Absorbed dose, dose equivalent and LET distributions in cylindrical phantoms irradiated by a collimated beam of monoenergetic neutrons.

TLDR: Dose from recoil ions averaged over a region 1 cm thick, immediately outside the volume of the incident beam, was found to be lower by- about an order of magnitude as compared with dose at comparable depths within the beam.

Abstract: A Monte Carlo code was used to study dose and LET distributions, in a specially designed tissue-equivalent cylindrical phantom, due to cylindrical beams of neutrons collimated to diameters of 5 cm. Dose due to ions produced by neutron interactions and dose due to (n, gamma) interactions for neutrons of energy 0.025 eV and 1, 5, 7 and 14 MeV are determined separately and are compared with broad-beam results. LET spectra are given for selected regions inside the phantom. Doses at the front of the phantom, in the center of the restricted beam, were found to approximate those of a broad beam for 14 MeV neutrons and to decrease with decreasing neutron energy, converging to a value of 80–85% of those for broad beams at 1 MeV. Dose from recoil ions averaged over a region 1 cm thick, immediately outside the volume of the incident beam, was found to be lower by- about an order of magnitude (for all energies except thermal, where a factor of 4 was observed) as compared with dose at comparable depths within the beam.