|Year : 2014 | Volume
| Issue : 3 | Page : 169-175
Thermoluminescent dosimeter-direct reading dosimeter dose discrepancy: Studies on the role of beta radiation fields
Munish Kumar1, RB Rakesh1, Anil Gupta2, SM Pradhan1, AK Bakshi1, D.A.R. Babu1
1 Radiological Physics and Advisory Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India
2 Health Physics Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India
|Date of Web Publication||10-Apr-2015|
Radiological Physics and Advisory Division, Bhabha Atomic Research Centre, Mumbai - 400 085, Maharashtra
Source of Support: None, Conflict of Interest: None
Dosimetry studies pertaining to thermoluminescent dosimeter (TLD) and direct reading dosimeter (DRD) have been performed for photons, beta fields and mixed field of photons and beta particles. In lab conditions, for pure photon radiation fields, the doses estimated using DRD and TLD match within the acceptable limits whereas in the mixed fields of photons and high energy beta particles, it has been found that the DRD doses are always higher than the corresponding whole body doses estimated by the TLD. This is due to the fact that DRD responds to high energy beta particles and the typical response of the DRD to high energy beta particles is observed to be in the range of 15-30%. This may lead to TLD-DRD dose discrepancy at workplaces where the skin doses received by the radiation workers from high energy beta sources in a given monitoring period are significant. The paper also provides a comparison of three different TLD-DRD discrepancy identification criteria available in literature for exposure conditions with a significant dose due to beta radiations. In addition, estimate of threshold beta dose which may lead to discrepancy as per the criteria have been studied. The results reported in this paper would be helpful in understanding the discrepancy arising out of variable response of DRD to beta radiations and will be useful in resolving the discrepancy in such cases.
Keywords: Beta dose, discrepancy identification criteria, response of dosimeters to various beta sources, thermoluminescent dosimeter-direct reading dosimeter dose discrepancy
|How to cite this article:|
Kumar M, Rakesh R B, Gupta A, Pradhan S M, Bakshi A K, Babu D. Thermoluminescent dosimeter-direct reading dosimeter dose discrepancy: Studies on the role of beta radiation fields. Radiat Prot Environ 2014;37:169-75
|How to cite this URL:|
Kumar M, Rakesh R B, Gupta A, Pradhan S M, Bakshi A K, Babu D. Thermoluminescent dosimeter-direct reading dosimeter dose discrepancy: Studies on the role of beta radiation fields. Radiat Prot Environ [serial online] 2014 [cited 2020 Jun 6];37:169-75. Available from: http://www.rpe.org.in/text.asp?2014/37/3/169/154880
| Introduction|| |
Active dosimeters used simultaneously with passive/legal dosimeters at various radiation institutions are recommended for day-to-day control of radiation exposure incurred to occupational workers and also in prospective planning. IAEA recommends that "where dose equivalent rates encountered in the workplace may vary by more than a factor of ten, an additional, direct reading dosimeter (DRD) and/or a warning device should be issued for dose control purpose."  In nuclear fuel cycle facilities, the radiation levels may vary significantly at adjacent locations and also with time at same location. Therefore, the use of DRD and/or a warning device is always recommended. Commonly used active dosimeters are ionization chamber (DRDs) and semiconductor-based electronic dosimeters whereas popular passive dosimeters are based on thermoluminescence (TL), optically stimulated luminescence, film badge etc. Ionization chamber based DRD being used at different radiation installations have been studied by many researchers. Jackson has suggested various guidelines for optimizing the performance of DRDs and presented data to support the fact that DRDs when properly used are accurate and precise instrument whose small size, ruggedness, resistance to severe environments, hermeticity and self-reading capabilities can be a decided advantage.  Use of TL based passive dosimeters is well-established and is a worldwide accepted system for personal monitoring. It is worth mentioning that in India, CaSO 4 :Dy Teflon based TL dosimeter is used for personal monitoring applications. ,
It is expected that the individual doses arrived from the summation of day-to-day dose values recorded by the DRDs over the monitoring period should be comparable to the dose values estimated using thermoluminescent dosimeter (TLD). Various factors such as least count, reporting level, charge leakage of DRDs, improper charging of DRDs, response to mechanical shock to DRDs, parallax error in reading DRD dose, more number of DRD transactions with doses below the readable dose, proximity of either of the dosimeters to a radiation hotspot, possible contamination of either dosimeters, wearing of the dosimeters at different locations, nonuse of either dosimeters, exposure of TLD to chemicals/heat/dust, different period of use, radiation backscattering, response to single/mixed radiation fields, energy dependence, nonhomogeneity of the radiation field and response to radiation not intended to be measured, contributes to difference between two measured individual doses. , It is worth mentioning that the active and passive dosimeters are expected to give similar results under laboratory conditions for photon radiations. However, under field conditions due to various factors mentioned above, the deviation among the dose values evaluated using active and passive dosimeters increases making the correlation among these dosimeters slightly poor. The unacceptable difference between the two sets of individual doses received in a given monitoring period as defined by authorities is identified as discrepancy. Kadwani and Sharma performed inter-comparison of TL and DRDs and found both the systems to be satisfactory.  Viswambharan et al. studied the performance of TLD-DRD pairs under various exposure conditions and found that the doses measured by both the dosimeter systems lie within the allowed limit in lab condition but mentioned fewer cases of TLD-DRD dose discrepancy in field conditions.  Further analysis of the TLD-DRD discrepancy cases for few years showed that in the majority of the cases, the nontechnical factors were mainly responsible for the discrepancy. Both the dosimeter systems, performed within the allowed limit under controlled as well as field conditions and technical factors such as dosimeter response were not responsible for the dose discrepancy in majority of the cases. Pradhan et al. investigated the relative responses of DRD, electronic pocket dosimeter and CaSO 4 :Dy based thermoluminescence dosimeter (TLD) badge to 60 Co and 137 Cs gamma ray sources and found that the difference between the doses evaluated using these dosimeters was within ±12%. 
Rakesh et al. performed comparison of individual gamma doses assessed by CaSO 4 :Dy Teflon disc based officially recognized TLD badge and the integrated individual dose assessed by the quartz fiber DRD over the same monitoring period.  The authors analyzed TLD-DRD monthly dose data for three Nuclear Power Plants and reported on an average 5 discrepancies annually based on prevalent identification criteria at each plant. , The performance of DRDs was also studied by Bergen et al. and found that the average percent standard deviations ranged from (5% to 14%) for various exposures.  Fernanda et al. studied dose-response curves of the DRDs for 60 Co and 137 Cs sources with and without the presence of the phantom.  The results showed a difference between two exposure conditions with DRD reading higher when backed by phantom due to the presence of radiation backscattering. 
Above-mentioned studies were performed in laboratory or field conditions, where mainly photon radiation was considered. ,,,,,, Pujari et al. performed relative response studies on different dosimeters viz. TLDs, DRDs and electret dosimeter to beta radiation.  The study was initiated in view of continued disagreement amongst doses recorded by TLDs and DRDs at work locations in which photons and high energy beta radiations were present.  In fact at such locations, the DRD doses are expected to be consistently higher than the whole body doses measured using CaSO 4 :Dy based TLD badge. , Further Kumar et al. have also investigated the response of ionization chamber based pocket dosimeter to beta radiation in the maximum beta energy from 0.224 MeV to 3.54 MeV and provided quantitative estimate of the values of the response of DRDs to various beta sources. 
In India, the identification criteria of TLD-DRD dose discrepancy were arrived in 1991, according to which the difference between two sets of individual doses is identified as discrepancy if the difference between the two dose values measured using active and passive dosimeters is more than 1 mSv and the difference is more than 50% of the higher dose.  Rakesh et al. have also proposed discrepancy identification criteria based on extensive analysis of TLD and DRD doses for a large number of workers studied over a period of 2 years and recommended the following criteria: 
Where, H 0 is the reporting value of the dose and H 1 is the lower of the two doses. As per the above criteria, the ratio of the higher to the lower dose falling outside the curve represented by above equations may be treated as a discrepancy. Comprehensive details about the same can be had from publication of Rakesh et al. 
More recently, International Standards Organization (ISO-15690) has issued recommendations for dealing with the discrepancy between personnel dosimeter systems used in parallel.  According to ISO-15690, for dose evaluations summed for the required wearing period for the dosimeters with the longest wearing period, it is recommended to research explanations for differences in dosimeter results, if estimated dose is >1 mSv (one or both dosimeters), and low reading dosimeter dose is <0.7 × high reading dosimeter dose. Further details can be had from ISO-15690. 
In view of above and continued prevalence of TLD-DRD dose discrepancy, investigations with pure as well as mixed field of photons and beta particles were performed. Investigations were also performed to arrive at threshold beta doses in pure beta and mixed field of photons and beta radiations, which may lead to discrepancy. It is worth mentioning that the threshold beta dose is the minimum value of the beta dose which will lead to discrepancy in TLD-DRD dose values. This study also provides a way to find out the method to identify the discrepancy arising out of variable response of DRD to beta radiations and also methods to resolve the same. In addition, a comparison amongst the three criteria mentioned above has also been performed.
| Materials and methods|| |
Ionization chamber based DRDs (Stephen and arrow-tech make) used in the present study are based on the gold leaf electroscope principle and a quartz fiber is displaced electrostatically by charging it to a potential of about 200 V. An image of fiber is focused on a scale and is viewed through a lens at one end of the instrument. Exposure of the dosimeter to radiation discharges the fiber, thereby allowing it to return to its original position. The amount discharged and consequently the change in the position of the fiber is proportional to the radiation exposure. It is worth mentioning that the wall thickness of the DRDs is ~280 mg/cm 2 and is commonly made of stainless steel. , Due to this, low energy photons (<30 keV) are heavily attenuated and doses recorded by DRDs may be under-estimated. Also for beta radiations, the maximum energy of the beta particles should be such that the range is more than the wall thickness of ~280 mg/cm 2 , which implies that beta particles having maximum energy Emax <0.8 MeV are unable to reach the sensitive volume of the DRD and are not recorded. As personnel monitoring in India is based on CaSO 4 :Dy based Teflon embedded TLD badge, having three filter regions viz. 1 mm Cu +1 mm Al (D 1 ), 1.5 mm thick Perspex (D 2 ) and open window (D 3 ) is capable of measuring photons and beta radiations. Additional details about the TLD badge and associated TLD reader system can be had from Vohra et al. and Kulkarni et al.,
One major difference between the CaSO 4 :Dy based TLD badge and ionization chamber based DRDs is that the TLD badge has multiple detector elements along with various filters, and it is possible to separate as well as measure deep/whole body and shallow (skin) doses. It is also worth mentioning that the beta particles belong to weakly penetrating radiations and deposit skin dose. Similar is the case with photons having energy <15 keV whereas photons having energy >15 keV are treated as strongly penetrating radiations and deposit deep dose in addition to the contribution toward skin dose.
In the present paper, results are reported for various beta sources viz. 147 Pm, 85 Kr, 204 Tl, 32 P, 90 Sr/ 90 Y, nat U and 106 Ru/ 106 Rh and were also compared with the response of dosimeters to photons from 137 Cs source. Based upon the response values amongst DRD and TLD dosimeters, a detailed analysis of the TLD-DRD discrepancy identification criteria's have been investigated. In addition, the values of the threshold beta doses that may lead to TLD-DRD discrepancy in pure beta and beta gamma fields are estimated. TLD-DRD discrepancy investigations have been performed for mixed field of photons and beta radiations.
| Results and discussions|| |
From the study, it has been found that the dose values estimated using DRDs and CaSO 4 : Dy based TLD badge are in good agreement for irradiations performed with 137 Cs photons as is reflected by the ratio of the doses estimated by TLD and DRD, which was found to be ~1.00 ± 0.10 under free in air irradiation conditions for normal incidence. In such situations, no discrepancy between the doses measured using TLDs and DRDs prevail as was observed by previous workers. ,,,,, The results of the study on the response of the DRDs to various beta sources viz. 147 Pm, 85 Kr, 204 Tl, 32 P, 90 Sr/ 90 Y, nat U and 106 Ru/ 106 Rh are given in [Figure 1]. As expected, the DRDs do not show response to beta particles from 147 Pm, 85 Kr, 204 Tl. The typical values of the response of DRD are ~5%, 14% and 27% for 32 P, 90 Sr/ 90 Y and 106 Ru/ 106 Rh beta sources respectively.  It may also be noted that the response of the DRD to beta particles from nat U is ~8%.  Further, these values may be influenced by the source to DRD distance, geometry and encapsulation/shielding of the source etc. For lower beta energy that is, Emax ~1 MeV, the DRDs may exhibit some/measurable response especially at very high dose values which arises from beta particles corresponding to maximum range as well as from bremsstrahlung radiation.
|Figure 1: Relative response of the ionization chamber based pocket dosimeter to beta particles from 32P, 90Sr/90Y and 106Ru/106Rh|
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From [Figure 1], it may be noted that the typical response of the DRDs to high energy beta particles may vary from (20% to 30%). This implies that the DRDs will detect high energy beta particles and this may be interpreted as photon/whole body dose, e.g. for a dose of 20 mSv from high energy beta sources, the DRD reading will be ~5 mSv or so. Further for a mixed fields of photon and beta radiations that is, a dose delivery of 2 mSv with 137 Cs (photons) and 50 mSv with 90 Sr/ 90 Y (beta particles), the net response of the DRD will be ~9 mSv. The dose assessment using CaSO 4 :Dy based TLD badge for the situation mentioned above, the whole body and beta doses comes to be ~2 mSv (photon) and ~50 mSv respectively as TLD separates photon and beta doses.
In view of above, the discrepancy amongst dose values measured using DRD and TLD badge may arise in mixed fields of photons and high energy beta particles. In fact such situations have been encountered many times at radiation facilities where radiation workers receive doses from photons as well as from beta particles and the DRD reading have been found to be more than the TLD reading. ,, This difference was drastically reduced with the use of Perspex shields or aluminum sleeves/shields of appropriate thickness, depending on the range of beta sources present. ,,, This clearly shows that the DRDs may exhibit much higher reading compared to the whole body dose value measured using CaSO 4 :Dy based TLD badge.
In [Table 1], various threshold doses, which may lead to TLD-DRD discrepancy in pure beta fields, are given. The threshold doses for 32 P, nat U, 90 Sr/ 90 Y and 106 Ru/ 106 Rh beta sources are ~20 mSv, 12.5 mSv, 7.20 mSv and 3.75 mSv respectively. Also for mixed field of photons and beta radiations, it has been found that the threshold dose value for the beta radiations should be more than 20, 12.5, 7.20 and 3.75 times the gamma dose for TLD-DRD discrepancy to be identified for 32 P, nat U, 90 Sr/ 90 Y and 106 Ru/ 106 Rh beta sources respectively. The threshold beta dose for observable TLD-DRD discrepancy in pure beta fields for various beta sources and prevalence of discrepancy as per three TLD-DRD discrepancy investigation criteria is shown in [Table 2]. Further in [Table 3], investigation of the discrepancy as per three TLD-DRD discrepancy investigation criteria have been performed for a mixed photon and beta fields.
|Table 1: Threshold beta dose for observable TLD-DRD discrepancy for various beta sources |
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|Table 2: Threshold beta dose for observable TLD - DRD discrepancy in pure beta fields for various beta sources |
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|Table 3: Threshold beta dose for observable TLD - DRD discrepancy in mixed field of photons and beta radiations |
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The study indicates that significantly high exposure to beta radiations is one of the factors that may cause TLD-DRD dose discrepancy. However, in the presence of mixed fields that is, photons and high energy beta fields, it is expected that the DRD doses are always greater than the corresponding whole body doses reflected by the TLD measurements and same have been confirmed in this study.
It is important to note that DRD dose will always be higher than the TLD dose in the TLD-DRD dose discrepancy due to the response of DRD to beta radiations. For the investigation of such discrepancies, the information about the source and nature of exposure will be required. The beta dose estimated by TLD and the energy of the beta source (beta energy) can be further used to estimate the contribution of beta radiations toward total dose estimated by the DRD and this will further resolve the discrepancy.
| Conclusions|| |
Dosimetry studies pertaining to TLD and DRD have been performed for photons, beta fields and mixed field of photons and beta particles. In lab conditions, for pure photon radiation fields, the doses estimated using DRD and TLD match within the acceptable limits whereas in the mixed fields of photons and high energy beta particles, it has been found that the DRD doses are always higher than the corresponding whole body doses estimated by the TLD due to response of DRD to high energy beta particles and inability of DRD to discriminate between beta and gamma doses. Typical response of the DRDs to high energy beta particles is observed to be in the range of 15-30%. For workplaces where the skin doses received by the workers from high energy beta sources in a given monitoring period are significant, TLD-DRD dose discrepancy may be observed. In such cases, the beta dose estimated by TLD can be used to estimate its contribution toward dose estimated by DRD that is, by taking into account its response to particular beta radiation. This will further help in resolving the TLD-DRD dose discrepancies.
| Acknowledgments|| |
Authors are thankful to Dr. D. N. Sharma, Director, HS and EG, BARC for encouragement.
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[Table 1], [Table 2], [Table 3]