A machine learning approach for correcting glow curve anomalies in CaSO4:Dy-based TLD dosimeters used in personnel monitoring.

The study presents a novel approach to analysing the thermoluminescence (TL) glow curves (GCs) of CaSO4:Dy-based personnel monitoring dosimeters using machine learning (ML). This study demonstrates the qualitative and quantitative impact of different types of anomalies on the TL signal and trains ML algorithms to estimate correction factors (CFs) to account for these anomalies. The results show a good degree of agreement between the predicted and actual CFs, with a coefficient of determination greater than 0.95, a root mean square error less than 0.025, and a mean absolute error less than 0.015. The use of ML algorithms leads to a significant two-fold reduction in the coefficient of variation of TL counts from anomalous GCs. This study proposes a promising approach to address anomalies caused by dosimeter, reader, and handling-related factors. Furthermore, it accounts for non-radiation-induced TL at low dose levels towards improving the dosimetric accuracy in personnel monitoring.

ESTIMATION OF ACTUAL DOSE BASED ON BAYESIAN PROBABILISTIC APPROACH USING PERSONNEL MONITORING DOSE RECORDS.

In this study, the Bayesian probabilistic approach is applied for the estimation of the actual dose using personnel monitoring dose records of occupational workers. To implement the Bayesian approach, the probability distribution of the uncertainty in the reported dose as a function of the actual dose is derived. Using the uncertainty distribution function of reported dose and prior knowledge of dose levels generally observed in a monitoring period, the posterior probability distribution of the actual dose is estimated. The posterior distributions of each monitoring period in a year are convoluted to arrive at actual annual dose distribution. The estimated actual doses distributions show a significant deviation from reported annual doses particularly for low annual doses.

MACHINE LEARNING ALGORITHMS FOR IDENTIFICATION OF ABNORMAL GLOW CURVES AND ASSOCIATED ABNORMALITY IN CaSO4:DY-BASED PERSONNEL MONITORING DOSIMETERS.

In the present study, machine learning (ML) methods for the identification of abnormal glow curves (GC) of CaSO4:Dy-based thermoluminescence dosimeters in individual monitoring are presented. The classifier algorithms, random forest (RF), artificial neural network (ANN) and support vector machine (SVM) are employed for identifying not only the abnormal glow curve but also the type of abnormality. For the first time, the simplest and computationally efficient algorithm based on RF is presented for GC classifications. About 4000 GCs are used for the training and validation of ML algorithms. The performance of all algorithms is compared by using various parameters. Results show a fairly good accuracy of 99.05% for the classification of GCs by RF algorithm. Whereas 96.7% and 96.1% accuracy is achieved using ANN and SVM, respectively. The RF-based classifier is recommended for GC classification as well as in assisting the fault determination of the TLD reader system.

MAPPING OF WORKPLACE RADIATION FIELDS IN DIAGNOSTIC RADIOLOGY FACILITIES USING PERSONNEL MONITORING TLD BADGES.

The radiation protection programme is aimed at safe usage of radiation at workplace, ensuring minimum possible dose to radiation workers, patients and members of the public. Verification of the adequacy of protective measures in actual workplace is important, especially for diagnostic radiology facilities, as a substantial number of suspected overexposures are reported from these facilities. To address this issue, a study was conducted for mapping workplace radiation field at various locations in nine Diagnostic Radiology Facilities of two hospitals in India. The cumulative doses were measured for a period of 1-3 months, using personnel monitoring TLD badges. The dosemeters were placed at positions representative of two exposure situations: (1) probable locations of workers during procedures, leading to genuine exposure and (2) inappropriate storage locations of personal dosemeters at user end for probing claims of nongenuine exposures. The results indicate that the measured doses at locations (1) were just a fraction of the permissible dose, provided all safety practices are adhered to. However, the measured doses at certain locations (2) exceeded the investigation levels and indicate that any inadvertent storage of the dosemeters at such locations could lead to reporting of the overexposure from these radiology facilities. The outcome of the study will be useful for the investigation of such exposures and better understanding of the readout patterns of TLD badges in radiology workplaces.

ESTIMATION OF UNCERTAINTY IN MEASUREMENT OF DOSE EQUIVALENT AT LABORATORY LEVEL USING CASO4 :Dy-BASED TLD BADGE SYSTEM IN INDIA.

The objective of this paper is to estimate the combined uncertainty in the measurement of dose equivalent at laboratory level using CaSO4:Dy-based thermoluminescent dosemeter badge system by including variations in the components of the system. The variability of performance of the system is analysed using random effects one way analysis of variance model. The model enables estimation of the overall variance of the performance of the sampled population. The population in the study comprises all possible indicated dose equivalents on irradiation of dosemeters to a specific dose equivalent and radiation quality. Coefficient of variation and combined uncertainty at 95% level of confidence in the measurement of Hp(10) due to S-Cs radiation quality are found to be 6.6 and 14.3%, respectively, at the dose level of 5.31 mSv. The above parameters in the measurement of in-use quantity, i.e. whole body dose or photon dose equivalent are found to be 7.4 and 16.4%, respectively. The performance of the monitoring system on relative response has been observed to be satisfactory. Various factors affecting the variability of performance of the system are identified for further improvement in coefficient of variation.

STUDY OF EFFECT OF CONSECUTIVE HEATING ON THERMOLUMINESCENCE GLOW CURVES OF MULTI-ELEMENT TL DOSEMETER IN HOT GAS-BASED READER SYSTEM.

The objective of this paper is to study the effect of consecutive heating of TL elements of a thermoluminescence dosemeter (TLD) card in hot N2 gas-based TLD badge reader. The effect is studied by theoretical simulations of clamped heating profiles of the discs and resulting TL glow curves. The simulated temperature profile accounts for heat transfer to disc from hot gas as well as radiative and convective heat exchanges between the disc and the surrounding. The glow curves are simulated using 10 component glow peak model for CaSO4:Dy using the simulated temperature profile. The shape of the simulated glow curves and trend in total TL signal of the three discs were observed to match closely with the experimental observations when elevated surrounding temperature was considered for simulation. It is concluded that the readout (heating) of adjacent TLD disc affects the surrounding temperature leading to the changes in temperature profile of the next disc.

DEVELOPMENT OF AN ALGORITHM TO ESTIMATE EYE LENS DOSE IN TERMS OF OPERATIONAL QUANTITY Hp(3) USING HEAD TLD BADGE.

In view of the recommendations of International Commission on Radiological Protection for reduction of the occupational annual dose limit for eye lens from 150 mSv to 20 mSv/y, questions have been raised on the adequacy of monitoring for the quantities Hp(10) and Hp(0.07). As an immediate requirement, in the present situation, where there is no exclusive eye lens dosemeter in India, the existing chest TLD badge was modified to be used as head badge (head dosemeter) by including a strap to enable wearing on the forehead. In order to estimate the eye lens dose in terms of the operational quantity Hp(3), the prevalent algorithm of chest badge was also modified. The modified algorithm was applied to estimate Hp(3) for dosemeters irradiated to various beta and photon radiations including mixtures. The Q values (estimated/delivered dose equivalent) were found to be within ±20% for most of the photon beams.

Study of Residual TL of CaSO4:Dy-based Thermoluminescence Dosemeter.

Thermoluminescence (TL) signal obtained during second readout of a TL dosemeter disc previously exposed to ionising radiation is termed as residual TL. The origin of residual TL has not been discussed in detail so far in the literature. In this work, experimentally obtained residual TL signal and its origin is studied for CaSO4:Dy-based TL dosemeter through numerical simulation and role of deeper traps has been discussed.

Development of a technique for improving coefficient of variation of CaSO4:Dy teflon-based TLD personnel monitoring system in low-dose region.

In view of the importance of zero-dose background (null signal) in influencing the coefficient of variation in low-dose region, a technique for the estimation of the same from composite (gross) signal is developed for CaSO4:Dy-based personnel monitoring system being used in India. The technique is based on simple analysis of glow curves (GCs) of unexposed and exposed dosemeters, evolution of trend/model for the zero-dose curves, generation of simulation protocol for individual zero-dose curves, establishment of characteristics of GCs of exposed dosemeters and finally preparation of an algorithm to segregate the components from composite signal. The technique offers the separation of real-time background and gives superior results over other method of approximation of the background. The results also prove efficiency of the empirical trending and simulation protocol of background GCs. The proposed technique can be implemented in routine monitoring without any extra man hours and reader time.

Response of ionization chamber based pocket dosimeter to beta radiation.

Quantitative estimate of the response of ionization chamber based pocket dosimeters (DRDs) to various beta sources was performed. It has been established that the ionization chamber based pocket dosimeters do not respond to beta particles having energy (Emax)<1 MeV and same was verified using (147)Pm, (85)Kr and (204)Tl beta sources. However, for beta particles having energy >1 MeV, the DRDs exhibit measureable response and the values are ~8%, ~14% and ~27% per mSv for natural uranium, (90)Sr/(90)Y and (106)Ru/(106)Rh beta sources respectively. As the energy of the beta particles increases, the response also increases. The response of DRDs to beta particles having energy>1 MeV arises due to the fact that the thickness of the chamber walls is less than the maximum range of beta particles. This may also be one of the reasons for disparity between doses measured with passive/legal dosimeters (TLDs) and DRDs in those situations in which radiation workers are exposed to mixed field of gamma photons and beta particles especially at uranium processing plants, nuclear (power and research) reactors, waste management facilities and fuel reprocessing plants etc. The paper provides the reason (technical) for disparity between the doses recorded by TLDs and DRDs in mixed field of photons and beta particles.

A method of identification of abnormal glow curves in individual monitoring using CaSO4:Dy teflon TLD and hot gas reader.

In the present study, a method of identifying abnormal glow curves to electronically screen the glow curves of TL readout is presented. The method is based on the fact that the shape of an abnormal glow curve differs from the shape of a normal one. A few criteria for defining the normal shape of glow curves are arrived at by analysing the glow curves of dosemeters exposed to various doses in laboratory conditions and read at different elapsed time post irradiation. About 300 glow curves of dosemeters used for monthly monitoring were analysed as per these criteria and the effectiveness of the method is observed for total counts as low as 150 µSv equivalent.

Study of minimum detection limit of TLD personnel monitoring system in India.

Personnel monitoring of radiation workers in India is carried out using a thermoluminescence dosemeter (TLD) system based on CaSO(4):Dy Teflon TLD disc. The dose due to occupational exposure for a majority of radiation workers is very small and hence is reported as zero. In view of this the detection of low levels of occupational dose over and above a variable background assumes great importance. The present values of reporting levels are based on the standard deviations of annealed dosemeters and therefore are fixed irrespective of period of use and background radiation levels. The validity of these levels is investigated under laboratory conditions. The laboratory values of standard deviations cannot be used as an indication of the imprecision that occurs during service. Therefore, the validity of the reporting levels is also investigated for control dosemeters used in routine service.

Development of an algorithm for evaluating personal doses due to photon fields in terms of operational quantities for TLD badge system in India.

In order to evaluate and report the personal doses in terms of personal dose equivalent, the performance of the CaSO(4):Dy based thermoluminescence dosemeter (TLD) badge used for countrywide personnel monitoring in India is investigated using monoenergetic and narrow spectrum radiation qualities equivalent to those given in ISO standards. Algorithms suitable for evaluating H(p)(10) and H(p)(0.07) within +/- 30 % are developed from the responses of dosemeter elements/discs under different filters for normal as well as angular irradiation conditions using these beams. The algorithm is tested for TLD badges irradiated to mixtures of low- and high-energy ((137)Cs) beams in various proportions. The paper concludes with the results of test of algorithm by evaluation of badges used in the IAEA/RCA intercomparison studies and discussion on inherent limitations.