Abstract / Summary
Organ absorbed-dose-rate coefficients for natural radionuclides in soil depend on radionuclide emissions, source geometry and the anatomical model used for radiation transport. Using Geant4 and the ICRP Publication 110 adult female and male voxel phantoms, we calculated coefficients for 40 K, the 238 U series and the 232 Th series for activity distributed uniformly in a soil cylinder of 25 m radius and 3 m depth. Only primary nuclear gamma rays were emitted by the source. We did not include source-emitted characteristic X rays, charged particles, or neutrons. This also excludes the annihilation photons and bremsstrahlung that would have arisen from the omitted charged particles. Secondary particles generated during transport were tracked using the configured physics and production settings, and their energy deposition was included in the scoring. The soil was partitioned into radial and depth intervals to quantify the spatial contribution to each coefficient. Across the eight scored quantities, the coefficients ranged from 0.0231 to 0.0326, 0.2378 to 0.3412, and 0.3241 to 0.4789 nGy h −1 per Bq kg −1 for 40 K, the 238 U series and the 232 Th series, respectively. Ratios between the male and female reference-phantom coefficients ranged from 0.90 to 1.09, with the largest differences for the kidneys. Ninety percent of each coefficient originated within outer radii of 8–18 m and within the upper 0.30 m of soil. The results provide reference-phantom-specific organ absorbed-dose-rate coefficients and identify the soil volume contributing most strongly to external organ irradiation.