Calculation of Counting Efficiency for 137Cs Using Voxel Phantoms with Lungs and a Skeleton

Calibration of whole-body counters for 137 Cs is generally conducted with physical phantoms which are uniformly filled with 137 Cs solutions. To check the conventional calibration, two types of mathematical voxel phantoms were prepared; the phantom with uniform density and that with lungs and a skeleton. Five different sizes, from newborn to adult were prepared for each type of the voxel phantoms. As for a whole-body counter at NIRS (Japan), counting efficiencies for these voxel phantoms were calculated using a Monte Carlo simulation code. The counting efficiency for the phantom with the lungs and skeleton was larger than that for the uniform density phantom for each size. The difference of counting efficiency ranged from 3 to 5% for five sizes. For the adult, it was about 5%. This indicates that the conventional calibration phantoms, assuming uniform activity distribution and uniform density, were acceptable for 137 Cs whole-body counting.