Annihilate positrons below the positron energy cutoff

[GuySten]

Description

Positrons created by pair production are handled at the photon collision site by process_charged_secondary. That function returned immediately for any charged particle below its energy cutoff, before reaching the positron annihilation step. With a nonzero energy_positron cutoff, positrons below the cutoff therefore never produced their two 511 keV annihilation photons. That energy was silently lost from the photon field. Positrons from a source were not affected, since sample_positron_reaction always annihilates.

The energy cutoff now only suppresses thick-target bremsstrahlung for a charged particle below the cutoff; its kinetic energy is deposited locally as before. A positron always annihilates, whatever its kinetic energy. Behavior with the default positron cutoff of 0 is unchanged.

I also clarified the energy_positron cutoff description in the settings documentation.

The new test test_positron_cutoff_annihilation runs a 10 MeV photon source in lead with local energy deposition, without a positron cutoff and with one above the source energy. With local energy deposition the positron cutoff should have no effect, so the photon flux tallies must match. On develop, the flux in the bin containing 511 keV drops from 0.33 to 0.004 when the cutoff is set; with this fix the two runs give identical results.

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 18) on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)