When nbr_split > 1 and Russian Roulette is on, split fluorescent photons resulting from electron impact ionization during Moller.

[blakewalters]

Describe the bug
Currently, when uniform bremsstrahlung splitting (UBS) is turned on with Russian Roulette, we do not split fluorescent photons resulting from atomic relaxations induced by electron impact ionization (EII) during Moller scattering of phat electrons. This results in the emission of phat photons.

To Reproduce

1. Use BEAMnrc to simulate a low energy (e.g., 225 keV) electron beam incident on a tungsten target using uniform brem splitting (UBS) with a splitting number (nbr_split) of, say, 100, and with Russian Roulette on. Ensure that atomic relaxations and EII are both ON.
2. Score phase space below the target
3. Run many histories (>= 10e6)
4. Observe photons with weight=1 in the phase space data

Expected behavior
Scored photons should all have weight 1/nbr_split. Scored charged particles should all have weight 1.

Operating system

• [x ] Linux
• Windows
• [ x] macOS
• Other:

EGSnrc version
develop branch

Other
This showed up when testing the egs++ DBS algorithm against BEAMnrc. Using BEAMnrc with UBS and nbr_split=100, I found that when I used a photon weight cutoff of 0.01 (i.e. eliminating phat photons), the energy spectrum with Russian Roulette on was up to ~40% lower than that with Russian Roulette off for energies < 40 keV.