Parabolic Mirror Example

Upcoming in the next release.

This example builds a telescope-like optical mirror from two Geant4 G4Paraboloid solids. A parallel bundle of optical photons enters an optical vacuum volume, reflects from the inner surface of the mirror shell, and is collected by a small gPhotonDetector panel near the nominal focus.

Quickstart

Copy the example to your current directory, create the geometry, and run one event:

cp -r $GEMC_HOME/examples/optical/parabolic_mirror .
cd parabolic_mirror
./parabolic_mirror.py
gemc parabolic_mirror.yaml -n=1

Geometry

The mirror is a thin boolean shell. The geometry script defines two construction operands:

outer_dish - the outside G4Paraboloid;
inner_dish - a slightly smaller G4Paraboloid that defines the empty cavity.

Both operands are published with exist set to 0. They are not placed as detector volumes, but their solids remain available to the final boolean operation:

mirror = GVolume("parabolic_dish")
mirror.mother = "optical_space"
mirror.solidsOpr = "outer_dish - inner_dish"
mirror.set_position(0, 0, mirror_z)
mirror.set_rotation(180, 0, 0)
mirror.material = "G4_Al"
mirror.mirror = "parabolic_reflector"
mirror.publish(cfg)

G4Paraboloid requires the radius at -dz to be smaller than the radius at +dz. The final shell is rotated 180 degrees around x so the open side faces the incoming photons.

Interactive viewer:

Parabolic mirror simulation with 1 generated optical-photon event.

Mirror Surface

The mirror coating is a GMirror skin surface attached to the final boolean shell:

reflector = GMirror("parabolic_reflector")
reflector.type = "dielectric_metal"
reflector.finish = "polished"
reflector.model = "unified"
reflector.border = "SkinSurface"
reflector.photonEnergy = "2.0*eV 3.0*eV 4.0*eV 5.0*eV 6.0*eV"
reflector.reflectivity = "0.95 0.95 0.95 0.95 0.95"
reflector.publish(cfg)

mirror.mirror = "parabolic_reflector"

See the Mirrors documentation for the mirror surface fields.

Photon Detector

The detector is placed near the nominal focal point and uses gPhotonDetector:

detector = GVolume("focus_detector")
detector.make_tube(0, 60, 1, 0, 360)
detector.set_position(0, 0, focus_z)
detector.material = "opticalVacuum"
detector.digitization = "gPhotonDetector"
detector.set_identifier("panel", 1)
detector.publish(cfg)

The steering card enables optical physics and launches optical photons into the mirror aperture:

phys_list: FTFP_BERT + G4OpticalPhysics

gparticle:
  - name: opticalphoton
    multiplicity: 200
    p: 3*eV
    vz: -42*cm
    delta_vx: 17*cm
    delta_vy: 17*cm

Geant4 may warn that generated optical photons have zero polarization and assign a random polarization. That warning is expected for this simple source.