We report on the first results obtained from our development project of focusing gamma-rays (>60 keV) by using Laue lenses. The first lens prototype model has been assembled and tested. We describe the technique adopted and the lens focusing capabilities at about 100 keV

We report on the feasibility study of a Laue lens for hard X-rays (> 60 keV) based on mosaic crystals, for astrophysical applications. In particular we discuss the scientific motivations, its functioning principle, the procedure followed to select the suitable crystal materials, the criteria adopted to establish crystal dimensions and their distribution on the lens in order to obtain the best lens focusing capabilities, and the criteria for optimizing the lens effective area in a given passband. We also discuss the effects of misalignments of the crystal tiles due to unavoidable mechanical errors in assembling the lens. A software was developed to face all these topics and to evaluate the expected lens performance. SCIENTIFIC MOTIVATIONSThe role of hard X-ray astronomy (> 10 keV) is now widely recognized. The numerous results obtained with the most recent satellite missions (BeppoSAX , Rossi-XTE ) on many classes of X-ray celestial sources have demonstrated the importance of the broad band (0.1 ÷ 300 keV) spectroscopy in order to derive an unbiased picture of the celestial source physics, like to establish the source geometry, the physical phenomena occurring in the emission region, the radiation production mechanisms, an unbiased separation of the contribution of thermal emission phenomena from the phenomena due to the presence of high energy plasmas (thermal or not thermal) and/or magnetic fields and/or source rotation.In spite of the excellent performance of the high energy instrument PDS (Phoswich Detection System) aboard BeppoSAX , 1 the most sensitive instrument ever flown in the 15-200 keV energy band, even in the case of the strongest Galactic (e.g., Cyg X-1 in soft state, Her X-1) and extragalactic (e.g., 3C373, MKN 3) X-ray sources, the statistical quality of the measured spectra becomes poor in the highest part of the instrument passband (> 80 keV). Thus the development of focusing optics in this band and, more generally, in the entire passband covered by the BSAX/PDS and possibly beyond it, is of key importance to overcome the limitations of the direct viewing telescopes (with or without masks) and to allow the study of the high energy spectra of the celestial sources with the same detail which is achieved at lower energies (< 10 keV), where focusing optics are available.In fact, X-ray mirrors, based on the external reflections with very high focal lengths (≥ 50 m), or 'supermirrors' based on Bragg diffraction from multilayers of bi-strates made of high and low Z materials (e.g., Joensen et al. 2 ), can overcome the sensitivity problem up to about 70 keV. For higher energy photons, an efficient focusing is a much more challenging task.Goal of our project is the development of a focusing telescope which efficiently focus hard X-/gamma-rays in a broad continuous band, from 70 keV to ≥ 300 keV, by exploiting the Bragg diffraction from mosaic crystals in Laue configuration.

Abstract-Results of reflectivity measurements of mosaic crystal samples of Cu (111) are reported. These tests were performed in the context of a feasibility study of a hard X-ray focusing telescope for space astronomy with energy passband from 60 to 600 keV. The technique envisaged is that of using mosaic crystals in transmission configuration that diffract X-rays for Bragg diffraction (Laue lens). The Laue lens assumed has a spherical shape with focal length f . It is made of flat mosaic crystal tiles suitably positioned in the lens. The samples were grown and worked for this project at the Institute Laue-Langevin (ILL) in Grenoble (France), while the reflectivity tests were performed at the X-ray facility of the Physics Department of the University of Ferrara.

A breakthrough in the sensitivity level of the hard X-/gamma-ray telescopes, which today are based on detectors that view the sky through (or not) coded masks, is expected when focusing optics will be available also in this energy range. Focusing techniques are now in an advanced stage of development. To date the most efficient technique to focus hard X-rays with energies above 100 keV appears to be the Bragg diffraction from crystals in transmission configuration (Laue lenses). Crystals with mosaic structure appear to be the most suitable to build a Laue lens with a broad passband, even though other alternative structures are being investigated. The goal of our project is the development of a broad band focusing telescope based on gamma-ray lenses for the study of the continuum emission of celestial sources from 60 keV up to >600 keV. We will report details of our project, its development status and results of our assessment study of a lens configuration for the European Gamma Ray Imager (GRI) mission now under study for the ESA plan Cosmic Vision 2015-2025.

A gamma-ray telescope mission concept [gamma ray imager (GRI)] based on Laue focusing techniques has been proposed in reply to the European Space Agency call for mission ideas within the framework of the next decade planning (Cosmic Vision 2015–2025). In order to optimize the design of a focal plane for this satellite mission, a CdZnTe detector prototype has been tested at the European Synchrotron Radiation Facility under an ∼100% polarized gamma-ray beam. The spectroscopic, imaging, and timing performances were studied and in particular its potential as a polarimeter was evaluated. Polarization has been recognized as being a very important observational parameter in high energy astrophysics (&gt;100 keV) and therefore this capability has been specifically included as part of the GRI mission proposal. The prototype detector tested was a 5 mm thick CdZnTe array with an 11×11 active pixel matrix (pixel area of 2.5×2.5 mm2). The detector was irradiated by a monochromatic linearly polarized beam with a spot diameter of about 0.5 mm over the energy range between 150 and 750 keV. Polarimetric Q factors of 0.35 and double event relative detection efficiency of 20% were obtained. Further measurements were performed with a copper Laue monochromator crystal placed between the beam and the detector prototype. In this configuration we have demonstrated that a polarized beam does not change its polarization level and direction after undergoing a small angle (&lt;1°) Laue diffraction inside a crystal.