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 withSDUM= METAL andWHAT(3)< -99. OPHBDX: to set optical properties of a boundary surface. The call is activated by card OPT-PROP withSDUM= 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 withSDUM= 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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