Implement Multi-Process Interaction Support

Summary

We need to extend the current probing setup to support multiple physical interaction processes (e.g., Compton scattering, photoelectric absorption, Rayleigh scattering). This change will serve as a preliminary for the transport engine to more realistically model energy-dependent and material-specific photon interaction

Motivation

Currently, the probing setup supports only a single differential cross section model. However, real materials have multiple competing interaction processes, each with its own cross section and physics model. To simulate realistic particle behavior, we must:

• Sample the interaction type based on relative cross sections.
• Call the appropriate physical model for that interaction.

Tasks

• Use the abstract interface for AbstractProcessDefinition, AbstractModelDefinition and AbstractPhaseSpaceLayout.

• Implement the following subtypes:

  • Thomson
  • PhotoelectricAbsorbtion
  • Rayleigh

• Create DifferentialCrossSection setup that holds:

  • A partial cross section (total cross section for the single process)
  • compute of the differential cross section

• Modify the probing setup to hold a list of DifferentialCrossSection entries.

• Implement interaction selection logic based on \sigma_i/\sum_i\sigma_i (e.g., roulette wheel selection).

• Update the particle transport loop to:

  • Sample distance based on total \sigma_t = \sum_i\sigma_i
  • Select and apply the correct interaction.
  • Handle energy and momentum, or particle absorption.

Acceptance Criteria

• A single voxel can contain multiple interaction processes.
• The interaction chosen during transport reflects the statistical weight of its cross section.
• All physical interactions behave as expected (e.g., energy loss in Thomson, absorption in photoelectric).
• The simulation terminates correctly for absorbed particles.
• Code is modular and extensible for future processes (e.g., pair production, fluorescence).
• Unit tests for all components

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Notes

Consider adding logging/debug flags to print the selected process during development.