Physics of Energetic Particle Interactions

Abstract

This paper is designed to serve as a bridge between preceding papers describing the space radiation environment and ensuing papers dealing with specific effects. Particles of interest include both primary particles present in the space environment (such as protons, heavy ions and electrons) and secondaries (such as neutrons, mesons, X-rays and gamma rays), which are generated by interactions in spacecraft materials. In addition neutrons, X-rays and gamma-rays are frequently used in ground testing to simulate the effects of primary radiation, and are emitted by nuclear power sources.

Relevant reactions are reviewed for all the above and include interactions with atomic electrons leading to ionization and excitation, and nuclear reactions leading to scattering, displacement and the production of secondary particles and induced radioactivity. The terminology used to quantify the interactions is defined and includes stopping power, linear energy transfer, range and reaction cross-section. The dependencies of these on incident particle and target material parameters are described and relationship is made with the parameters used to describe effects such as dose and equivalent displacement damage fluence. Thick target situations require radiation transport calculations to deal with the build-up of secondaries. Such methods are described and illustrated by calculations performed for the Gamma Ray Observatory. In such a heavyweight spacecraft effects are magnified some twentyfold by the build-up of secondaries. Hence predictions for such a system are a good test of the codes. Comparison with early data is very encouraging.