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. - Highlights: • Conversion factors γ-to-dose for soil was studied. • An updates of these γ-to-dose conversion factors, taking into consideration importantly, updated branching ratios. • MCNPX simulations are applied