Focussing crystals for use in broad band hard X/soft gamma-ray Laue lenses

Hard X-/soft gamma-ray astronomy is a crucial window for the study of the most energetic and violent events in the Universe. To fulfil the scientific requirements in this regime, a new generation of telescopes with a broad operational band extending from tens up to several hundreds of keV and exploiting unprecedented sensitivity (50-100 times better that current instruments) is required. We report on diffractive bent crystals made of Gallium Arsenide (GaAs) that are suitable for the construction of high sensitivity X-/gamma-ray Laue lens space telescopes. Laue lenses, made of sets of diffractive crystals working in transmission, offer one possibility, albeit technically challenging, to build a new generation of focusing telescopes that can extend the energy band far beyond the 80 keV limit for current multilayer concentrators. In particular, we present the results obtained from the characterization of crystals that will be used to realise a broad band Laue demonstrator. They have been studied in terms of focusing capability and diffraction efficiency by using a flat X-ray panel imager and an HPGe spectrometer as focal plane detectors. The GaAs tiles, bent via a surface lapping procedure, have been developed at the IMEM/CNR in Parma (Italy) in the framework of the LAUE project funded by the Italian Space Agency. The main goal of the project was to build a broad band Laue lens demonstrator for hard X-/soft gamma-rays (80-300 keV).