Design and optimization around 1 MeV of a Tracker for a CubeSat Mission

Gamma-ray astrophysics is quite a young field, especially in comparison to the long history of optical observations or even radio-astronomy. Starting with the first observations of telescopes like OSO3 (1967-1969) and SAS2 (1972-1973), in the last two decades many space observatories obtained considerable results: FERMI-LAT (2008-), in the energy range from 20 MeV to 300 GeV AGILE (2007-), in the energy range from 30 MeV to 50 GeV, COMPTEL (1991-2000), in the energy range from 1 to 30 MeV, and INTEGRAL-IBIS (2002-), in the energy range from 15 keV to 10 MeV, only to name but a few gamma-ray observatories. At present, the worse sensitivity occurs in the range 1-10 MeV, where the dominant interaction mechanism of gamma rays with matter is Compton scattering. The scientific interest towards this area of the electromagnetic spectrum is actually remarkable. In fact, the MeV regime can provide unique informations several cosmic accelerators and sources like pulsars, supernovae and active galactic nuclei, in particular thanks to the detailed study of several emission lines. For these reasons the next generation of space observatories for gamma-ray astrophysics will focus on the energy range around 1 MeV. The best instrument to date in this range, COMPTEL onboard CGRO, flew in the 1990's but with a technology dating to a decade earlier. The operating principle of the detector was based on Compton interaction in one of a series of liquid scintillators, and the consecutive absorption in a second plane of NaI scintillators at the distance of 150 cm. Given the huge leap in technology that occurred since, such as the development of semiconductor detectors and the experience gathered with other gamma-ray observatories like the FERMI-LAT, the performances of a future Compton telescope is expected to improve at least by an order of magnitude with respect to COMPTEL. However, several issues affect this optimistic picture of the situation: the external gamma-ray background produced by the interaction of charged particles with the atmosphere represents a remarkable complication, the internal background due to material activation produces events in the energy range of signal, and the Compton track reconstruction and event analysis are, generally, quite complex. For these reasons a careful optimization of instrument design and operation is required. Therefore, the goal of this thesis is a preliminary analysis of the design and the performances of a small Compton detector, with a payload's dimension of 2-3U, contained development costs and relatively quick design phase. This instrument should be used as a pathfinder for Compton space missions recently proposed (like the ESA mission ASTROGAM and the NASA mission COMPAIR), to be presented again in an updated version in the coming years. The time scale for an M-class mission is around 10 years, with a cost 500 M€, so the realization of a small pathfinder test instrument is of great interest. The work is focused mainly on the study and optimization of the silicon tracker which will be the heart of a future Telescope optimized in the MeV energy range.