Determination of the body burden using a whole-body counter

Abstract

A whole-body counter (WBC) of' chair geometry configuration has been developed and calibrated for in-vivo measurement of human radioactive body burdens enabling assessment of internal radiation doses. The counting system consists of NaI (T1) crysta 1 of 12.7 cm x 12.7 cm and standard assdciated electroniris including a multichannel analyser. In order to measure low levels of body radioactivity compar?ble to annual limits of intake (ALI) within a reasonable c~unting time of 30 mins, the crystal detector .has been shielded by an appropriate combination of Pb and Fe of 5 cm total thickness of annular shape. The optimum shielding thickness has been chosen 'from a series of experiments carried out using various thickness and combination of Fe and Pb. The seating chair has also been covered by 1.5 cm thick Fe sheet which acts as a back shielding. Under the,above mentioned shielding arrangement a background reduction upto a factor of 6 depending on the energy has been achieved. The calibration studies of the detecting system were made using a human whole-body phantom and two other partial body phantoms namely Trunk-Pelvis and Pelvis simulating roughly the human body.and parts the~eof respectively. The calibrat10n data of sensitivity, efficiency etc. of the dtecting system and the minimum detectable amounts (MDA) of various gamma emitting radioisotopes covering a wide range of energies from 0.30 1;0 2.62 MeV have been obtained under various experimental ","~-;,".~' conditions. The estimated values of MDA of different radioisotopes with of 95.5% confidence limit were within the ALI values for a countinq time of 30 mins. The detecting system has also been used for the d~termination of radioactive body burdens in human subjects working in gas mantle. industries using thorium nitrate. The detecting system has been calibrated for the determination of total body potassium from the measurement of 1.46 MeV Gamma of 40K.