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External_Sensitive_Detectors

README.md

External sensitive detectors

A minimal example showing how to add sensitive detectors to o2-sim without compiling anything into O2. Two artificial detectors are injected purely from data:

name geometry (runtime macro) DetID slot sensitive action
ACYL geometry_innerCylinder.macro ITS built-in entrance/exit action
BDISK geometry_outerDisk.macro TST custom action sensitive_action.macro

Both are instances of the single compiled o2::ext::ExternalDetector class and produce the single generic hit type o2::ext::Hit. They differ only in their data: geometry, the DetID slot they occupy, and (optionally) a sensitive-action macro.

Run

./run.sh

This transports a few boxgen events and prints the per-detector hit counts, e.g.

External sensitive detector hits:
  ACYLHit                : ~430 hits over 5 events, mean radius 20.0 cm  [o2sim_HitsITS.root]
  BDISKHit               : ~120 hits over 5 events, mean radius ...  cm  [o2sim_HitsTST.root]

How it works

  • Geometry — each "macro" is a ROOT macro exporting get_builder_hook_unchecked(), the same symbol O2_CADtoTGeo.py emits when converting CAD (STEP) files to TGeo. The macros here are hand-written so the example is self-contained (no CAD binaries). The volumes whose names match "sensitiveVolumes" are registered as sensitive.

  • Sensitive action — ACYL has none, so it uses the built-in action that records an entrance/exit hit per track. BDISK points "sensitiveMacro" at a macro whose sensitiveAction() returns the per-step callable; it is JIT-compiled at runtime via o2::conf::GetFromMacro (the same mechanism as the generator / stepping hooks — no recompilation, no ACLIC). The callable has the full TVirtualMC singleton and a few helpers (currentSensorID(), currentTrackID(), addHit()).

  • Persistence — each detector is tied to a free DetID slot (ITS, TST here, chosen because no real detector of that name is active). The DetID is the scarce resource: it fixes the hit-file name (o2sim_Hits<DetID>.root) and lets the hit merger instantiate a matching receiver, so hits are written in parallel multi-worker mode just like for any built-in detector. The branch keeps the detector's own name (ACYLHit, BDISKHit).

Adding more detectors

Append entries to externalDetectors.json (each on a different free DetID slot) and list their names in detectorlist.json. The mechanism is fully data-driven; nothing needs to be rebuilt.

See also Detectors/External and scripts/geometry/O2_CADtoTGeo.py.