Update of 40K and 226Ra and 232Th series γ-to-dose conversion factors for soil

Over the last four decades, the dose conversion factors used to calculate external exposure to γ radiation from soil from 40K and the 226Ra and 232Th chains have been 0.62, 0.042 and 0.46 (nGy/h per Bq/kg), respectively. This work updates these γ-to-dose conversion factors, taking into consideration the composition, depth and radius of the soil source, and importantly, updated branching ratios, with MCNPX simulations. The new conversion factors are 0.036, 0.357 and 0.482 (nGy/h per Bq/kg) for 40K and the 226Ra and 232Th chains, respectively.