In general, since Xrays are very penetrating they probe the depth of the target, sampling the information in a large volume. This makes it very probable that secondary radiation may suffer further interactions before leaving the medium where it started to interact, giving place to a multiple scattering process where the photons change energy and angle distribution according to the number and type of the collisions undergone. Therefore, a multiple scattering scheme within photon transport theory seems to be the simplest approach to study the diffusion of Xrays in a dense material. This approach has been used successfully by this research group to analyse the buildup of Xray spectra emitted after photon excitation (fluorescence), improving the knowledge and interpretation of experimental Xray spectra in materials of high complexity (multicomponent mono and multilayers). The reached high degree of detail in the description of the spectra has been due, essentially, to two factors: an accurate knowledge of the interaction cross sections for the prevailing processes in the Xray regime: photoelectric effect, Rayleigh and Compton scattering; and a detailed description of the evolution of the polarisation state across interactions by using an exact solution of the vector transport equation. The approach has been extended to photons in other energy regimes (i.e. to the LIDAR in the visible range, by using appropriate crosssections in that range), and to photons produced by excitation with other kind of particles (i.e. by electrons in the microprobe, or by protons in particle accelerators). In this last case, a more complex problem arises because the transport description requires to find the solution to a coupled system of transport equations, one for the charged particles, and one for the photons. Some computer codes were developed during these studies for both, a numerical evaluation of the deterministic solutions and a Monte Carlo simulation of polarised photons, which constitute a good basis for supporting any further serious software development involving photon transport. Polarisation is the most important and promising property of the radiation in view of the interests of industry in Xray imaging and chemical quantitation. The multiple scattering description of polarised photon diffusion represents a unified approach to describe attenuation of photon beams maintaining all the optical properties which are related to the polarisation state. In fact, a correct description of the angular distribution of the radiation is essential for a correct interpretation of Xray images, and for the moment, such a description is only possible with recourse to the vector transport model for polarised radiation. It is worth mentioning that such a description covers photons having all the possible states of polarisation, in particular those that born unpolarised but become polarised as a consequence of scattering. This represents one of the interest cases for application of Xray imaging in computed tomography, archaeometry, airpollution/environment and element mapping with microbeam techniques. Several codes for photon transport with different degrees of refinement on the description of the interactions photonatom, the composition of the sample, the polarisation, etc. have been developed along the years by our group. All these codes use a simple geometry since they have been mainly designed for testing the deterministic models, and symmetrically, the more valid variance reduction techniques to apply in Monte Carlo simulations.
In this site it is possible to find also some useful utility codes.


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