DUNE integration

[sethrj]

This is the primary tracking issue for integrating Celeritas with the simulation workflow for DUNE. The primary Celeritas contact points are the "assignees" to the right.

Core capabilities

• Optical photon physics: Add optical photon simulation #886
• ORANGE for platform portability: Convert Geant4 geometry to ORANGE #1167

[sethrj]

Notes from today's meeting (compiled an hour later so hopefully I remember the important bits):

• Cherenkov is neglected in current simulations; it's only a few percent, and the detectors weren't designed for it. (There's currently no directional capabilities, the detector geometry doesn't favor capturing the light, and the photodetectors can't discriminate against UV light.)
• The optical photon mapping includes time dependence, but a finely resolved grid (necessary for higher fidelity) requires too much memory
• Currently the framework decouples energy deposition from optical responses at a framework level: energy deposition from steps are gathered during the Geant4 simulation, and afterward another stage of processing does the optical response calculation
• Future work could move back to a model where optical photons are simulated during the main simulation
• DUNE uses GDML for input (we have a copy)
• They have a limited closed-source standalone simulation we might be able to try out
• Surface physics is important; there will soon be reflective materials to worry about

[sethrj]

@whokion You can forward this issue page to Erica with a link to our (almost current) status? https://indico.cern.ch/event/1466097/contributions/6216512/attachments/2965789/5217806/celeritas-optical-photon.pdf