Improved calculations of energy deposition from fast neutrons

Increased interest in fast neutrons for radiotherapy has led to a need for accurate measurements and calculations in the energy region up to 100 MeV. Experimental neutron cross sections on elements of biological interest in the region above 15 MeV are sparse. Moreover, until recently, no secondary particle production spectra had been measured in this energy range. New data from the University of California at Davis has allowed cross-sectional calculations to be more firmly based. This in turn results in improved calculations of dose and energy deposition in small sites from high-energy neutrons. Secondary particle production spectra due to neutrons incident on carbon and oxygen are calculated using an intranuclear cascade code which takes into account specific nuclear properties of carbon and oxygen (in particular, alpha-particle clustering). It also allows excited nuclei to deexcite by Fermi breakup rather than the more frequently used statistical evaporation mechanism, which is not applicable to low-mass nuclei. The code yields double-differential secondary particle spectra in energy and angle which show good agreement with the experimental data from the University of California at Davis at 27, 40, and 61 MeV