The StandFast whole body counter: Efficiency as a function of BOMAB phantom size and energy modeled by MCNP5

The StandFast whole body counter has been modeled using Monte Carlo simulations to examine the effect of phantom size, photon energy, and position of the phantom within the counting enclosure on the counting efficiency. The first geometry, the manufacturer's recommended positioning, was found to have the higher counting efficiencies and the most dependence on phantom size. The second position, where the phantom is at the back of the counting enclosure, had lower counting efficiencies, and hence higher minimum detectable activities, by a factor of between 1.3 to 2.1 when compared with the first geometry; however, for emergency response where accuracy is to be preferred over sensitivity, this geometry would be the better choice. A unified calibration equation was also developed for the StandFast so that it is possible to predict the counting efficiency as a function of photon energy and size to within 11%.