Helium and hydrogen generation in pure metals irradiated with high-energy protons and spallation neutrons in LANSCE

High-power spallation neutron sources will require accurate estimates of cross-sections for generation of He and H in structural materials. At high-proton energies, very high levels of gas atoms are generated in all constituents of typical iron-based and nickel-based structural alloys, with He typically ∼150 appm/dpa and H at levels ∼3–5 times higher. Improved estimates of these cross-sections have been derived from a series of irradiations conducted at relatively low temperatures (<100 °C) in the Los Alamos Neutron Science Center as part of a test program supporting the Accelerator Production of Tritium Program. Pure metal dosimetry foils were irradiated in two different spectra ranging from ∼800 MeV protons to a mixed distribution of both protons and spallation neutrons. Most of the gas production was due to spallation reactions with the proton beam, although gas and especially damage production from lower-energy spallation neutrons became more significant at the mixed proton/neutron location. The measured He concentrations are similar to those derived in other proton environments, but larger by about a factor of two than those calculated using the LAHET/MCNPX code system. Unlike He, the measured H retention levels are affected by diffusional losses, but H is still retained at rather high concentrations, allowing a lower bound estimate of the H generation cross-sections.