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.

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.

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A Laue lens for focusing X-ray photons with energies above 60 keV for astrophysical applications is being developed. The lens is based on mosaic crystals of Cu (111) produced at the Institute Laue-Langevin. A feasibility study has allowed to establish lens geometry and crystal properties required. The test of the crystals has provided very satisfactory results. We are now developing a Demonstration Model (DM) of the lens in order to establish the best assembling technique of the crystals. We will discuss the status of the project and its prospects. SCIENTIFIC MOTIVATIONSThe role of hard X-ray astronomy (> 10 keV) is now widely recognised. The numerous results obtained with the most recent satellite missions (BeppoSAX , Rossi-XTE , INTEGRAL) on many classes of X-ray celestial sources have demonstrated the importance of the high (> 60) energy band in order to derive an unbiased picture of the radiation emission mechanisms of the celestial sources, their nature, their evolution with time, their contribution to the Cosmic X-ray Background (CXB). In spite of the excellent results obtained thus far, the statistical quality of the measured spectra with the best instrumentation of the current generation becomes poor at these energies. Thus only the strongest sources can be accurately studied. The development of focusing optics at these energies and, more generally, in the entire passband covered by the BSAX/PDS and beyond, is of key importance to study 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.X-ray mirrors based on Bragg diffraction from multilayers of bi-strates made of high and low Z materials, 1 can overcome the sensitivity problem up to about 70 keV. At higher energies the Bragg diffraction from mosaic crystals in transmission configuration (Laue geometry) appears the most efficent technique for focusing photons. A Laue lens is a set of properly oriented crystals that focus X-ray photons via Bragg diffraction in Laue geometry. X-ray diffraction can occur in crystals because the lattice structure is periodic and its period is comparable with the wavelength λ of the X-rays: crystals are a natural diffraction grating for X-rays. However, standard (almost perfect) crystals are not suitable for building lenses for astrophysical investigations. Indeed they allow a high reflection efficiency in a very narrow energy band (in the keV fraction range) and a very narrow angular range of photon acceptance (in the arcsecond range), while for studying properties of celestial sources, a good reflection efficiency is required in a relatively broad energy band (tens to hundreds of keV, depending on science goal), along with a relatively large field of view (in the arcminute range). For these types of lens mosaic crystals appear the best solution.