A solution for 40K interference from K internal to the human body, in an underground high sensitivity whole body counter at the Waste Isolation Pilot Plant, Carlsbad, NM, USA

The objective of the present work is to find a solution for the interference arising from 1461 keV photons emitted by 40K in natural K internal to the human body in the detection of 241Am and 210Pb activities at the level of 37 mBq (1 pCi). The response of a broad energy Ge (BEGe) reference crystal to 1461 keV photons was simulated using GEANT4 code. Simulations were performed for a point source without and with shielding, as well as for a Bottle Manikin Absorption (BOMAB) phantom in the shielded whole-body counting chamber (WBC). The WBC facility is located underground at the Waste Isolation Pilot Plant, Carlsbad, NM, USA. The simulation results were discussed with regards to the interference in detecting the 241Am and 210 Pb at the level of 37 mBq (1 pCi). The specific conclusion is that the interference counts from 40K activity at the natural levels present in the human body in typical clinical settings were found to be greater than the 210Pb and 241Am signal counts from 37 mBq (1 pCi), when a 38.1 mm diameter, 25.4 mm thick BEGe reference crystal was used. Our solution for minimizing the interference to the desired precision at the specified levels was found by sacrificing the broad energy response of the counting system.