Estimation of uncertainty in lead spallation particle multiplicity and its propagation to a neutron energy spectrum

This paper presents an approach to uncertainty estimation of spallation particle multiplicity of lead (Pb-nat), primarily focusing on proton-induced spallation neutron multiplicity (x(pn)) and its propagation to a neutron energy spectrum. The x(pn) uncertainty is estimated from experimental proton-induced neutron-production double-differential cross-sections (DDXs) and model calculations with the Particle and Heavy Ion Transport code System (PHITS). Uncertainties in multiplicities for (n, xn), (p, xp), and (n, xp) reactions are then inferred from the estimated x(pn) uncertainty and the PHITS calculation. Using these uncertainties, uncertainty in a neutron energy spectrum produced from a thick Pb-nat target bombarded with 500 MeV proton beams, measured in a previous experiment, is quantified by a random sampling technique, and propagation to the neutron energy spectrum is examined. Relatively large uncertainty intervals (UIs) are observed outside the lower limit of the measurement range, which is prominent in the backward directions. Our findings suggest that a reliable assessment of spallation neutron energy spectra requires systematic DDX experiments for detector angles and incident energies below 100 MeV as well as neutron energy spectrum measurements at lower energies below similar to 1.4 MeV with an accuracy below the quantified UIs.