3.2.1 (n,alpha)-reactions

This chapter describes the production processes and production rates of helium. Among the numerous neutron-induced nuclear transmutations, (n, α)-reactions are considered to be important sources for helium production in materials in a neutronic environment. In the (n, α)-reactions between fast neutrons in the MeV energy range and metal nuclei of atomic weight and number, the created α-particles are ejected from the reacting nuclei with energies of some MeV. The corresponding cross-sections depend on the neutron energy. Of particular interest are values for 14 MeV neutrons which result from the deuterium-tritium fusion reaction. Although the fast neutron cross-sections vary from isotope to isotope, they are substantial for all nuclei. The chapter lists helium generation cross-sections for 14.8 MeV neutrons measured by gas mass spectroscopy for various materials. Typical examples for helium and damage production rates are calculated for different first wall materials in conceptual designs of fusion reactors. All rates are normalized to a neutronic wall loading of 1 MW/m² and assume a plant factor of 100%. Light ion irradiation experiments are of increasing interest after it has been recognized that protons, deuterons and α-particles in the 5 to 50MeV energy range can in many cases efficiently be employed to simulate radiation damage effects caused by fast neutrons. Since most of these effects (e.g. mechanical property changes, void swelling) depend on the helium production rate, a knowledge of the nuclear reactions (p, α) and (d, α) cross-sections is essential for comparing data obtained by light ion and neutron irradiations.