MONTE CARLO STUDY ON UNCERTAINTY DUE TO POSITIONING IN A FULL BODY COUNTER (Atena Editora)

Abstract

The objective of this study was to evaluate the uncertainties due to the displacement of the occupationally exposed individual (IOE) in a full-body counter using the MCNP6 Monte Carlo code. The whole body counter (CCI) of an in-house dosimetry laboratory was modeled in MCNP6. The adult male ICRP reference phantom (RCP_AM) was positioned within the CCI at sixteen different positions along the longitudinal axis of the model at 5 cm intervals. The detection efficiency was evaluated at each position for monoenergetic photons of 100, 300, 500, 1,000, 1,500 and 2,500 keV. Two biodistribution configurations were simulated: source uniformly distributed in the soft tissues or lungs of the RCP_AM. In each case, the position with the highest efficiency was considered a reference for in vivo monitoring, and the error of the other positions in relation to the reference was calculated. Greater efficiency was found in the center of the abdomen in the lumbar spine region (P11) for soft tissue sources. Shifts of up to 10 cm from the RMP resulted in differences of less than 2.5% for all energies tested, except for 2500 keV, where the differences were less than 5%. In cases with a source in the lungs, IOE positioning errors were greater. Errors of less than 5% were observed only in the 5 cm band around the monitoring reference position, which was located over the sternum (P5), except for 100 keV photons, for which errors reached 6%. It was possible to conclude that errors due to displacement in the longitudinal axis will depend on biodistribution and must be greater when the activity is concentrated in a specific organ. Other biodistributions must be studied in the future, as well as errors arising from displacements of transverse axes.