Environmental absorbed-dose quantities for natural radionuclides in soil using a simplified block phantom with an ICRP 110 comparison.

Natural radionuclides in soil produce an outdoor photon field, but an organ absorbed dose also depends on source depth, body geometry, scored mass, emission data and charged-particle transport settings. This work calculates absorbed-dose quantities in defined scoring regions for 40K, the 238U decay series and the 232Th decay series with Geant4 and a simplified block phantom. The Geant4 source uses the full nuclear gamma-emission library recovered from the adopted RadioactiveDecay6.1.2 and PhotonEvaporation6.1.2 data. The production calculation consisted of 20 independent replicas and 4.0×109 source histories. Source-specific results are absorbed-dose-rate coefficients, while the adopted-mixture results are scored-region dose rates for the stated activity mixture. The outdoor photon field was compared with a matched FLUKA calculation. The median FLUKA/Geant4 spectral ratio was 1.001 for the silicon-reference case and 0.993 for the realistic-soil case, with most matched nonzero spectral bins within a factor of two. The Geant4 scored-region results give 8 strong, 11 acceptable, 7 low-precision and 2 unresolved scored-region/source quantities by independent-replica uncertainty. Low-precision and unresolved quantities are reported with their precision labels and are not used as stable coefficients. A complementary calculation with the ICRP 110 adult male and female reference phantoms compared the lungs, stomach and colon regions; the ICRP 110/simplified-phantom reconstructed-dose ratios ranged from 0.98 to 1.22. The main coefficient set remains specific to the simplified geometry and is not presented as a set of ICRP reference-phantom coefficients. The added ICRP 110 calculation is limited to the comparison of corresponding lung, stomach and colon regions.