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.
