  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.
