Add Multi-Particle Transport Capabilities

Background

MC/DC should support multi-particle transport workflows where different particle types can be transported within the same simulation and one particle type may produce another through physical interactions.

This requires both particle-specific physics support and shared infrastructure for managing coupled particle transport.

Goal

Add multi-particle transport capabilities to MC/DC, including infrastructure for coupled particle production and transport, plus particle-specific support for electrons, protons, photons, and heavier charged particles.

Scope

This work should include:

• Designing the core infrastructure for multi-particle transport.
• Supporting simulations with more than one transported particle type.
• Supporting interactions where one particle type can create another particle type.
• Defining particle-type abstractions for shared transport behavior and particle-specific behavior.
• Updating particle banks, source definitions, event handling, tally support, and output data structures as needed.
• Updating configuration and user-facing APIs to select transported particle types and coupled physics models.
• Adding documentation and examples for multi-particle simulations.

Proposed Sub-Issues

• Add electron transport capability.
• Add proton transport capability.
• Add photon transport capability.
• Add heavier charged particle transport capability.
• Add multi-particle transport infrastructure and abstractions.
• Add coupled particle-production handling.
• Update source, bank, tally, and output infrastructure for multi-particle workflows.
• Add tests and benchmarks for single-particle and coupled multi-particle problems.

Acceptance Criteria

• MC/DC can represent and execute simulations involving multiple particle types.
• A transported particle can generate one or more secondary particles of another supported type.
• Electron, proton, photon, and heavier charged particle workstreams are tracked as separate sub-issues.
• Shared infrastructure avoids duplicating transport logic unnecessarily across particle types.
• Particle-specific physics can be added or extended without requiring broad changes to unrelated particle types.
• Tests cover both single-particle and coupled multi-particle workflows.
• Documentation explains how users configure and run multi-particle simulations.

Notes

This should be organized as an umbrella issue. The infrastructure and abstraction work should be coordinated with the particle-specific implementation issues so that early particle support does not lock MC/DC into an interface that becomes difficult to extend.

No files, tests, or entry points are named; start by breaking this umbrella issue into the proposed infrastructure and particle-specific sub-issues, then inspect the existing source, bank, event, tally, output, configuration, and API structures. Done means the acceptance criteria are met, including coupled particle production, tests and benchmarks, and user documentation.