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Setting optical properties

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Paola Ferrario edited this page Apr 23, 2024 · 7 revisions

This page tries to give some guidance on setting the optical properties of material and surfaces used in nexus simulations as it may be tricky in certain aspects.

General peculiarities:

  • All optical properties are specified as a function of the photon energy within Geant4 and have to be sorted by rising energy values. Thus, if a property spectrum is available as a function of the photon wavelength and/or sorted by falling energy values, the user has to convert the spectrum into a function of rising photon energy, before passing it to Geant4.

  • All optical properties have to be specified on the same energy range. Otherwise errors will occur, e.g. if a scintillation photon is created with an energy for which no refractive index is specified in the material.

Material properties:

  • The spectra of the efficiency, reflectivity, the refractive index, the attenuation length and the WLS attenuation length are interpolated by a straight line between two given points of the spectrum. For energies lower than the first bin, the value considered is the one of the first bin; and in the same way, for energies higher than the last bin, the last bin value is considered. In general, when defining absorption lengths it is recommended to use for the first and last bins, values corresponding to total absorption (abs_length = 0), or null absorption (abs_length >> 0).

  • In contrast, emission spectra are interpolated by applying the mean of the values of two neighboring points as constant value between these points. So, for an accurate modeling of the spectra, a high density of data points must be used. For energies below the first bin and higher than last bin, there is no emission.

  • For WLS materials, two different attenuation length variables can be specified: In addition to the ABSLENGTH variable, which defines the usual absorption of optical photons, the WLSABSLENGTH variable corresponds to absorption which triggers the WLS process. Both variables can be combined for one material, e.g. to simulate WLS material with a limited WLS absorption range in combination with an absorption spectrum for the created WLS photons. If both spectra overlap, both are taken into account, i.e. the process with the shorter attenuation length is more likely in the overlap region.

  • The Quantum Efficiency of the WaveLength Shifting (WLS) process is specified by the constant property: "WLSMEANNUMBERPHOTONS". Geant4 doesn't allow to specify different values for different energies.

Surface properties:

  • If no G4OpticalSurface is defined, the reflection is simulated as geometric reflection at a perfectly smooth optical surface, i.e. applying Snell’s law. When introducing a G4OpticalSurface without specifying the parameters of the reflection model, the default reflection model is diffuse reflection.

  • Optical surfaces can be defined via two ways: Between two specific volumes (G4LogicalBorderSurface) or around one volume (G4LogicalSkinSurface). The following table summarizes the effective G4OpticalSurfaces, which are responsible for the surface properties, for all possible combinations of G4OpticalSurfaces. Thereby, the photons cross the surface from volume 1 to volume 2, “skin” stands for a G4LogicalSkinSurface around the corresponding volume, “border” represents a G4LogicalBorderSurface between both volumes (where the properties are specified for photons that approach the border from the volume corresponding to the column), and “a” or “b” distinguish different surfaces of the same type. Optical_Surfaces

  • If you want to specify a surface property via G4LogicalBorderSurface for photons in both directions, you must add 2 different border surfaces: "volume1-volume2" & "volume2-volume1".

  • For direct specification of the reflectivity of G4OpticalSurfaces, two optical property variables can be set and have to be distinguished. One is the REFLECTIVITY variable. In case of a surface between a dielectric and a metal, it does exactly what its name suggests: The optical photons are reflected with the probability that has been set via the REFLECTIVITY variable (i.e. bypassing the Fresnel equations). Not reflected photons are absorbed. Strictly speaking, this is a discrepancy with respect to the behavior of “realistic” surfaces, which do not absorb photons. Photons that are not reflected at such a surface enter the next volume and undergo absorption corresponding to the attenuation length of the material, which is typically very short for optical photons in metals. However, in case of a surface between two dielectrics, the result is not as the variable name suggests: The photons are absorbed with a probability of 100 %−r, where r is the specified “reflectivity” value. The remaining photons are then reflected/refracted according to the Fresnel equation. Thus, in this case, the REFLECTIVITY variable simulates something like absorbing dirt on the surface. To specify the reflectivity of a surface between two dielectrics, the TRANSMITTANCE variable has to be used, where the reflectivity R derives from the transmittance T by R = 100 % − T . One should also note that Geant4 does not perform any optical process at a surface between two dielectrics that consist of the same material.

  • For the typical situations one may find in NEXT geometries, follow the next recipe:

1 Implement TEFLON with a skin surface.

2 Implement TPB with a skin surface.

3 The only exception to previous rules is when TPB is directly applied over TEFLON. In that case, TEFLON should have a Skin surface, and TPB should have a Border surface with the neighbor volume (the other than teflon), in both directions.

  • If TEFLON has its skin surface, there is no need to add its OpticalMaterialProperties to the bulk material. (This may be revised if TRANSMITTANCE is defined).

  • TPB has the same effect over materials if it is placed inside or outside the volume it is applied to. Typically internal placements are easier to implement.

Other considerations:

  • The optical material properties table set to a given material is shared by all the geometry. So be careful if you plan to use same material with different optical properties in different places of your geometry.

  • It is very important in Generic Sensors to set the refractive index of their windows to the one of the neighbor material. In any other case the overall sensor PDE wouldn't match the one set.

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