CALCULATION OF EFFECTIVE DOSE-EQUIVALENT RESPONSES FOR EXTERNAL EXPOSURE FROM RESIDUAL PHOTON EMITTERS IN SOIL

Effective dose-equivalent responses have been calculated for external exposure from residual photon emitters in soil. The calculations are based on the assumption that the receptor is located 1 m above the contaminated ground. A Monte Carlo algorithm was developed to perform the photon transport calculation for the soil/air configuration, in which the soil constituents were assumed to be similar to those of the earth's crust. Photon cross-sections for soil were based on the latest cross-sectional information generated by the U.S. National Institute of Standards and Technology. For every incident photon at the receptor, an estimate of the air-absorbed dose was calculated first and then converted into the effective dose equivalent. The effective dose equivalent is based on the concept of weighted organ doses, as recommended by the ICRP. The ICRP's latest conversion coefficients were used to transform point air-absorbed doses into effective dose equivalents. Baseline effective dose responses were obtained for monoenergetic photon sources assumed to be distributed uniformly in soil for energies ranging from 0.01 to 10 MeV, soil thicknesses from 0 to 5 mean-free-path (mfp) lengths, and soil densities from 1 to 2 g cm-3. On the basis of the calculated dose responses, empirical relationships were obtained for correlating the dose responses with the soil depths and densities for each source energy. The derived correlations contain a convenient exponential form that describes the depth-dose relationship for an estimated accuracy within about 15%; soil densities show an inverse relationship with dose responses. Results of the effective dose-equivalent response can be used to calculate effective dose-equivalent responses for gamma-emitting radionuclides that are commonly identified as residual radioactive materials in soil.