Dose-rate conversion factors for external exposure to photon emitters in soil.

Dose-rate conversion factors have been calculated for external exposure above ground to monoenergetic photon emitters in soil. These factors give external dose rates per unit source concentration in soil. The calculations are based on the point-kernel integration method and assume that the source concentration at any depth in soil is uniform over an infinite surface parallel to the ground plane. Dose-rate factors in air at a height of 1 m above ground are tabulated for discrete photon energies between 0.01 and 10 MeV and for source depths in soil between 0 and 300 cm. Application of the results for plane sources in soil to the calculation of photon dose rates from distributions of sources with depth in soil is described, and dose-rate factors are tabulated for the particular cases of uniform slab sources of finite thickness and sources which are exponentially distributed with depth. We also demonstrate how dose-rate factors in air for monoenergetic photon sources are used to estimate dose-rate factors for body organs of exposed individuals and for the spectrum of photons from radioactive decay. The calculations in this paper show that allowing for downward migration of radionuclides in soil can result in significant reductions in external dose compared with the usual assumption that radionuclides which are deposited on the ground surface remain there until removal by radioactive decay.