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

Abstract

This work compares simulated and measured neutron time-of-flight spectra for a cold neutron moderator with varying para-hydrogen concentrations (25%, 50%, 90% and 99.9%) embedded in a polyethylene thermal moderator. The primary neutrons are generated from the interaction of 45MeV protons with a tantalum target. The simulations were performed using several Monte Carlo codes (MCNP, PHITS, McStas, VITESS, and KDSource) together with nuclear data from the ENDF/B-VII.1 and JENDL\(-\)5.0 libraries. The simulated primary neutron yields had deviations from experimental measurements ranging from 0.3 to 16% depending on the code and the nuclear data used. The neutron moderation in the para-hydrogen moderator coupled with a neutron guide was then modeled. The neutron time distribution was measured by a \(^3\)He detector at the end of the guide. Comparison with experimental data showed good agreement, with relative differences of less than 15%. For the 99.9% para-hydrogen concentration, simulations with 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 code and 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.