Application of optimized geometry for the Monte Carlo simulation of a gamma-ray field in air created by sources distributed in the ground

摘要

An optimized geometry for the Monte Carlo simulation of a gamma-ray field in air created by any source distribution in the ground is applied. The soil air medium of propagation is modeled using an optimized geometry that depends on the soil depth as the sole parameter. When this geometry is implemented, it is possible to track only those photons that are most likely to be detected. The variance in the radiation estimators is consequently reduced. The absorbed dose rates in air at 1 m from the ground for a uniform source distribution are studied. The conversion factors for the natural series of Th-232, U-238 and K-4 are calculated and compared with previous calculations and with experimentally deduced values. The conversion factors for Cs-137 sources exponentially distributed in the ground are calculated based on the results for planar sources at various depths. According to the optimization performed here, infinite planes are reduced to discs of decreasing radius. The obtained results are in agreement with the reference studies, although the size of the geometry-detector system was considerably reduced in this work. For the uniform distribution, the best agreement was achieved with the Monte Carlo results adopted by the International Commission on Radiation Units and Measurements (ICRU), although the emission volume of soil used in our case was 30,000 times lower. (C) 2014 Elsevier Ltd. All rights reserved.