Optical properties¶

Each “physics material” (i.e., a combination of material and physics options) can have an associated “optical material” (compatible with optical photons).

class MaterialParams : public celeritas::ParamsDataInterface<MaterialParamsData>¶

Manage properties for optical materials.

Each “physics material” (the combination of a geometry-specified material and a user-specified region) can map to a single optical material. Many materials—especially those in mechanical structures and components not optically connected to the detector—may have no optical properties at all.

Optical volume properties are imported from Geant4 into the ImportData container. The celeritas::MaterialParams class loads the mapping of PhysMatId to OptMatId and makes it accessible via the main loop’s material view. By combining that with the GeoMaterialParams which maps volumes to PhysMatId, this class maps the geometry volumes to optical materials for use in the optical tracking loop.

Users can define “boundary” and “interface” surfaces representing, respectively, the entire boundary of a volume (all points where it touches the parent or child volumes) and the common face between two adjacent volume instances. See Geometry for a discussion of these definitions and Geant4 geometry for their translation from Geant4.

Bulk properties¶

When importing from Geant4, each optical material corresponds to a single G4MaterialPropertiesTable that has a RINDEX material property. (It also provides a special case for water if no material table is associated, allowing Rayleigh scattering by default by providing an isothermal compressibility and assuming STP.)

Celeritas translates many Geant4 material properties into its internal physics input parameters. It also allows material-specific user configuration of Celeritas-only physics, using properties listed in the following table.

Table 10 Celeritas-only properties, with the `CELER_` prefix omitted.¶
Name	Description
`SCINTILLATIONLAMBDAMEAN`	Mean wavelength of the Gaussian scintillation peak [mm]
`SCINTILLATIONLAMBDASIGMA`	Standard deviation of the Gaussian scintillation peak [mm]

Surface properties¶

Geant4 surfaces may be configured with the following variables:

Table 11 Geant4 surface parameters.¶
Name	Description
model	Model used for optical surface physics
finish	Level of a surface’s roughness
type	Type of optical materials on either side of the surface
value	Used by the surface finish to parameterize the surface roughness

Table 12 Model independent surface properties.¶
Name	Description
`REFLECTIVITY`	One minus the probability the photon is absorbed on the surface.
`TRANSMITTANCE`	Probability the photon is transmitted across the surface without change.
`EFFICIENCY`	Quantum efficiency of the detector attached to the surface. If a photon is absorbed, this is the probability it is subsequently detected.

GLISUR Model¶

Geant3’s default model, referred to as GLISUR, supports a uniform smear roughness for both dielectric-dielectric and dieletric-metal surfaces.

Table 13 Mapping of GLISUR model finishes to Celeritas inputs (in the `celeritas::inp` namespace).¶
Finish	Roughness	Reflection Mode
`polished`	No roughness	`ReflectionForm::from_spike`
`ground`	Smear roughness with roughness `1 - value`	`ReflectionForm::from_lobe`

Table 14 Mapping of GLISUR model types to Celeritas inputs (in the `celeritas::inp` namespace).¶
Type	Interaction
`dielectric_dielectric`	`DielectricInteraction::from_dielectric`
`dielectric_metal`	`DielectricInteraction::from_metal`

UNIFIED Model¶

The UNIFIED model provides a probabilistic set of possible types of reflection a photon may undergo. These are stores as properties on a surface’s property table, which is loaded into Celeritas as a celeritas::inp::ReflectionForm

Table 15 Description of UNIFIED reflection modes.¶
Name	Description
`SPECULARSPIKECONSTANT`	Probability to reflect with respect to the surface’s global normal (no roughness sampling)
`SPECULARLOBECONSTANT`	Probability to reflect with respect to a facet normal as sampled by the surface roughness
`BACKSCATTERCONSTANT`	Probability to directly back-scatter (opposite direction and polarization)

As with the GLISUR model, the surface type determines the Celeritas interaction model:

Table 16 Mapping of UNIFIED model types to Celeritas inputs (in the `celeritas::inp` namespace).¶
Type	Interaction
`dielectric_dielectric`	`DielectricInteraction::from_dielectric`
`dielectric_metal`	`DielectricInteraction::from_metal`

Both of which support the polished and ground finishes as well:

Table 17 Mapping of UNIFIED model basic finishes to Celeritas inputs (in the `celeritas::inp` namespace).¶
Finish	Roughness	Reflection Mode
`polished`	No roughness	`ReflectionForm::from_spike`
`ground`	Gaussian roughness with `sigma_alpha = value`	Populated from UNIFIED reflection modes

The UNIFIED model further supports front-painted and back-painted finishes for dielectric-dielectric interfaces. A painted surface is entirely reflective with a fixed reflection mode.

Table 18 Mapping of UNIFIED model dielectric-dielectric front-painted finishes to Celeritas inputs (in the `celeritas::inp` namespace).¶
Finish	Roughness	Interaction
`polishedfrontpainted`	No roughness	`ReflectionMode::specular_spike`
`groundfrontpainted`	No roughness	`ReflectionMode::diffuse_lobe`

Back-painted finishes are used to represent surfaces such as a “crystal - air gap - wrapping” where there’s an interstitial material between the initial material and the wrapping. The RINDEX present on a surface material property table represents the refractive index of the interstitial material when applying the Fresnel equations between the initial and interstitial materials. Furthermore, if reflectivity grids are defined on the surface, they only apply to the initial-interstitial interface and not the painted surface for back-painted finishes.

Back-painted surfaces are represented as combinations of the above surfaces, with the interstitial material determined by the RINDEX of the surface material property table.

Table 19 Surface construction of UNIFIED model dielectric-dielectric back-painted finishes in Celeritas.¶
Finish	Initial-Interstitial Surface	Interstitial-Final Surface
`polishedbackpainted`	UNIFIED dielectric-dielectric ground	Polished front-painted
`groundbackpainted`	UNIFIED dielectric-dielectric ground	Ground front-painted