Monte Carlo simulations of cold neutron spectra for various para- and ortho-hydrogen ratios using different codes and nuclear data libraries

This work compares simulated and measured neutron time-of-flight spectra for a cold neutron moderator with varyingpara-hydrogen concentrations (25%, 50%, 90% and 99.9%) embedded in a polyethylene thermal moderator. The primary neutronsare generated from the interaction of 45MeV protons with a tantalum target. The simulations were performed using several MonteCarlo codes (MCNP, PHITS, McStas, VITESS, and KDSource) together with nuclear data from the ENDF/B-VII.1 and JENDL−5.0libraries. The simulated primary neutron yields had deviations from experimental measurements ranging from 0.3 to 16% dependingon the code and the nuclear data used. The neutron moderation in the para-hydrogen moderator coupled with a neutron guide wasthen modeled. The neutron time distribution was measured by a 3He detector at the end of the guide. Comparison with experimentaldata showed good agreement, with relative differences of less than 15%. For the 99.9% para-hydrogen concentration, simulationswith JENDL−5.0 were in better agreement with the experimental data, while ENDF-B/VII.1 showed better agreement for the 25%para-hydrogen case. The analysis of the results obtained provides insights into the strengths and limitations of each Monte Carlo codeand nuclear data library combination. The observed discrepancies were analyzed, and possible sources of error were also identified.The analytical procedure followed in this work will help to improve the accuracy and reliability of neutron cold moderator design.