Dose and Fluence Distributions of the Primary and Secondary Particles in Biological Material Irradiated by $^{252}$Cf Fission Neutrons and d-Be Generated Neutrons

For understanding the biological effects of neutrons, predictions of the secondary charged particle distributions by neutron irradiation are needed in biophysical models. We have performed detailed Monte-Carlo simulations using the PHITS computer code of the the dose and fluence spectra of charged particles in the biological materials irradiated by neutron beams with energies below 10 MeV. We compare the results for two different neutron spectra used in radiobiology experiments; the spontaneous fission neutron spectrum of $^{252}$Cf, and a 4 MeV d-Be generated neutron spectrum. The results show that over 90% of the dose and fluence are from secondary protons, which are low energy (<2 MeV) and high LET, and indicate higher secondary charged particle fluence near the surface compared to the deep tissue regions in a mouse. It is also suggested that the different neutron sources considered result in largely similar types of secondary particles with modestly varying fluence distributions.