Kerma factors for calculating the absorbed dose of 15–150 MeV neutrons

Abstract

The absorbed dose is the principal characteristic of the radiative interaction of ionizing radiation with matter. It is of special importance to be able to predict accurately its distribution in the biological tissue of patients undergoing a course of radiation therapy and also to determine the operating lifetime of the particle detectors and electronic equipment operating under conditions of a high radiation level.The absorbed dose of neutrons with energies lower than t00-150 MeV (D, GR) is calculated with satisfactory accuracy in the kerma approximationwhere F(E) is the fluence energy distribution in units of 1 cm-2.MeV -t , and k(E) is the kerma factor in units of 1 GR.cm 2.A large number of experimental and calculated data on the neutron kerma factors are presently known for the principal elements of tissue (see for instance [1]). Although the minimum error of the estimated kerma factors for energies in the region of 50 MeV for tissue elements is -20%, based on analyzing data from more than 20 studies, the real errors are considerably greater. In addition the errors increase as the neutron energy increases. It is therefore very important to improve the accuracy of the kerma factors for carbon, oxygen, and nitrogen.Data on kerma factors for the elements from which the particle detectors and electronics are composed are extremely scarce and their reliability is poor. At the same time, one of the main factors limiting the possibilities of operating the existing and planned apparatus is the change in the physical and chemical properties of their materials under the action of radiation. It is therefore an urgent problem in radiation physics to obtain information to enable one to predict, while still in the planning stage, the radiation loadings and service lifetimes of the experimental equipment. METHOD OF CALCULATIONThe kerma factor is defined as the sum of the initial kinetic energy of the charged particles created in elastic and inelastic interactions of a neutron with a nucleus, normalized to unit mass of the substance and unit fluence.Elastic scattering introduces an important contribution to the kerma factor for light isotopes and for neutron energies below 70 MeV. For example, in the case of carbon and for E n = 20 MeV it constitutes about 25%. In the work now presented the elastic scattering component for the kerma factor was taken into account for neutron energies below 70 MeV. (In the case of hydrogen the kerma factor is completely determined by elastic scattering, independently of the neutron energy.) The recoil energy of the nuclei was calculated from the well-known kinematic relationships, taking account of their angular distributions defined in terms of the Glauber model. The cross sections for the elastic scattering of neutrons by different nuclei were taken from the Sadko-2 library of nuclear data [2]. However, in the investigated range of energies the kerma factors for all nuclei with the exception of hydrogen are determined by inelastic scattering.The kerma factors for neutrons of energy less tha...