The W-chair whole body counter: a Monte Carlo investigation

The W-chair whole body counting geometry, a derivative of the meter-arc counting geometry, has been examined using Monte Carlo simulations to investigate the effect of phantom size on the counting efficiency. Three detector positions were simulated (42 cm, 71 cm, and 100 cm from the chair), with one being very close to the physical counting geometry in use at the Whiteshell Laboratories, to investigate the effects of detector-chair distance. The agreement of the simulations with observed counting efficiencies was within 7% over the energy region of 122 keV to 1,173 keV. The optimum counting geometry was found to be the 71-cm detector-chair distance as this balances both sensitivity and counting efficiency. The range of relative counting efficiencies is 0.94 to 1.13 for all energies studied (126 keV to 2,754 keV) and selected phantom sizes relative to the Reference Man phantom. This range also represents the extent of the activity estimate's uncertainty if no size correction factors are applied during routine counting and shows the W-chair counting geometry to be relatively subject-size independent.