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

• We apply an optimized geometry model for Monte Carlo simulation of gamma radiation field in air due to ground sources.
• Emission geometry is considerably reduced.
• We calculate conversion factors for different source configurations and for different emission energies.
• We achieve better accuracy than the calculations based on classical geometry.
• We achieve good agreement with the Monte Carlo results adopted by ICRU.

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 232Th, 238U and 4K are calculated and compared with previous calculations and with experimentally deduced values. The conversion factors for 137Cs 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.