Calibration of Phoswich detector for the measurement of natural uranium in lungs

Abstract

The occupational workers of fuel fabrication and reprocessing facilities at Indira Gandhi Center for Atomic Research (IGCAR) have a potential for internal exposure to natural uranium which is hazardous both in chemical and radiological aspects. Hence, in vivo monitoring of the radiation workers has to be carried out to ensure safe working conditions. In IGCAR, the in vivo monitoring of natural uranium is being carried out using Phoswich-based lung monitor. The measurement and quantification of internal exposure due to natural uranium is done using 63 and 93 keV photons emitted by 234 Th, immediate daughter of 238 U. Realistic anthropomorphic Lawrence Livermore National Laboratory (LLNL) phantom is used for the calibration of the system. As natural uranium-loaded lung set is not available currently in the laboratory, lung set loaded with indigenously prepared natural uranium source capsules were used. Efficiency curve for 238 U was established for varying muscle equivalent-chest wall thickness (MEQ-CWT) and the efficiency values were in the range of 5.604E-03 to 8.601E-03 CPS/Bq. Simulation results of LLNL voxel phantom having uniform lung distribution of natural uranium agreed within 9% with the measured one which is comparable with the error associated with measurement. This confirms that the distribution pattern of 12 capsules in each lung in the given geometry closely resembles the uniform distribution. The efficiency and minimum detectable activity (MDA) values for Indian population were found to be ranging from 1.011E-03 to 7.931E-03 CPS/Bq and 12–18 Bq, respectively. The efficiency value for 238 U established from the measurement using Japan Atomic Energy Research Institute (JAERI) phantom having uniform source distribution agreed with that of LLNL phantom measurement having capsule source distribution for the same MEQ-CWT thickness (33.8 mm) within 3%. This further reaffirms that the adopted capsule distribution is close to uniform distribution.