12} Generating and propagating optical photons

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 FLUKA can be used to generate and propagate optical photons of Cherenkov,
 scintillation and transition radiation light. Light generation is switched
 off by default and is activated and totally controlled by the user by means
 of data cards and user routines. This is true also for the optical properties of
 materials. These include the refraction index as a function of wave-length
 (or frequency or energy), the reflection coefficient of a given material,
 etc.

 In this respect, the user has the responsibility of issuing the right input
 directives: the code does not perform any physics check on the assumptions
 about the light yield and the properties of material.

 Optical photons (FLUKA id = -1) are treated according the laws of
 geometrical optics and therefore can be reflected and refracted at
 boundaries between different materials. From the physics point of view,
 optical photons have a certain energy (sampled according to the
 generation parameters given by the user) and carry along their polarisation
 information. Cherenkov photons are produced with their expected
 polarisation, while scintillation photons are assumed to be unpolarised. At
 each reflection or refraction, polarisation is assigned or modified
 according to optics laws derived from Maxwell equations.

 At a boundary between two materials with different refraction index, an
 optical photon is propagated (refracted) or reflected with a relative
 probability calculated according to the laws of optics.

 Furthermore, optical photons can be absorbed in flight (if the user
 defines a non zero absorption coefficient for the material under
 consideration) or elastically scattered (Rayleigh scattering) if the
 user defines a non zero diffusion coefficient for the material under
 consideration).

 In order to deal with optical photon problems, two specific input cards
 are available to the user:

  OPT-PROP: to set optical properties of materials.
  OPT-PROD: to manage light generation.

 See the corresponding detailed description of these options and of their
 parameters in Chapter 7}.

 Some user routines are also available for a more complete representation
 of the physical problem:

   RFRNDX: to specify a refraction index as a function of wavelength,
           frequency or energy
   RFLCTV: to specify the reflectivity of a material.
           This can be activated by card OPT-PROP with 
SDUM
 = METAL and
           
WHAT(3)
 < -99.
   OPHBDX: to set optical properties of a boundary surface. The call
           is activated by card OPT-PROP with 
SDUM
 = SPEC-BDX.
   FRGHNS: to set a possible degree of surface roughness, in order to have
           both diffusive and specular reflectivity from a given material
           surface.
   QUEFFC: to request a detailed quantum efficiency treatment. This is
           activated by card OPT-PROP with 
SDUM
 = SENSITIV, setting the
           0-th optical photon sentitivity parameter to a value lesser
           than -99 (
WHAT(1)
 < -99).

 All running values of optical photon tracking are contained in the TRACKR
 common block, just as for the other ordinary elementary particles.

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