Depth-dose relationships near the skin resulting from parallel beams of fast neutrons

Monte Carlo calculations were undertaken to permit comparison of kerma and absorbed dose within the first 0.5 cm of penetration of fast neutrons incident normally on a tissue slab phantom. Various neutron energies from 0.1 to 20 MeV were considered. Dose-equivalent values were also calculated. It was shown that only for incident neutron energies of 5 MeV and above is there any marked difference between absorbed dose and kerma (in equivalent units). The variation is shown to be due largely to lack of equilibrium for recoil protons until the penetration distance equal to the range of the most energetic proton is reached. Skin absorbed dose for 14-Mev incident neutrons, for example, is calculated to be about 38% of the kerma (in equivalent units). This is somewhat lower than the experimental figure previously published, but qualitative trends are similar