62387 requirements for the tests described above. One of the main advantages of the BeO-OSLDs over the LiF-TLDs is their lower energy dependency and higher
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reusability. They are also easier to manipulate, read and prepare. On the other hand, advantages of the LiF-TLD system over the BeO-OSLD system are the more accurate energy estimations provided by the CND dose algorithm, which are used to correct the higher energy dependency, and the glow curves provided by the TL readers. Conclusion
The LiF-TLDs and BeO-OSLDs tested satisfy the dosimetric requirements for their use as passive personal dosimeters in photon fields. For this reason, a study of the simplicity, reusability, amount of dosimeters read each month or automation in the handling and reading
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process can be helpful to decide the best option. The choice between one of them from a dosimetric point of view might also depend on the importance given to the different criteria here studied.
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Key words: OSLD; TLD; BeO; LiF; personal dosimetry; IEC 62387.
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1.
INTRODUCTION
External exposures to ionizing radiation can be measured with passive dosimeters, which should be worn on a representative site on the surface of the body (ICRP, 2007). Thermoluminescent
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dosimeters (TLD) have been widely used in recent decades for personal external dose monitoring. However, optically stimulated luminescence detectors (OSLD) have shown advances that have contributed to it being considered an effective alternative. A survey performed in 2012 by the European Radiation Dosimetry (EURADOS) group showed that 40% of dosimeters issued in Europe for whole body dosimetry are based on TL, whereas OSL only
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contributes with 3% (Gilvin, 2015). However, its use is increasing.
A disadvantage of the reading process in TL is that it involves heating the dosimeter. In some TL
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materials, the luminescence efficiency decreases as the temperature of the material is increased, and the TL sensitivity becomes dependent on the heating rate. Among others, it is observed in Al2O3:C and LiF:Mg,Cu,P as reported by Pradhan (1995). This problem is not found if light is used instead of heat to stimulate the luminescence. Another disadvantage of the TL is that the act of measuring the dose destroys most of the signal. In contrast, with optically stimulation, it is possible to control the amount of signal emitted per reading, allowing for multiple readings.
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Jursinic (2007) reported a 0.05% signal decreasing per reading with Al2O3:C OSLDs. Nevertheless, a recognized advantage of the TL over the OSL commercially available for personal dosimetry, is the capability to generate glow curves, which can be used to identify a
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defective reading of the dosimeter.
Theoretical and numerical comparisons for OSL and TL systems have been reported in the
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literature (Moscovitch, 2003; Romanyukha, 2016). However, to our knowledge, this comparison cannot be found for an OSLD system based on BeO. The Spanish National Dosimetry Centre (Centro Nacional de Dosimetría, CND) offers an inhouse whole-body personal dosimeter based on four LiF:Mg,Ti (TLD-100) detectors (Thermo Fisher Scientific, USA) and four filters (Casal, 2015). This TLD system meets the technical and dosimetric requirements established by the standard 61066 (IEC, 1991) from the International Electrotechnical Commission (IEC). That publication has been replaced by the newer IEC 62387 (IEC, 2012), which establishes the requirements needed for personal and environmental passive integrating dosimetry systems.
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Althought BeO has been used for decades (Bulur, 1998), in the last few years, the beryllium-oxid OSL dosimeters, named BeOSL, have been introduced in the market (Sommer, 2008; Sommer, 2011; Hübner, 2014). Each dosimeter is available with either two or four detectors, each of them having a different filter material. Extended information about the 2-element version of the
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dosimeter can be found elsewhere (Sommer, 2011; Jahn, 2014). However, only little data about the dosimetric response of the 4-element version have been published in the literature. This version allows energy estimations and provides a dose estimation with less energy dependency (Haninger, 2016).
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The first goal of this study was to perform the tests stated in the IEC 62387 to verify that both, the 4-element version of the BeOSL system together with the TLD system provided by the CND
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satisfy the requirements established in this publication. The second goal was to perform a comparison between both systems according to these results.
2.
MATERIAL AND METHODS
2.1. Dosimetry systems tested
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The TL dosimeter tested (named LiF-TLD in this study) is based on a detector card with four LiF:Mg,Ti (TLD-100) detectors, each one sandwiched in between two equal filter plates (Casal, 2015), as shown in Figure 1a. Each detector is 4.5 mm in diameter and 0.6 mm thick, whereas each filter is 11.8 mm in diameter. The following filter materials are used: 4 mm aluminum, 3
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mm copper in between 0.5 mm thick plastic slices, 3.9 mm plastic PTFE (Teflon), and air (open window). The card and the filter plates are wrapped with aluminized plastic, which protects the
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detectors from light and contamination. The energy is estimated from the ratios of the readings of the four detectors and the dose reading of each element is corrected to take into account the energy dependency and the attenuation of the filter. The dose readings are then used to compute the deep dose Hp(10) and the shallow dose Hp(0.07). The specific dose calculation algorithm has already been described by Casal et al. (2015).
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(c)
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Figure 1. Components and filters of the 4-element (a) LiF-TLD and (b) BeO-OSLD. (c) Radiation incident angle for the BeO-OSLD (top) and the LiF-TLD (bottom) in the photon angular dependency test.
The LiF-TLD are read with an automatic Harshaw 8800 reader (Moscovitch, 1990). Each reading cycle consists of a pre-heating of 9 seconds at 160ºC, followed by the reading period of 30 seconds at 300ºC, with a heating rate of 12ºC/s. In order to ensure that the dosimeters are
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completely erased and to recover the sensitivity of the detectors, they are annealed in an oven for 18h at 80ºC, unless doses higher than 18 mSv are accumulated, in which case the annealing is done at a temperature up to 300ºC.
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The OSL dosimeters tested (named BeO-OSLD in this study) are made of a card with four BeO OSL detectors that are also sandwiched in between two filter plates (Figure 1b). The following filter materials are used: copper, a sandwich of lead with low content of 210Pb and tin (the tin is
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between the Pb and the detector, in order to reduce self-irradiation caused by lead impurities), 2.4 mm PTFE (Teflon) and a 0.5 mm thick plastic window. Each element is a square of 4.7 mm size and 0.5 mm thick. The detector card and the filter plates are inside a black holder that protects the dosimeters from light and contamination. For Hp(0.07) the dose measured by the plastic window detector is assigned, whereas Hp(10) is computed as a linear combination of the reading of the four elements. The linear coefficients used were those that minimized the energy dependency of the dose measurement, and were provided by Dosimetrics. Independently of the dose reading, which is not corrected by energy dependency, the energy of the radiation beam is
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estimated using the relative readings of all four elements. Unlike the LiF-TLD system by the CND, in which an integer energy value is estimated by the algorithm, the BeOSL software provides a range of possible energy values in which the energy is classified.
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The OSL reader is based on a photodiode controlled blue LED and a photo sensor module, with the addition of optical filters between the LED and the BeO detector. Additional details have been published by Sommer et al. (2011). Each OSL measurement consists of five offset measurements (without stimulation, to get the actual offset signal of the photo sensor module) and five dose measurements, totaling 1 second. Offset and dose measurements are equally long
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(100 ms). These 10 measurements have to pass a consistency check, otherwise a warning message is shown by the software. The eraser bleaches all four elements during several seconds
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(10 s were used in this study) and then the dosimeter is re-read in order to evaluate the reusability of the dosimeter and if it needs to be re-erased or not.
2.2. Dosimetric assays
The following dosimetric assays for photon fields, reported by the IEC 62387 standard and
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described below, were performed for both systems: coefficient of variation, non-linearity, aftereffects, reusability, energy dependency, angular dependency, dose build-up, fading, selfirradiation, and response to natural radiation. Furthermore, although not included in that standard, the accuracy of the energy estimation was also evaluated. In addition, for the BeO-
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OSLDs, the consistency between several re-readings was scored. For each test, both Hp(10) and Hp(0.07) were evaluated.
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Most of the LiF-TL and BeO-OSL dosimeters were irradiated at the ionizing radiation metrology laboratory by the CND. These were placed on the surface of a standard ISO slab phantom, placed 250 cm away from the radiation focus. The beam qualities used conform to ISO 4037 (ISO, 1999) and IEC 61267 (IEC, 2005). For the energy dependency test, the irradiations with a 137Cs source were performed at CIEMAT (Madrid, Spain) for the LiF-TLDs, and at the Helmholts Zentrum (Muenchen, Germany) for the BeO-OSLDs.
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3.
RESULTS
3.1. Coefficient of variation and non-linearity Five groups were irradiated to different dose values, ranging from Hp(10) = 0.08 mSv to Hp(10)
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= 41.0 mSv. Six dosimeters were used for each group. The broad beam quality W-300 was used for all the irradiations. For each group, the coefficient of variation was calculated as the standard deviation divided by the mean dose, and was compared to the threshold value established by the IEC 62387, which depends on the irradiation dose.
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each group normalized by the linearity of the reference group, which in this case was that irradiated to 10 mSv. Additional details on the equations and parameters used can be found in the IEC 62387. Uncertainties are used with coverage factor k = 2.
For the 4-element version of LiF-TLDs and BeO-OSLDs tested in this study, Figure 2 shows the coefficient of variation as a function of the nominal dose. All the values are below the threshold
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established by the IEC 62387 for this test.
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Figure 2. Coefficient of variation for (a) Hp(0.07) and (b) Hp(10), as a function of the nominal dose for the 4-element version of LiF-TLDs and BeO-OSLDs tested in this study. The solid line shows the upper limit.
Figure 3 shows the results of the non-linearity test, compared with the limits defined by the IEC 62387. In the LiF-TLDs for dose values below 0.1 mSv the uncertainty exceeds the threshold values. However, doses below 0.1 mSv are not reported in the CND, nor in most of the European
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countries according to a recent survey (Gilvin, 2015) . It is also noticed that only six dosimeters were irradiated for this dose group. Generally, if the number of dosimeters is increased, the relative uncertainty due to reproducibility is decreased. Therefore, this result is acceptable for the
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dose range in which the dosimeters are used.
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Figure 3. Linearity test results of (a) Hp(0.07) and (b) Hp(10), as a function of the nominal dose for the 4-element version of LiF-TLDs and BeO-OSLDs tested. Solid lines show the tolerances.
3.2. After-effects and reusability
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Four groups composed of six dosimeters were used for this test. The reference group (group 1) was irradiated to 10.2 mSv. Groups 2 to 4 were irradiated to 0.08 mSv, 10.2 mSv and 41 mSv respectively; afterwards they were erased and irradiated again with the minimum detectable dose of roughly 0.05 mSv. Then, the test values were obtained by introducing the second readings in
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The results for each group here defined are shown in Figure 4. The test values for the BeO-
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OSLDs meet the limits fixed by the IEC 62387, whereas in case of the LiF-TLDs with previous doses above 10 mSv the condition is not fulfilled. Since in the CND dosimeters with doses above 4 mSv are subject to a reusability test, these dosimeters would have been evaluated before reuse and, depending on the result, either they would have been rejected or a new sensitivity factor would have been assigned to them. Therefore, this is not a case of a wrong estimation in the dose reported by the LiF-TLD system, but rather a case of time efficiency and more reusability of the BeO-OSLDs compared to the LiF-TLDs.
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Figure 4. Reusability test for each group, i, for (a) Hp(0.07) and (b) Hp(10) for the 4-element version of LiF-TLDs and BeO-OSLDs tested in this study. Solid lines show the tolerances.
3.3. Photon energy dependency
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The variation of the photon energy response at normal incidence was tested. Groups of three dosimeters were irradiated with the following beam qualities:
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Cs, N-300, N-200, N-150, N-
100, N-80, N-60, N-40, N-30 and RQR-M1.
The energy dependency for LiF-TL and BeO-OSL dosimeters is shown in Figure 5. Both systems met the IEC 62387 requirements. The flat energy response observed for the LiF-TLDs is
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due to the fact that the energy is estimated and the reading is corrected by the dose calculation algorithm. Due to the low energy dependency of the BeO-OSLDs, the manufacturer decided not
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Figure 5. Photon energy dependency for (a) Hp(0.07) and (b) Hp(10) for the 4-element version of LiF-TLDs and BeO-OSLDs tested in this study. Solid lines show the tolerances, which are not continuous since they depend on the uncertainty of the nominal dose value.
3.4. Photon angular dependency
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The variation of the response with the radiation incident angle was studied for two beam qualities, N-40 (low energy) and N-300 (high energy). The irradiation was performed, for the angles θ and ϕ represented in Figure 1c, at θ= ± 60º in both dosimetry systems, at ϕ= ± 60º for
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the LiF-TLD and only at ϕ= + 60º for the BeO-OSLD due to the symmetry of the system. Groups of three dosimeters were irradiated with doses of Hp(10) = 3 mSv (a few groups required 6 extra dosimeters due to the dispersion of the measurements). The analysis of the dose readings was performed according to IEC 62387. The reference group is the one irradiated with normal incidence. Although the IEC 62387 requires the test to be done for the three lowest energies in
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rated energy range, in this study, for simplicity, a low beam quality was chosen. However, in order to compare the angular dose response of both dosimetry systems in a broader energy range,
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The irradiation conditions of the incident angles and beam qualities for each group are shown in Table 1, whereas results are shown in Figure 6. For both systems the condition established by the IEC 62387 is met.
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Table 1. Angle of incidence and beam quality in the irradiation for each group of dosimeters for the angular dependency test.
Beam quality
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Group i
Mean photon energy (keV) 33 33 33 33 33 250 250 250 250 250
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ϕ (°)
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0 0 0 60 -60 0 0 0 60 -60
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Figure 6. Photon angular dependency for (a) Hp(0.07) and (b) Hp(10) for the 4-element version of LiF-TLDs and BeO-OSLDs tested in this study. Solid lines show the tolerances.
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3.5. Dose build-up, fading, self-irradiation and response to natural radiation Eight groups of dosimeters were used for this test. The number of dosimeters compounding each one is in Table 2, as well as the irradiation dose for Hp(10). Hlow is the lower limit for the measuring range, which in this case is 0.05 mSv.
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Table 2. Number of dosimeters, n, and irradiation doses for each group for the dose tests: build-up, fading, self-irradiation and response to natural radiation.
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n 6 27 6 25
Nominal dose 7 x Hlow=0.35 mSv approx. Hlow=0.05 mSv approx. Unexposed Unexposed
Groups 1 and 5 were read 24 hours post irradiation, groups 2 and 6 were read 7 days post irradiation, and groups 3, 4, 7 and 8 were read 28 days post irradiation. The corresponding background accumulated in groups 5 to 8 was subtracted from each dosimeter in groups 1 to 4, obtaining groups G1’ to G4’ respectively:
,
,
,
,
,
,
,
, where
G is the mean value of the background readings in group i. According to IEC 62387 (IEC, 2012), expression 1 is the one that groups 1’ to 3’ have to meet, where rmin and rmax depend on the irradiation dose and Ucom is the uncertainty associated to ′ / ′ . Group 4’ is analyzed according to expression 2. Expression 3 is applied to group 8’, where Cnat is the theoretical background - 13 -
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accumulated until the reading date, and Um is the half-width of the confidence interval of the mean dose. For this study a natural radiation rate of 0.1 µSv/h was assumed, which leads to 0.067 mSv accumulated in 28 days.
≤
H'
(
G ±U G
7·G G
!≤r !≤r
±U
≤G ±U
C
"*
(1)
"#
"#
≤ +H'
(
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r
≤
(2)
(3)
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r
Test results for groups 1’ to 4’, described above, are between 0.93 and 1.04, which are within the
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tolerance levels 0.9 and 1.1. For group 8’, results are included in Table 3, for which the test value (± the uncertainty) must be included in the range defined by ± Hlow. It can be observed that both systems, the BeO-OSL and the LiF-TL, match the requirements established by the IEC 62387 for these dose dependencies.
Table 3. Response to natural radiation test results for the 4-element version of LiF-TLDs and BeO-OSLDs tested in this study.
± Hlow
Hp(0.07)
-0.004
0.006
±0.057
Hp(10)
-0.017
-0.007
±0.059
Hp(0.07)
-0.008
0.006
±0.057
-0.007
-0.007
±0.059
BeO-OSLD
LiF-TLD
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Dosimetric magnitude Test value - Um Test value + Um
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System
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Hp(10)
3.6. Accuracy of the energy estimation The LiF-TLD system provides a range of continuous integer values for the energy estimated, while the BeO-OSL system provides an interval in which the energy value is in between. Intervals at which the energies are classified are: “very soft” (for energies below 20 keV), “soft” (for energies between 20 and 55 keV), “medium” (for energies between 55 and 200 keV) and “hard” (for energies above 200 keV). Irradiations at different energy values and angles were performed and the nominal energies provided by the irradiation laboratory were compared with
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the estimations provided by each system. Irradiations were performed in groups of three dosimeters for the following beam qualities (the mean energy is noted in parentheses):
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Cs (662 keV), N-300 (250 keV), N-200 (164 keV), N-
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150 (118 keV), N-100 (83 keV), N-80 (65 keV), N-60 (48 keV), N-40 (33 keV), N-30 (24 keV) and RQR-M1 (16,11 keV). The irradiations were performed at normal incidence for each beam quality, and at θ= ± 60 º , and ϕ= ± 60 º for the beam qualities N-300 and N-40.
For all dosimeters tested, the energy interval estimated by the BeO-OSLD system was correct for
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both, normal incidence and for different angles. For the LiF-TLD system, the deviation between the nominal and the estimated mean photon energy was below 4.2% in all cases, except for one dosimeter irradiated with the beam quality N-80 where it reached 7.7%. For radiation incidence
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at different angles, the deviation in the energy estimated increases up to 30%. However, this leads to a maximum deviation of only 3% in the dose correction factor.
3.7. Consistency between consecutive re-readings
Although it is not part of IEC 62387 standard, the possibility to read a dosimeter more than once
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was also tested for the 4-element OSLD based on BeO. For these detectors, only a small part (roughly 3% in the BeOSL system) is erased. The software by Dosimetrics takes this into account and corrects the re-reading signals by the loss due to the previous readings. In fact, as stated by the manufacturer, the signal loss per read is an individual figure for each detector,
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which is determined during calibration. The signal loss also depends on the number of re-read. The re-reading capability of LiF-TLDs has been published elsewhere (Moscovitch, 2011;
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McKinlay, 1980) and is not evaluated in this study. Groups of six dosimeters were irradiated with Hp(10) doses ranging from 0.08 mSv to 51.1 mSv. Each dosimeter was read six times (the first five times in 10-minute intervals and the last one four weeks later). For each dose reading the deviation to the first reading was computed. Results are shown in Figure 7.
For doses above 1 mSv, deviations are lower than 3%, increasing up to 20% for lower dose values. It should be emphasized that the re-read capability seems to be more useful in cases of high dose levels, close to or beyond the dose limits. These are clearly higher than 1 mSv, where
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the BeOSL re-reading system has shown to provide high accuracy.
(a)
(b)
4.
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Figure 7. Deviation for consecutive dose readings for (a) Hp(0.07) and (b) Hp(10) for the 4-element version of BeO-OSLDs tested in this study. For a clearer representation, the sixth reading of a dosimeter irradiated to 0.08 mSv was omitted in both figures, which showed deviations of 120% and 75% for Hp(0.07) and Hp(10), respectively.
DISCUSSION
The dosimetric systems and dose algorithms that have been studied here satisfy the IEC 62387
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requirements for the tests described above, which makes them suitable to be used as personal dosimeters for photon fields. However, there are some important differences that should be emphasized. It should be also noted that this discussion is true for the systems here studied, but
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may be different for other TL or OSL systems.
The BeO-OSLDs show less energy dependency than the LiF-TLDs. Hence, the required dose algorithm can be simpler and, therefore, provides less options for making operational mistakes.
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In the LiF-TLDs this characteristic is compensated with the CND dose calculation algorithm by estimating the energy (with 1 keV resolution) and applying a dose correction (Casal, 2015). It provides a flatter response with energy variations than in the BeO-OSLD system. However, this correction has the disadvantage that the estimation of the dose depends on the estimation of the energy, and if a wrong energy value is obtained, it leads to a wrong dose correction factor of the LiF-TLD reading. This can be important, for example, in mixed field irradiations (i.e. irradiations of both photons and electrons). Nevertheless, this higher dependency allows for more precision in the estimation of the energy radiation, providing a continuous range of values
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for the estimated energy, instead of intervals in which it is classified. Energy dependency for Hp(10) for the 4-element version of the OSLDs based on BeO was already reported by Haninger et al. (2016). For the Hp(0.07) element of these dosimeters,
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graphics for the energy and angular response are provided by Sommer et al. (2011). The results of the current study for these specific tests are consistent with data published in the literature. For instance, a variation in the relative energy response below 0.3 for energies comprised between 20 and 300 keV is observed in both cases. In addition, in the case of Hp(10) the results in both studies show a local minimum between 40 and 50 keV and a local maximum between 100 and
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200 keV. For the Hp(0.07) estimation, a local maximum between 30 and 40 keV is observed as well as a slight increase in the dosimeter's response with energy above 90 keV.
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In this study, it is verified that, in comparison to the 2-element version of the BeOSL system reported by Haninger et al. (2016), the 4-element version flattens the response with the energy variation. Therefore, it provides more accurate dose estimations. As shown in section 3.6, it also provides an estimation of the energy, which is not possible with the 2-element version. An advantage of the BeO-OSLD system over the LiF-TLD system is that the reading and erasing
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process consume less power. In addition, as shown in section 3.2, they are more reusable, since they meet the requirements to be reused after having been irradiated to higher doses. This is an important factor taking into account the large amount of workers that are monitored by the CND every month (more than 42 000 dosimeters are read monthly).
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Another advantage of the BeOSL dosimeters is that they can be read easily more than once. This provides the possibility of repeating dose readings if a questionable value has been obtained.
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Dosimeters with high dose values can also be archived to re-read them in a future. In this study it has been proven that the tool works well above 1 mSv for the system by Dosimetrics. Notwithstanding, glow curves are obtained in the LiF-TLD system during the reading process. A pattern with evident glow peaks is a sign that the TLD signal is not spurious, but is a result of a real radiation exposure. These curves can also be used to identify a defective element in the dosimeter or a malfunctioning reader. This is not possible with the current state of the BeOSL system. However, this is partly compensated by the fact that each dose value provided by the BeOSL system is the average of 5 measurements, and a consistency check is made. Either one system or another will be more appropriate depending on which of the analyzed - 17 -
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aspects is more important for the intended purpose.
5.
CONCLUSIONS
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This study verifies that, for photon beams, the LiF-TLD system by the National Dosimetry Centre and the BeO-OSLD system by Dosimetrics meet the requirements established in the IEC 62387 to be used as passive personal dosimeters. Among the main differences between them, the BeO-OSLDs show less energy dependency, while the higher dependency of the LiF-TLDs is
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compensated by the dose calculation algorithm of the CND, which provides more accurate energy estimations. The BeOSL dosimeters are more reusable than the LiF-TLDs, they also can
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simpler, faster and consumes less energy.
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be read easily more than once with good accuracy, and their reading and erasing process is
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ACKNOWLEDGMENTS We would like to thank Dosimetrics for kindly providing the BeO-OSLD system used in this
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CONFLICTS OF INTERESTS
This research did not receive any specific grant from funding agencies in the public, commercial,
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or not-for-profit sectors. The authors declare that they have no conflicts of interests.
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Casal E, Soriano A, Alabau J, Palma J. (2015).Results obtained by the personnel dosimetry service of the Spanish health department in the EURADOS intercomparison 2012 for whole body photon dosemeters. EURADOS International Conference on Individual Monitoring of ionising radiation, Bruges -Belgium.
Gilvin PJ, Alves JG, Cherestes C, van Dijk JWE, Lehtinen M, Rossi F, Vekic B, Chevallier MA.
Survey 2012. EURADOS Report 2015-04.
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Haninger T, Hödlmoser H, Figel M, Köning-Meier D, Sommer M, Jahn A, Ledtermann G, Eßer R. (2016). Properties of the BeOSL dosimetry system in the framework of a large-scale personal monitoring service. Radiation protection dosimetry,170: 269-273. Hübner S (2014). Introduction of a new dosimetry system based on optically stimulated luminescence (OSL) in our personal monitoring service. ISSSD. Cusco, Peru. International Commission on Radiological Protection (ICRP) 103 (2007). The 2007
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recommendations of the International Commission on Radiological Protection. International Electrotechnical Commission (IEC) 61066 (1991-12). Thermoluminescence dosimetry systems for personal and environmental monitoring. Edition 1.0.
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International Electrotechnical Commission (IEC) 61267 (2005-11). Medical diagnostic X-ray equipment - Radiation conditions for use in the determination of characteristics. Edition 2.0. International Electrotechnical Commission (IEC) 62387 (2012-12). Radiation protection
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instrumentation – Passive integrating dosimetry systems for personal and environmental monitoring of photon and beta radiation. Edition 1.0. International Organization for Standardization, ISO 4037-3. (1999). X and gamma reference radiation for calibrating dosemeters and dose rate meters and for determining their response as a function of photon energy. ISO 4037-3. Jahn A, Sommer M, Henniger J (2014). Environmental dosimetry with the BeOSL personal dosemeter – State of the Art. Radiation Measurements, 71: 438-441.
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Jursinic PA (2007). Characterization of optically stimulated luminescent dosimeters, OSLDs, for clinical dosimetric measurements. Medical Physics, 34:4594-4604. McKinlay AF, Bartlett DT, Smith PA (1980). The development of photo-transferred
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thermoluminescence (PTTL) technique and its application to the routine re-assessment of absorbed dose in the NRPB automated personal dosimetry system. Nuclear Instruments and Methods, 175: 57-59.
Moscovitch M, Szalanczy A, Bruml WW, Velbeck KJ, Tawil RA. (1990). A TLD system based on gas heating with linear time-temperature profile. Radiation ProtectionDosimetry, 34:
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disadvantages of optically stimulated luminescence dosimetry and thermoluminescence dosimetry. Radiation Protection Dosimetry, 104: 263-270.
Moscovitch M, Benevides L, Romanyukha A, Hull F, Duffy M, Voss S, Velbeck K. J, Nita I, Rotunda J. E. (2011). The Applicability of the PTTL dose re-analysis method to the harshaw Lif:Mg,Cu,P material. Radiation Protection Dosimetry, 144: 161-164. Pradhan AS (1995). Influence of Heating Rate on the TL Response of LiF TLD-700,
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LiF:Mg,Cu,P and Al2O3:C. Radiation Protection Dosimetry, 58 (3): 205-209. Romanyukha A, Grypp MD, Faifchild GR, Williams AS (2016). Performance comparison of OSLD (Al2O3:C) and TLD (LiF:Mg,Cu,P) in accreditation proficiency testing. Radiation
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1 2 3 4 5 6 7 8 9 10
N-40 N-40 N-40 N-40 N-40 N-300 N-300 N-300 N-300 N-300
Mean photon energy (keV) 33 33 33 33 33 250 250 250 250 250
θ (°)
ϕ (°)
0 60 -60 0 0 0 60 -60 0 0
0 0 0 60 -60 0 0 0 60 -60
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Nominal dose 7 x Hlow=0.35 mSv approx. Hlow=0.05 mSv approx. Unexposed Unexposed
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System
Dosimetric magnitude Test value - Um
Test value + Um
± Hlow
Hp(0.07)
-0.004
0.006
±0.057
Hp(10)
-0.017
-0.007
±0.059
Hp(0.07)
-0.008
0.006
±0.057
Hp(10)
-0.007
-0.007
±0.059
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