Calculation of Radiation Dose from Protons to 400 MeV

Abstract

The Monte Carlo technique has been used to obtain estimates of dose and dose equivalent in a homogeneous tissue slab irradiated by a broad beam of normally incident monoenergetic protons and also for an isotropic flux of protons incident on a homogeneous parallelepiped of tissue. Incident proton energies as high as 400 MeV are considered. The dose is essentially uniform above 200 MeV, where average primary proton ranges become large compared with the mean track lengths of primary protons in the target. At all proton energies the ratio of rem and rad doses is generally about 1.4 if rem dose estimates are based on values of RBE in current use for long-term occupational exposure to ionizing radiation. The contributions to dose from various ranges of LET values which arise in the calculations are presented and discussed. The nuclear interactions considered are cascade and evaporation phenomena. The effect of the presence of pions is assumed to be negligible at incident proton energies not exceeding 400 MeV. An analysis of nuclear interactions is made from the standpoint of dose assessment. From the results of the detailed Monte Carlo calculations a simplified model is derived which permits a rapid and accurate estimation of proton dose. A method of handling RBE in a flexible manner is given also.