DIRC: add bar lateral surface with UNIFIED model micro-facet roughness#1113
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DIRC: add bar lateral surface with UNIFIED model micro-facet roughness#1113wdconinc wants to merge 1 commit into
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Add an explicit G4LogicalSkinSurface on bar_vol using the UNIFIED optical model with ground finish and sigma_alpha = 0.02 rad. This models the micro-facet roughness of polished quartz surfaces: photons that would otherwise undergo perfect total internal reflection at the lateral faces are deflected by randomly-oriented micro-facets, and a fraction whose angles exceed the critical angle refract out of the bar instead. This physically eliminates the unphysically long photon paths (up to 690 m / 113,000 steps with perfect TIR) without any artificial hard cutoffs. The sigma_alpha = 0.02 rad (≈ 1.1°) is the standard value for polished quartz used in hpDIRC simulations (BaBar DIRC, Belle II iTOP). Note: the DD4hep compact XML opticalsurface parser does not currently support the sigma_alpha attribute for the unified model. The value is therefore set programmatically via TGeoOpticalSurface::SetSigmaAlpha(), which the DDG4 Geant4Converter forwards to G4OpticalSurface. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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Pull request overview
This PR adds a DIRC quartz bar optical surface intended to model micro-facet roughness with Geant4’s UNIFIED ground surface behavior, reducing unphysical long photon paths from idealized total internal reflection.
Changes:
- Adds
DIRC_BarOpticalSurfaceto the optical materials XML. - Looks up the new surface in
DIRC_geo.cpp, setssigma_alpha = 0.02, and attaches it tobar_vol.
Reviewed changes
Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.
| File | Description |
|---|---|
src/DIRC_geo.cpp |
Applies the new DIRC bar optical surface programmatically with SetSigmaAlpha(0.02). |
compact/optical_materials.xml |
Defines the new UNIFIED/ground dielectric optical surface for DIRC bars. |
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| SkinSurface bar_skin(desc, det, Form("dirc_bar_optical_surface"), bar_surf, bar_vol); | ||
| bar_skin.isValid(); |
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While correct, this is the part of the implementation I'd rather leave to the DIRC experts to undertake.
wdconinc
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Summary
Add an explicit
G4LogicalSkinSurfaceonbar_vol(the quartz bars) using the UNIFIED optical model withgroundfinish andsigma_alpha = 0.02 rad.Physics motivation
Currently the bar lateral faces have no explicit optical surface, defaulting to perfect
glisur/polishedtotal internal reflection. This causes an unphysical simulation pathology: optical photons can bounce indefinitely (up to 690 m / 113,000 steps in profiling runs), limited only by the bulk absorption length.In a real polished quartz bar, surface micro-roughness causes photons near the TIR critical angle to be deflected onto micro-facets at slightly different angles. A fraction of these exceed the critical angle and refract out of the bar instead of reflecting. The UNIFIED model with
groundfinish andsigma_alpha(Gaussian std-dev of micro-facet normal angle) correctly models this physics.sigma_alpha = 0.02 rad(≈ 1.1°) is the standard value for polished quartz used in hpDIRC simulations (BaBar DIRC, Belle II iTOP papers).Implementation note
DD4hep compact XML
<opticalsurface>does not currently support thesigma_alphaattribute for the UNIFIED model (onlyvalue→polishfor glisur is parsed inCompact2Objects.cpp). The value is therefore set programmatically viaTGeoOpticalSurface::SetSigmaAlpha(), whichDDG4::Geant4Converter::handleOpticalSurface()correctly forwards toG4OpticalSurface::SetSigmaAlpha().Files changed
compact/optical_materials.xml: addDIRC_BarOpticalSurfacedefinition (unified/ground/dielectric_dielectric)src/DIRC_geo.cpp: load bar surface, set sigma_alpha, apply as SkinSurface on bar_vol