Conclusions

In this thesis I have described the development of a new model of the gamma-ray emissivity (in the MeV - GeV range) of interstellar medium of our galaxy. The model follows the classical approach in which the gamma-ray emissivity is produced by the interaction of the cosmic-ray flux with the interstellar medium, through the process of proton-proton scattering (with production and decay of $\pi ^0$), Bremsstrahlung, and inverse Compton scattering. For this model I have obtained the three-dimensional distribution of diffuse matter (HI regions and molecular clouds) in the galactic plane, which, being based on recent radio survey, has a finer resolution and a larger extension with respect to distributions used by previous models.
The intensity of gamma-ray diffuse emission foreseen by this new model agrees quite well with the EGRET observations, both for spatial distribution and spectrum. The angular resolution of gamma-ray emission maps, that can be obtained using this model, are better of a factor of two with respect to previous works. This allows to better cope with the scientific requirements of a new generation of gamma-ray telescopes. The model presented here has been developed for the AGILE mission and is intended to be an analysis tool for its forthcoming observations. The model is crucial for the study of both diffuse emission and discrete gamma-ray sources. Concerning the study of diffuse emission I have proposed a method to extract information on the cosmic-rays Galactic distribution from the diffuse gamma-ray observations. This method, tested on EGRET data, produced the profile along the Galactic plane of the cosmic-ray (both protons and electrons) density enhancement, which shows a general gradient proceeding from the center to the anticenter region, and a superimposed finer modulation.
The study of gamma-ray point-like sources in the Galactic plane is performed using likelihood techniques that requires a Galactic diffuse emission model as input. The analysis of a simulated AGILE observation shows that more accurate is the diffuse emission model, more precise are the sources features reconstructed by the likelihood calculation.
The AGILE observations will carry new informations on the galactic gamma-ray diffuse emission, because especially of the large field of view and the good angular resolution of its gamma-ray imager. Other two features which need particular attention for a gamma-ray instrument are the energy resolution and the background rejection of spurious events due to charged particles or gamma radiation from terrestrial atmosphere. In the last part of this thesis I presented the study of the optimization of these features. Concerning the energy resolution, I have developed a technique for energy reconstruction of tracker events based on the measure of the multiple scattering suffered by the $e^+ e^-$ pairs. The application of this technique to Montecarlo-simulated data showed that is possible to use the AGILE imaging tracker also as a partial ''calorimeter'', obtaining good results in term of energy and spectral resolution. Concerning the background rejection I developed an algorithm able to made a basic reconstruction of tracker events. It produces a quite good estimate of the direction of gamma events, which permits to discriminate the events originated from the Earth's atmosphere. The same algorithm produces also a quality factor of the reconstructed events which can be used to discriminate about 70% of the residual background (both charged particles and albedo photons). Thanks to the good results obtained and its relative simplicity this algorithm as been chosen as part of the AGILE on-board software.