Spectral Unfolding of Mixed Proton/Neutron Fluences in the LANSCE Irradiation Environment

An analysis of activation foils from irradiations at the Los Alamos Neutron Science Center (LANSCE) was used to provide a basis for determination of combined proton and neutron fluxes. Materials samples were irradiated in the A-6 target area at LANSCE to explore the mechanical property changes in a high-energy proton/spallation neutron environment. Proper assessment of the effects of irradiation required quantifying the proton and neutron fluences. Foil stacks consisting of disks of Co, Ni, Fe, Al, Nb and Cu, were irradiated concurrently with mechanical property samples. The irradiation consisted of an 800 MeV, 1 mA proton beam, and a W target in the beam provided a source of spallation neutrons. The maximum proton fluence was 4×10²¹ p/cm² and the maximum neutron fluence was approximately 1×10²¹ n/cm². After irradiation, the foils were withdrawn and the radioactive isotopes sampled with gamma spectroscopy. Initial calculated estimates of the fluences we made with the Los Alamos High-Energy Transport (LAHET) Code System (LCS). The LCS code is a coupling of LAHET and MCNP (Monte Carlo N-Particle) to calculate transport of the high-energy protons, neutrons and photons. From the initial estimates and the measured activation products, STAYSL2 was used to produced a revised spectra of proton and neutron fluences. STAYSL2 is an extension of the STAY'SL code normally used for neutron activation analysis. STAYSL2 has been modified to include proton fluences in the analysis of activation to account for the mixed proton/neutron fluence in the target area. This methodology was applied to multiple foil stacks from different regions of the target in order to profile the spatial dependencies of the fluences.