Just an FYI if anyone else stumbles on to this post looking for reasons for discrepancies of gamma spectra between MCNP and OpenMC like I did:
I observed a noticeable difference, above a few percent, when comparing gamma doses with low energy source (Cs-137) calculated with MCNP and OpenMC. When comparing doses for higher energy sources this discrepancy wasn’t present - likely masked by the high energy contribution to the total dose.
Tracked the origin of the discrepancy down to the nuclear data - mcplib84 in MCNP and EPICS2017 based photo atomic data OpenMC used (Thanks @yrrepy for that insight) in the distribution of ENDF/B-VIII.0. Re-ran the problem with the recently released LANL EPRDATA25 ( EPRData25 | Nuclear Data ) and ENDF/B-VIII.1 from the OpenMC site ( Cross sections - OpenMC ) and the match is basically perfect.
I will take this opportunity to put forward a proposal/suggestion to the OpenMC ND team: A more transparent way to track the origin of the nuclear data would be super useful. I would both note it on the download site and, if possible, actually bake into the hdf5 files themselves. Maybe something to consider for future nuclear library releases? If other users agree, I’ll open a feature request.