Skip to content

physics.md

Latest commit

 

History

History
108 lines (91 loc) · 5.55 KB

physics.md

File metadata and controls

108 lines (91 loc) · 5.55 KB
Raw

Physics

This guide describes the optical-photon physics behavior relevant to Simphony's GPU propagation. For runtime configuration, GDML syntax requirements, application input modes, and persisted event output schemas, see Simulation inputs and outputs.

Optical surface models in Geant4

In Geant4, optical surface properties such as finish, model, and type are defined using enums in the G4OpticalSurface and G4SurfaceProperty header files:

These enums allow users to configure how optical photons interact with surfaces, controlling behaviors like reflection, transmission, and absorption based on physical models and surface qualities. The string values corresponding to these enums, such as ground, glisur, and dielectric_dielectric, can also be used directly in GDML files when defining <opticalsurface> elements for geometry. Geant4 parses these attributes and applies the corresponding surface behavior.

For a physics-motivated explanation of how Geant4 handles optical photon boundary interactions, refer to the Geant4 Application Developer Guide boundary process. The UNIFIED model section is especially useful when comparing Geant4 behavior with Simphony's GPU implementation.

<gdml>
  ...
  <solids>
    <opticalsurface finish="ground" model="glisur" name="medium_surface" type="dielectric_dielectric" value="1">
      <property name="EFFICIENCY" ref="EFFICIENCY_DEF"/>
      <property name="REFLECTIVITY" ref="REFLECTIVITY_DEF"/>
    </opticalsurface>
  </solids>
  ...
</gdml>

Optical surface support in Simphony

Simphony reads GDML through Geant4, so <opticalsurface> attributes such as model="glisur", model="unified", finish="ground", and type="dielectric_dielectric" can be present in input files. During geometry translation, Simphony also preserves the Geant4 surface metadata and material property table arrays in the serialized surface fold. The standard GPU propagation path, however, implements a smaller optical-surface model than Geant4.

For ordinary explicit surfaces, Simphony builds a four-probability surface payload:

(detect, absorb, reflect_specular, reflect_diffuse)

This is done in u4/U4SurfaceArray.h and consumed by qsim::propagate_at_surface in qudarap/qsim.h. The supported ordinary surface behavior is:

GDML input Standard GPU behavior
EFFICIENCY with any value greater than zero Treats the surface as sensor-like: detect with EFFICIENCY, absorb with 1 - EFFICIENCY, and do not reflect.
No nonzero EFFICIENCY, with REFLECTIVITY and a polished finish Specular reflection with probability REFLECTIVITY; otherwise absorption.
No nonzero EFFICIENCY, with REFLECTIVITY and a ground finish Diffuse/Lambertian reflection with probability REFLECTIVITY; otherwise absorption.
Boundary from a material with RINDEX to a material without RINDEX Adds an implicit perfect absorber, matching Geant4's stop-and-kill behavior for non-transparent boundaries.

When no explicit optical surface is present and both materials have RINDEX, Simphony uses the material refractive indices in qsim::propagate_at_boundary to perform Fresnel reflection or transmission with polarization handling. This corresponds to Geant4's smooth dielectric-dielectric boundary case.

Several Geant4 optical-surface features are currently parsed or preserved as metadata, but are not used by the standard GPU surface propagation:

  • model="glisur" and model="unified" are recorded, but ordinary GPU propagation is not selected from the full Geant4 model implementation.
  • The value attribute is stored as polish for glisur and as sigma_alpha for non-glisur models, but normal smearing from polish or sigma_alpha is not active in the ordinary surface path.
  • type, such as dielectric_metal or dielectric_dielectric, is recorded, but ordinary explicit surfaces are still reduced to the probability payload above.
  • UNIFIED-model properties such as SPECULARLOBECONSTANT, SPECULARSPIKECONSTANT, BACKSCATTERCONSTANT, and TRANSMITTANCE are not used by the ordinary GPU surface path.
  • RINDEX or GROUPVEL properties attached to an optical surface are preserved, but the standard boundary calculation uses the corresponding material properties.
  • LUT, DAVIS, dichroic, and coated/thin-film Geant4 models should not be treated as supported standard GDML surface features in Simphony.
  • Only the common polished and ground finish families are recognized by the current helper code: polished, polishedfrontpainted, polishedbackpainted, ground, groundfrontpainted, and groundbackpainted.

For example, in tests/geom/basic_detector.gdml, the surface named medium_container_bs... is a ground glisur surface with REFLECTIVITY. In the standard GPU path it becomes a diffuse reflector. The medium_drop_surface and MirrorPyramid... surfaces both define EFFICIENCY=1, so they become fully detecting surfaces in the simplified payload, even though they also define REFLECTIVITY.