[Moller_simulation] [EXTERNAL] Geant4 10.7 optical photons at boundary surfaces

Wouter Deconinck wouter.deconinck at umanitoba.ca
Thu Feb 2 10:56:11 EST 2023

Hi everyone,

In the pion detector working group we have recently discovered an issue
that may cause photocathodes and quantum efficiencies to be modeled
incorrectly in recent versions of geant4 (>= 10.7) when no changes are made
to older application code (<= 10.6). You may need to add a line to your
macros to enable the previous behavior.

## How do we model photocathodes?

In Geant4 we typically do not rely on an accurate model for the
photo-ionization and photo-emission processes to describe the combined
effects which we refer to as surface reflectivity and quantum efficiency,
i.e. the likelihood that an incident optical photon will generate a
photo-electron in the PMT that is subsequently detected. Instead, we often
rely on making the surface of the photocathode itself sensitive, without
requiring the optical photon to go into the bulk photocathode material.
This is handled in the G4OpBoundaryProcess for optical surfaces around
volumes that are set as sensitive detectors.

## What changed?

In versions of Geant4 until 10.6, the G4OpBoundaryProcess would by default
invoke the sensitive detector of the volume corresponding to the surface it
refers to if an optical photon was determined to have been transmitted
(i.e. not reflected, based on calculated or provided surface reflectivity)
and to have been detected (i.e. not simply absorbed, based on the provided
detection efficiency).

In versions of Geant4 starting with 10.7, this default was changed and now
doesn't invoke the sensitive detector without further action.

## What should I do?

If you are relying on optical boundaries to detect optical photons, and if
you are upgrading from 10.6 to a newer version, you may want to add the
following line to your macro to restore the behavior that was previously
the default:
/process/optical/boundary/setInvokeSD true

Wouter Deconinck (he/him) 🕪 <https://urldefense.proofpoint.com/v2/url?u=https-3A__www.name-2Dcoach.com_wouter-2Ddeconinck&d=DwIFaQ&c=CJqEzB1piLOyyvZjb8YUQw&r=hK0qfHtfdwUHlZG0DnK-QRJVYus_TzTB8u52ev2QBtI&m=g-GcBnTBt94yXMlvtmLtpBHfEsfwzXtL-U2dpSC-phs26N6Xm4dq0sQ8EsQqPPnn&s=X9Btfq83AGANpwJXfhIhbwHJxkS7vjZAPXK8tLcGnU0&e= >
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