The Monte Carlo Model

The Monte Carlo model of the BeppoSAX payload accounts for the geometry and the materials of the instruments and electronics (figg. ). The code used is MCNP4b (``Monte Carlo N-Particle'' code by J. F. Briesmeister et al. ); it describes the neutron, photon, electron, or coupled neutron/photon/electron transport. Photons with energies from 1 keV to 1 GeV can be used and particular attention has been paid to their interactions with matter, like photoelectric absorption, Thomson and Compton scattering and pair production.

Figure: Top panel: schematic top view of the payload model: the two couples of NFIs, the HPGSPC (right) and the WFC box (left) are clearly visible around the PDS/GRBM square box. Bottom panel: side view of the model: the orange box represents the electronics, while the pedestal (used during the on-ground calibration sessions) is violet (from [Calura et al., 2000]).

The response matrix derived from this model has been successfully tested with the on-ground calibrations ([Amati, 1998]) and by using the Crab Nebula as calibration source: its well known spectrum has been satisfactorily reconstructed with the 240 channel spectrum of the GRBM in-flight data. Besides, also the Crab count rates in the 40-700 keV and $>$ 100 keV energy bands at different directions with respect to the satellite local frame of reference, estimated through the Earth occultation technique ([Guidorzi et al., 1998]), have been used for testing the response matrix.