The code chosen to perform the simulation is MCNP (Monte Carlo N-Particles)8 version 4.2. It allows to transport photons (and neutrons and electrons) from 1 keV to 100 MeV through the matter. Every kind of three- dimensional geometry can be described, defining separate cells whose compositions and densities are chosen by the user. Photon interactions are treated very accurately. Coherent and incoherent scattering processes are treated according to the atomic form factors. Photo-electric absorption results in the generation of Auger electrons or fluorescence photons from the K and L shells (provided that the energy of the emitted photons is within the working range). The energy transferred to the electron is assumed to be lost locally and all the bremsstrahlung photons, that would have been created during the path of that electron, are assumed to be produced in the interaction point, with the direction of the electron (Thick Target Bremsstrahlung option of MCNP). A minor modification has been done to the code, to write the energy losses into an external file. The satellite has been modeled to a high level of detail. The process of refinement is still in progress. To date about three hundreds objects are present in the model (not counting as independent objects the collimator cells). In building the model the first obvious criterion has been that the closer to the GRBM where the components, the more detailed had to be the geometrical description. Where possible the exact material composition has been used. When it was impossible to detail tiny features, approximations have been made, but care was taken to preserve the overall opacity of the simplified components, following the other criteria that the masses should be the same of the real ones (changing the density accordingly) and the average atomic weight too.
Every object encountered by a gamma ray in its way to the scintillator slabs must be accurately described.