Effective dose equivalents for photon exposures from plane sources on the ground

The calculation of effective dose equivalents for photon exposures from plane sources on the ground has been improved by accounting for the energy-dependent shielding of rough surfaces, the energy and angular characteristics of the photon field, and the time dependence of the nuclide composition due to the production of daughter nuclides by radioactive decay. The spectral distribution of the photon energy fluence, with respect to energy and solid angle, from monoenergetic plane sources located at a depth of 3 mm in the soil was calculated for a height of 1 m above the air-ground interface. Organ doses were obtained by integration of the product of this distribution with conversion factors, which give the organ dose per energy fluence for a monoenergetic irradiation of an anthropomorphic phantom at a certain angle. The improvements lead to up to 40% higher organ dose conversion factors for source energies above 200 keV and to considerably lower conversion factors for source energies below 50 keV. The time-dependent contributions from daughter nuclides were calculated. (author)