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

Abstract : Ever since the dawn of astronomy, cosmic discoveries have been directly associated with instrumental progresses. This certainly applies also to the hard X-ray / soft gamma-ray domain where week photon cross-sections and strong background noise hamper instrument sensitivities. SPI, IBIS, and LAT, the main telescopes onboard INTEGRAL and SWIFT probably represent the pinnacle of what is possible using coded mask techniques.
How in this context can we keep improving our knowledge of the sub-MeV sky? A number of teams are currently developing second generation Compton telescopes that seem to be able to bring a part of the answer. At CESR, a complementary solution has been studied for over 10 years; it consists in the realization of a lens focusing soft gamma rays.

The Laue lenses we study focuses in the domain of 0.1-1 MeV thanks to Bragg diffraction in the volume of a large number of small crystal tiles. The focal length of a typical Laue lens system is of the order of 100 m. This requirement calls for two formation flying satellites maintaining lens and detector at the focal distance. The major breakthrough of Laue lenses is to decouple collecting area from detector area. Concentrating a signal from the large area of a Laue lens onto a small focal spot dramatically increases the signal over background ratio with respect to present technologies. Here's the reason for the long awaited leap in sensitivity.

The feasibility of the Laue lens concept for astrophysics has been demonstrated by the CLAIRE project between 2001 and 2003. The objective of the present thesis was to improve the concept, finding viable technical solutions towards a future space mission. Two aspects of the lens R\&D have been highlighted in this thesis : the first one is an analytical model of the lens that is used to calculate and improve the performance of a certain configuration, the second aspect concerns the search and the characterization of diffracting media of interest.

The lens model developed relies on a fast semi-analytical simulation library, permitting to build several design- and optimisation-tools. For the configuration of a given lens, this code computes the resulting effective area and point spread function in a handful of seconds. The model helps finding lens configurations (mass, pack ratio of the lens rings, ...) which are automatically refined to match with effective area and energy coverage constraints. These tools have been used to investigate various design aspects, such as the influence of focal length, size, mosaic spread, structure and materials of crystals, etc... The central evaluation criterion in the model is a figure of merit, based on the compactness of the focal spot and the intensity of the collected signal.

The second part of this work addresses the actual search and characterization of crystals potentially interesting for Laue lenses. Copper and germanium mosaic crystals, silicon-germanium alloys with a concentration gradient, and stacks of either silicon or germanium wafers have been studied in various facilities including the European Synchrotron Radiation Facility (Grenoble, France) and the GAMS 4 gamma ray spectrometer of the Institute Laue Langevin (Grenoble, France). These quantitative measurements permitted me to evaluate the quality of crystals currently available in the light of the requirements I had calculated with the above described to lens model.

The MAX and GRI mission concept studies, proposed to CNES in 2005 and to ESA in 2007 respectively, have largely benefited of this work. The design process and performance evaluation of MAX and GRI are presented in detail in this thesis.
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Submitted on : Monday, October 13, 2008 - 4:20:15 PM
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Nicolas Barrière. Développement d'une lentille de Laue pour l'astrophysique nucléaire. Planète et Univers [physics]. Université Paul Sabatier - Toulouse III, 2008. Français. ⟨tel-00329925⟩

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