Comparison of Indian BOMAB and ICRP voxel phantom for calibration of shadow shield whole body counter.

In-vivo whole body monitors are calibrated with various types of phantoms like Bottle Mannikin absorption phantom (BOMAB), IGOR phantom, Masonite cut sheet phantom, realistic numerical phantoms (NORMAN, ICRP voxel phantom). These phantoms contain either point sources, rod sources, uniform source or radionuclides distributed in soft tissues etc. In this study the efficiency values of Shadow Shield wholebody counter (SSC) for 137Cs and 60Co obtained theoretically using ICRP adult voxel male reference phantom (ICRP-AM) having sources distributed in soft tissues and muscles ICRP-AM(S) are compared with the measured efficiencies of Indian BOMAB phantom having mid axial source distribution (BOM-I(A)). The results show that the efficiency value of BOM-I(A) for 137Cs and 60Co is 30% and 20% lower respectively compared to that of ICRP-AM(S). This is due to the variation in size, composition and source distribution between the two phantoms. Study using Indian Voxel phantom shows that the increase in the size of ICRP-AM contributed to an increase of 14% for 137Cs. In case of 60Co the size did not have any influence. Uniform distribution has 9% and 17% higher efficiency than axial distribution for 137Cs and 60Co respectively as shown by the simulation study using uniformly filled Indian BOMAB phantom (BOM-I(U)). The actual tissue composition and source distribution in soft tissue as shown by Indian voxel has efficiencies 12-14% higher compared to BOM-I(U). The systemic error due to the axial source distribution is recognized and quantified to be 22-25% lower compared to that of a realistic phantom with radionuclides in soft tissue and muscle. This study has resulted in an efficiency of the system using Indian realistic Voxel phantom. The efficiencies are 0.65 CPS/kBq for 137Cs and 0.49 CPS/kBq for 60Co.

Monte Carlo simulation of an HPGe detector for detection of 239Pu in waste assay system.

This paper discusses the modeling of HPGe detector used in waste assay system using Monte Carlo code FLUKA to determine its detection limits. The modeled detector is validated using experimental point sources efficiencies in the energy range 60 keV-2000 keV. The numerical estimation of efficiency values for 239Pu present inside a 230 L cotton filled waste drum at various locations along the axis of the detector is discussed. Estimation of the minimum detectable activity (MDA) for 239Pu at these locations in the presence of 1 MBq of 137Cs/60Co again along the axis of the detector is examined. The efficiency of 239Pu decreases from 6.1 × 10-10 to 7.5 × 10-11 cps/Bq as the location of 239Pu moves away from the detector. MDA of 239Pu increases as the position of 239Pu and 137Cs/60Co in the waste drum shifts away from the detector while it decreases as the location of 137Cs/60CO moves away from 239Pu along measurement axis. It is observed that for all locations of 137Cs/60Co with 239Pu located up to 200 mm from the edge of the drum, the detection limit is within 140.3 mg (allowed limit).

Comparison of two anthropomorphic phantoms as a calibration tool for whole-body counter using Monte Carlo simulations.

The whole-body counting facility at the Indira Gandhi Centre for Atomic Research uses an in-house built Masonite cut-sheet phantom for the calibration of whole-body monitors. Recently, an Indian Adult BOMAB phantom was procured as an additional utility to augment the facility. The present study is to generate full-energy peak efficiencies (FEPE) of the shielded chair (SC) whole-body counting system using the new BOMAB phantom through Monte Carlo (MC) simulations. The values are compared with that of the Masonite phantom. First, the SC was modelled along with the Masonite phantom to estimate the FEPE values and Compton scattering factors (CSFs) for different energies. The simulated values were validated against the measurements using the Masonite cut-sheet phantom (Masonite phantom). The validated SC model was used along with the Indian adult BOMAB phantom to estimate the FEPEs and the CSFs. The simulated BOMAB phantom values were compared with the simulated Masonite phantom values. The maximum deviation for both the FEPEs and CSFs was ±10% validating the use of the Masonite phantom as a calibration tool representing an Indian adult.

Prediction of background in low-energy spectrum of Phoswich detector.

In vivo monitoring of actinides in occupational workers is done using Phoswich detector by measuring the low-energy X ray and gamma rays. Quantification of actinides like plutonium and americium in the lungs is extremely difficult due to higher background in the low-energy regions, which is from ambient background as well as from the subject. In the latter case, it is mainly due to the Compton scattering of body potassium, which varies person-to-person. Hence, an accurate prediction of subject-specific background counts in the lower-energy regions is an essential element in the in vivo measurement of plutonium and americium. Empirical equations are established for the prediction of background count rate in (239)Pu and (241)Am lower-energy regions, called 'target regions', as a function of count rate in the monitoring region (97-130 keV)/(40)K region in the high-energy spectrum, weight-to-height ratio of the subject (scattering parameter) and the gender.

Calibration of phoswich-based lung counting system using realistic chest phantom.

A phoswich detector, housed inside a low background steel room, coupled with a state-of-art pulse shape discrimination (PSD) electronics is recently established at Radiological Safety Division of IGCAR for in vivo monitoring of actinides. The various parameters of PSD electronics were optimised to achieve efficient background reduction in low-energy regions. The PSD with optimised parameters has reduced steel room background from 9.5 to 0.28 cps in the 17 keV region and 5.8 to 0.3 cps in the 60 keV region. The Figure of Merit for the timing spectrum of the system is 3.0. The true signal loss due to PSD was found to be less than 2 %. The phoswich system was calibrated with Lawrence Livermore National Laboratory realistic chest phantom loaded with (241)Am activity tagged lung set. Calibration factors for varying chest wall composition and chest wall thickness in terms of muscle equivalent chest wall thickness were established. (241)Am activity in the JAERI phantom which was received as a part of IAEA inter-comparison exercise was estimated. This paper presents the optimisation of PSD electronics and the salient results of the calibration.