Hydrogen and helium production in structural materials by neutrons

Hydrogen and helium are produced when energetic neutrons interact with materials, and these gases can lead to significant changes in materials properties such as embrittlement and swelling. Such effects have been seen in fission reactors and a significant effort has been made for the development of fusion reactors where the effects are expected to be larger because of the higher neutron energy. For the Advanced Fuel Cycle Initiative, new structural materials are proposed, and the amount of gas production must be known to assess the properties of these materials under radiation damage. We are measuring the production cross sections for these gases by neutrons in the energy range from threshold to 100 MeV at the Los Alamos Neutron Science Center/Weapons Neutron Research (LANSCE/WNR) spallation source of fast neutrons. We measure the light charged particles (protons, deuterons, tritons, 3He and alpha particles) emitted as a function of incident energy and angle and then integrate the angular distributions to obtain production cross sections. Results for the higher neutron energies are relevant to accelerated radiation damage facilities based on spallation neutron sources. The data measured for tantalum show a monotonic increase in the production cross sections of hydrogen and helium with neutron energy, whereas for iron and chromium, the cross sections flatten out above 50 MeV. Nuclear data evaluations often do not account well for the excitation functions. (author)