Optical Physics

Optical Photons

A full description of the tracking of optical photons is available in the Geant4 Physics Reference Manual and the Geant4 Guide for Applications Developers.

TOPAS allows to include optical physics by means of the g4optical module in the physics list. The available optical processes included in the g4optical module are: scintillation, Cerenkov radiation, wavelength shifting, optical absorption, Rayleigh scattering and boundary processes. However, the optical properties of the material of the volumes must also be defined (at the least the refractive index must to be defined). There exist two types of variables to define the optical properties: a vector based and constant based. The vector-based parameter allows to define a property (refractive index for example) as a function of the photon’s energy. While the constant-based parameters allows to define an scalar (scintillation yield for example).

To activate the optical properties in a material one must to set:

b:Ma/MyMaterial/EnableOpticalProperties = "True"

To set a property based on a vector, one must to define the energy of reference. For example to include the refractive index one must to define two parameters:

dv:Ma/MyMaterial/RefractiveIndex/Energies = 3 2.0 2.5 3.0 eV
uv:Ma/MyMaterial/RefractiveIndex/Values = 3 1.58 1.58 1.58

To set a property based on a scalar only one parameter is needed, for example:

u:Ma/MyMaterial/ScintillationYield = 1120 # in ph/MeV
d:Ma/MyMaterial/FastTimeConstant = 2.1 ns

The full list of parameters available is listed in the next table.

Type Parameter name
uv RefractiveIndex
uv ImaginaryRefractiveIndex
uv RealRefractiveIndex
dv AbsLength
uv FastComponent
uv SlowComponent
uv Miehg
uv SpecularLobeConstant
uv SpecularSpikeConstant
uv BackScatterConstant
uv WLSAbsLength
uv WLSComponent
uv Reflectivity
uv Efficiency
uv Transmittance
u ScintillationYield ( in photons/MeV)
u ResolutionScale
d FastTimeConstant
d SlowTimeConstant
u YieldRatio
u MiehgForward
u MiehgBackward
u MiehgForwardRatio
u WLSTimeConstant
u BirksConstant (in mm/MeV)

Optical Surfaces

If a perfect smooth interface is between two dielectric materials, the user only needs to provide the refractive index. In all other cases, a surface or optical boundary needs to be defined. There exist two kinds of surfaces: the border surface that delimits the boundary between two components; and the skin surface which surrounds one single component. Border surface is ordered in the sense that the order of the components matters, two border surfaces can exists between a pair of components. Thus, the follow parameters define two surfaces for a pair of components:

s:Ge/MyComponent1/OpticalBehaviorTo/MyComponent2 = "MySurface1"
s:Ge/MyComponent2/OpticalBehaviorTo/MyComponent1 = "MySurface2"

For skin surface only one surface can be defined per component:

s:Ge/MyComponent1/OpticalBehavior = "MySurface1"

Surfaces can be defined as follows (see next table for description):

s:Su/MySurfaceName/Type = "dielectric_dielectric" # or dielectric_metal

Next, choose the model for optical surfaces:

s:Su/MySurfaceName/Model = "Glisur " # Or Unified

Finally the finish:

s:Su/MySurfaceName/Finish = "Polished"

In addition, more detailed properties can be added by parameters described in the table below. In such a case, the way to define would be for example (with prefix Su instead of Ma):

dv:Su/MySurfaceName/Energies = 2 1.0 4.0 eV
uv:Su/MySurfaceName/Reflectivity = 2 0.8 0.8
Type Parameter name Possible values
string Type
string Finish
polished: smooth perfectly polished surface
polishedfrontpainted: smooth top-layer (front) paint
polishedbackpainted: same as polished but with a back-paint
ground: rough surface
groundfrontpainted: rough top-layer (front) paint
groundbackpainted: same as ground but with a back-paint
string Model
Unified: reference
Glisur: original GEANT3.21 model
unitless SigmaAlpha Between 0 and 1. By default 0