Monte Carlo simulation of a scanning whole body counter and the effect of BOMAB phantom size on the calibration

INTRODUCTION: In Vivo measurements use radiation detectors, such as NaI, that require calibration using a source that is both geometrically and materially similar to the actual sample to be measured. In the case of whole body counting, the sample being measured is a person. Traditionally, BOttle Mannikin ABsorber (BOMAB) phantoms have been used for this purpose. A BOMAB phantom usually consists of ten elliptical containers (bottles) so that when assembled it mimics a human form; the 95-percentile phantom consists of eleven containers, an adjustment made because of weight considerations. The BOMAB phantom has, by consensus opinion, been acclaimed as the de facto standard phantom for the calibration of whole body counting systems and an ANSI standard has since been published (American National Standards Institute/Health Physics Society. Specifications for the Bottle Manikin Absorption Phantom. McLean, VA: Health Physics Society; 1999). The use of different sized BOMAB phantoms is important as it allows a facility to establish a calibration data set that can account for the variability in size of its workforce. For example, a recent study in Canada has shown that physical characteristics of male workers in the Uranium Industry can vary significantly. This study found that the height of male workers varied from 1.5 2.0 m, the weight varied from 50 150 kg, and the chest wall thickness varied from 1.9 6.6 cm over an age range of 23 63 yrs. It is recognised that the purchase and use of a BOMAB phantom family can be both expensive and time consuming. The establishment of all the calibration curves corresponding to each phantom and over a wide energy range would requires a facility to: 1) purchase radioactive standards that cover the required energy range, 100 keV2 MeV. 2) Fill each phantom with a single radionuclide. 3) Develop the calibration sets. 4) Establish a correction factor methodology for persons of intermediate height and weight. However, there is an inexpensive alternative to the above: the counting efficiency of a whole body counter can be determined by Monte Carlo simulation. This paper has applied this technique to the HML’s scanning detector whole body counter using all of the phantoms in the BRMD BOMAB phantom family.