Mirrors

Mirrors are upcoming in the next release.

In GEMC any optical boundary is described as a mirror, regardless of its use or reflective quality: metal reflectors, painted walls, wrappings, or the interface between two transparent media. A mirror defines a Geant4 optical surface; each volume can reference one mirror by name through its mirror field.

All mirrors are created with the GMirror class:

from pygemc import GMirror

Defining a mirror

A GMirror is created with its name and four mandatory fields:

The available types and finishes are the ones defined by Geant4 in G4OpticalSurface: unknown strings are rejected at load time.

mirror = GMirror("polished_metal")
mirror.description = "Polished metal mirror"
mirror.type = "dielectric_metal"
mirror.finish = "polished"
mirror.model = "unified"
mirror.border = "SkinSurface"
mirror.photonEnergy = "2.0*eV 3.0*eV 4.0*eV 5.0*eV 6.0*eV"
mirror.reflectivity = "0.90 0.90 0.89 0.88 0.87"
mirror.publish(cfg)

Boundary optical properties

The optical behavior of the boundary comes from one of two sources:

Explicit tables

Energy-dependent tables evaluated at photonEnergy, exactly as for material optical properties:

Every provided table must have the same number of entries as photonEnergy; mismatches are rejected when the mirror is loaded.

The scalar sigmaAlpha sets the surface roughness used by the unified model with ground finishes. Note that with a ground finish the unified model reflects diffusely (Lambertian) by default: to obtain a blurred but still specular reflection, direct the reflection into the specular lobe with specularlobe and control the blur with sigmaAlpha:

rough = GMirror("rough_metal")
rough.type = "dielectric_metal"
rough.finish = "ground"
rough.model = "unified"
rough.border = "SkinSurface"
rough.photonEnergy = "2.0*eV 4.0*eV 6.0*eV"
rough.reflectivity = "0.90 0.89 0.87"
rough.specularlobe = "1.0 1.0 1.0"
rough.sigmaAlpha = 0.03
rough.publish(cfg)

A transmittance table turns a metal mirror into a half-silvered (semi-transparent) one: at the boundary the photon is reflected with probability R, transmitted with probability T, and absorbed otherwise:

semi = GMirror("semi_transparent")
semi.type = "dielectric_metal"
semi.finish = "polished"
semi.model = "unified"
semi.border = "beam_splitter_plate"       # border surface (see below)
semi.photonEnergy = "2.0*eV 4.0*eV 6.0*eV"
semi.reflectivity = "0.60 0.60 0.60"
semi.transmittance = "0.30 0.30 0.30"
semi.publish(cfg)

From a material

Alternatively, matOptProps names a material whose optical properties table is used as the boundary properties. The material does not have to be the material of either bordering volume - think of it as a thin coating or paint:

coated = GMirror("coated_metal")
coated.type = "dielectric_metal"
coated.finish = "polished"
coated.model = "unified"
coated.border = "SkinSurface"
coated.matOptProps = "mirror_coating"      # a GMaterial with optical properties
coated.publish(cfg)

Skin and border surfaces

The border field selects between the two Geant4 logical surface kinds:

• SkinSurface: the optical boundary covers the entire outside surface of every volume that references the mirror.
• A volume name: the optical boundary applies only to photons crossing from the referencing volume into the named border volume (both must belong to the same system). The direction matters: the surface acts on photons leaving the volume that carries the mirror field.

Assigning a mirror to a volume

Volumes reference a mirror by name through the GVolume mirror field:

plate = GVolume("flat_reflector")
plate.mother = "radiator"
plate.make_box(140, 440, 5)
plate.material = "G4_Al"
plate.mirror = "polished_metal"            # skin surface on this plate
plate.publish(cfg)

For a border surface, the mirror is attached to the volume the photons come from:

radiator.mirror = "black_paint"            # black_paint.border = "bottom_panel"

Any number of volumes can share the same mirror: GEMC creates one optical surface per mirror definition and reuses it for every volume that references it.

Storage

Like geometry and materials, mirrors are written by the Python API to the factory selected at geometry-creation time:

• sqlite: one row per mirror in the mirrors table, keyed by experiment, system, variation and run;
• ascii: one row per mirror in <system>__mirrors_<variation>.txt.

Both are loaded automatically by gemc together with the system geometry and materials.

Example

The Mirrors example shows all of the above in a working simulation: three identical electrons aim at three reflector plates (polished, rough, and semi-transparent), which reflect their Cherenkov photons onto three aligned photon detectors - identical conditions, so the panel counts directly compare the reflections. A fourth panel behind the semi-transparent plate counts the transmitted photons.

The Parabolic Mirror example shows a focusing mirror made from two G4Paraboloid solids. The final mirror volume is a boolean subtraction, outer_dish - inner_dish, and carries the GMirror skin surface so photons reflect from the inner cavity toward the focus detector.

cp -r $GEMC_HOME/examples/optical/mirrors .
cd mirrors
./mirrors.py
gemc mirrors.yaml -n=10