Développement d'une lentille de Laue pour l'astrophysique nucléaire

Gamma-ray astronomy addresses a wide range of fundamental astrophysical questions such as the life cycles of matter and the behavior of matter under extreme conditions. Whereas Gamma-rays present an extraordinary scientific potential for the study of the most powerful sources and the most violent events in the Universe, their observation has been held back by an intricate compound of experimental difficulties. Due to the atmospheric absorption of gamma-rays, instruments have to be carried above the atmosphere where they are exposed to Cosmic Rays. The bombardment of the instrument and spacecraft by Cosmic Ray particles generates high background rate in the detectors, limiting severely the sensitivity of gamma-ray telescopes. Despite these difficulties, Nuclear Astrophysics has made considerable progress in the last twenty years thanks to the instruments on the CGRO and INTEGRAL Observatories, i.e. instruments that are based on inelastic interaction processes, using either geometrical optics (e.g. coded masks) or quantum optics (e.g. Compton telescopes). Today, physical limits to the size of these instruments seem to be attained: pushing the performance further leads to several apparently unsolvable problems, ultimately owing to the belief that it is "impossible to reflect or refract gamma-rays". In this work, a new type of gamma-ray telescope is presented. It features a crystal diffraction lens that concentrates photons from a large photon collecting area onto a small detector. In such a Laue lens, a large number of crystals are disposed on concentric rings; each crystal can be considered as a little mirror which deviates gamma-rays through Bragg diffraction from the incident beam onto a common focal spot. The principle of the Laue lens for astrophysics was demonstrated with the balloon mission CLAIRE. The objective of this work was to develop the Laue Lens concept into a sensitive, space qualified optics for a future satellite mission. My contribution consisted of two main facets: a) finding appropriate crystal materials and improving the performance of the diffracting crystals, and b) develop a prototype segment for a space qualified lens. Exploring new diffracting media and improving the performance of individual crystals implied the development of numerical simulations of the diffraction process for various types of mosaic and CDP (Curved Diffraction Plane) crystals. These simulations were used to select suitable crystals to be grown and to be tested. Two different X- gamma ray facilities were used to probe the crystalline quality of our candidate materials: The European Synchrotron Radiation Facility (ESRF, France) and the high neutron flux reactor at ILL (France). During 10 beam-runs (and a total of 3 months of measurement), a large number of samples were tested, including Ag, Ir, Pt, Au, Pb, Rh, AsGa, SiGe, and Cu crystals. Outstanding performances were established for gold and silver crystals (> 500 keV), Cu and Ge (300-500 keV) crystals and SiGe CDP (