G4OpticalSurface: correctly setting the surface properties

Physics Processes, Models and Cross Sections
1

Dear all,

I am trying to understand how to properly describe the surface properties of a scintillating crystal wrapped with Teflon, in order to correctly take into account optical photons behavior at the boundaries.
For this reason, I am following what is described in the “Optical Photon Processes” section of the “Book for Application Developers”, but I don’t quite understand the meaning of some parameter settings for the G4OpticalSurface in listing 66:

http://geant4-userdoc.web.cern.ch/geant4-userdoc/UsersGuides/ForApplicationDeveloper/html/TrackingAndPhysics/physicsProcess.html#tracking-of-photons-in-processes-optical

I had also a look at the example under:

examples/extended/optical/OpNovice

but I am left with the same doubts.

In particular, to what do the values of photon energies and rindex refer to?

const G4int NUM = 2;

G4double pp[NUM] = {2.038*eV, 4.144*eV};

G4double rindex[NUM] = {1.35, 1.40};

Why are there two values specified for each parameter? Are they referring to volume1 and volume2 respectively? I am confused about this, first of all because the rindex values would be already defined in the properties of the materials filling the two volumes, and secondly because they would be limited to just one value of the photon energy for each volume……so, there is something I am not getting!

Just to get a bit more concrete, in my particular setting I guess that groundbackpainted finish is what I am looking for: this is what I understand from fig. 17, with Teflon as wrapping. The two volumes, volume1 and volume2, would be the crystal and the experimental hall (filled with air), respectively. What is in this case the values I should specify for pp and rindex?

Any help would be very much appreciated!

Best regards,
Antonio

2

Hi,

First suppose you have two materials (scintillator and teflon) in close contact. The surface finish will either be polished or ground. The refractive index needs to be specified for both materials over the energy range of interest. Specify the refractive index at a few energies and interpolation will be done automatically. Put these values in the material property table for each material.

The “painted” surface finishes simulate three materials. e.g. if the scintillator is wrapped in a dielectric that has a metal coating. In this case one needs to specify the refractive index of the surface (the dielectric part of the wrapping). This is what is done in listing 66 in the Book for Application Developers.

OpNovice example is confusing here. The surface type is LUTDAVIS, so the surface refractive index is not specified by the user. For some reason, the lines you quote are part of a surface material property. However this surface material property is not actually attached to the surface (it is commented out).
https://geant4.kek.jp/lxr/source/examples/extended/optical/OpNovice/src/OpNoviceDetectorConstruction.cc#L336

3

The “painted” surface finishes simulate three materials. e.g. if the scintillator is wrapped in a dielectric that has a metal coating. In this case one needs to specify the refractive index of the surface (the dielectric part of the wrapping). This is what is done in listing 66 in the Book for Application Developers.

ok, thanks. It sounds better now. Other (naive) questions:

  1. why only two values are specified? Shouldn’t be more precise by setting more values, which are then interpolated, like for the standard materials of the volumes in contact? I saw this in different examples: always a minimum and maximum values are set;

  2. just to be sure about my next step in the simulation: ground-back-painted means:

    • ground: the actual wrapping, a ground mirror with only Lambertian reflection (like the teflon I want to simulate);
    • back-painted: there is something else in between the wrapping and the crystal, for which I need to specify the rindex and the optical properties (specular_spike, specular_lobe, backscattering, sigma_alpha) of the surface between the crystal and the “something else”;

    is that correct?

OpNovice example is confusing here. The surface type is LUTDAVIS, so the surface refractive index is not specified by the user. For some reason, the lines you quote are part of a surface material property. However this surface material property is not actually attached to the surface (it is commented out).
Geant4/examples/extended/optical/OpNovice/src/OpNoviceDetectorConstruction.cc

ok, good. I will also try to use the LUTDAVIS, I would expect the simulation to better reproduce a real setup. I feel that is quite difficult to fine-tune the parameters in the UNIFIED model.

Thanks for your help!
Antonio

4

Sure, add more.

I admit I find the terminology confusing. Suppose you stick a piece of aluminized mylar on your crystal. If it’s aluminum side out and the aluminum is smooth, it’s polishedbackpainted. If the Al is rough it’s groundbackpainted. Turn the aluminized mylar around so the Al side is touching the crystal and you’ve got frontpainted (assuming no air gaps).

Please try to puzzle through the figure:

http://geant4-userdoc.web.cern.ch/geant4-userdoc/UsersGuides/ForApplicationDeveloper/html/TrackingAndPhysics/physicsProcess.html#fig-unified-surface-model