MCNP6 simulations of gamma line emissions from fission products and their comparisons to plutonium and uranium measurements

Aqueous solutions containing microgram quantities of special nuclear materials (SNMs) were irradiated for 60 s using a SLOWPOKE-2 reactor, and their delayed gamma (DG) emissions were recorded as a function of count time up to three minutes after irradiation. The irradiation and counting processes were simulated using the production release of MCNP6.1 and the more recent MCNP6.1.1β version, and the line emission option was used to create DGs. In every simulation MCNP6 successfully predicted the 25 most prominent measured fission product peaks for each SNM solution in the 0.1-1.6 MeV energy range. Fission products with gamma emissions greater than 0.6 MeV were selected for a detailed comparison of their relative measured and simulated magnitudes. There were significant fission product peak outliers, several of which were attributed to the outdated ENDF/B VI gamma line data used by MCNP6 for DG emissions. The updated time-bin structure used by MCNP6.1.1β for DG production and its effect on time-dependent measurements are also discussed. Discrepancies between measurements and simulations were further resolved in a final comparison which used MCNP6.1.1β alongside updated gamma line data files.