Refactor physics tables for correct material-dependent bounds

[sethrj]

Refactor physics tables for correct material-dependent bounds

[amandalund]

Currently the model finder always returns a valid model: using the integral approach, the process xs is recalculated at the post-step energy and if it's zero (no applicable process) then no model is returned before actually trying to select a model. We might be able to use your suggestion if the post-step process xs is zero and resample a process, but I don't think we would need material-dependent model applicabilities.

[amandalund]

the applicability doesn't allow us to limit correctly the lower energy of models based on their production cutoff, so we end up sampling inapplicable discrete models (most seriously in the case of stopped positrons)

Is this because of the applicability, or because the process is sampled from the cross sections calculated at the pre-step energy and the particle might lose energy over the step (possibly making the sampled process/model inapplicable at the post-step energy)?

[sethrj]

It's because the way I implemented applicability requires the same lower bound for all materials. If we could do material-dependent, then we would be able to select the actually applicable models from the post-slowing-down energy.

[amandalund]

But won't we still be sampling the (possibly now inapplicable) process from the pre-step energy?

[sethrj]

Right: we calculated the process cross sections using pre-step energies. Well, perhaps we could adjust the logic at the end of select_discrete_interaction so that if the model finder returns "no model" (which it can do with the post-step energy), the associated per-process xs is set to zero (and the macro xs reduced accordingly) and we return to the top of the function to resample a process? Unless there are processes with upper energy bounds, this will allow us to always sample a process that has a valid model.