Calculated Depth Dose Curves in Tissue for Broad Beams of Fast Neutrons

Abstract

When tissue is irradiated by fast neutrons with energies extending up to 10 MeV, the neutrons lose their energy primarily by elastic collisions with the atoms composing the tissue. For neutron energies above a few MeV inelastic scattering may occur and increases in importance as the neutron energy increases, but below 10 MeV inelastic scattering should not be of major importance since (1) it is entirely absent for scattering with hydrogen and collisions with hydrogen predominate, and (2) even for such collisions the energy released by the corresponding γ ray is, on the average, a small fraction of the energy carried by the recoil atom. In this study inelastic scattering has been neglected since elastic collisions are the principal process by which the neutron loses its energy. As a result of an elastic collision the struck atom acquires kinetic energy which is dissipated by ionization, excitation and elastic collisions with other atoms in tissue. More rarely a nuclear reaction takes place, which results in the emission of a γ ray, a proton, or an α particle. The recoil atoms, protons and α particles lose their energy within a small fraction of a centimetre of the point of collision and thus the energy imparted by a neutron may be considered as absorbed at the collision site.