Monte Carlo study of the neutron time emission spectra at the Manuel Lujan Jr. Neutron Scattering Center

A campaign of neutron-beam characterization measurements was carried out in 2001–2002 at the Manuel Lujan Jr. Neutron Scattering Center (Lujan Center) at the Los Alamos Neutron Science Center (LANSCE), Los Alamos National Laboratory. These experiments provided a valuable comprehensive data set to define the neutron-beam characteristics of the Lujan Centerʹs target–moderator–reflector–shield assembly. To validate the current computer models of the target assembly, we carried out multiple Monte Carlo calculations of the neutron time emission pulses. The results of our simulations are directly compared with the experimental data characterizing five of the six moderators installed in the Lujan target–moderator–reflector–shield system. Our results not only present an important benchmark test of the MCNPX Monte Carlo transport model of the target assembly, but also show the capabilities of the MCNPX code in designing upgrade and future spallation neutron sources.