Erroneous/Unexpected Behavior with Photon Scattering Tallies

[nelsonag]

Bug Description

Tests of the photon tally results were performed to understand how close the photon tallying system is to being used to create MGXS library. The following deficiencies were noted:

• Pair Production is slightly non-zero when it should be zero (addressed by Fix for erroneously non-zero tally results of photon threshold reactions #3242)
• Scatter production (nu-scatter) scores are zero. To be consistent with the neutron score, they should be equal to or greater the result received with the scatter score.
• P_1 scores are clearly erroneous (~1E-310) for scatter, coherent-scatter, incoherent-scatter, photoelectric, pair-production (addressed by Enable the LegendreFilter filter to be used in photon tallies for orders greater than P0. #3245)

Steps to Reproduce

See the attached .zip file for a model.xml, orig_tallies.out (before #3242) and new_tallies.out (after #3242)

Environment

• Running on an up-to-date Arch Linux with GCC 14.2.1.
• Executing a locally compiled instance of the develop branch of OpenMC, up-to-date as of 1/5/2025.
• Run using the endf b 7.1 data set from www.openmc.org

[nelsonag]

The scatter production issue (box 2) is because p.mu is not used during photon kinematics routines and instead a local mu variable is used. That's an easy fix. I can't fathom why p.mu isn't used besides this just being lost in the transition from Fortran to C which was happening at nearly the same time as photon physics were added.

I can get this fixed soon.