Charged Particle Slowing Down Spectra and Energy-Loss Distribution Resulting from Fast-Neutron Irradiation of Water

Energy spectra have been computed for protons, alpha particles, and their secondary electrons slowing down in water irradiated by 14.6-MeV neutrons. Spectra for protons and alpha particles were based on continuous slowing down theory. Anisotropy of the proton-recoil reaction and elastic nuclear collisions of charged particles were found to have negligible influence on energy spectra and the energy-loss distribution. Partitioning of the neutron first-collision dose rate among the three particles was found to be very sensitive to the cutoff energy for production of secondary electrons. An analysis based on treatment of a collisional energy loss of less than 200 eV as localized energy dissipation along a particle track showed that localized electronic energy loss is distributed among protons, alpha particles, and their secondary electrons in the respective fractions 0.530, 0.112, and 0.358.