tokens: of passive materials, e.g. the lens frame and satellite housing. The sensitivity was calculated as follows: where is the exposure time, set at 100 ks. The results for the sensitivities are shown in Fig. 9 b. The best sensitivities are achieved for cases 2 and 3. Finally, to compare the three focal spots, it was resorted to the S/N , as in Guidi et al. (2011) , where is the signal count within a specified encircled area with radius , is the background of the area, and is the useful volume of the detector. For a Laue lens, the size of the detector determines its field of view for off-axis sources. Here, an on-axis source was simulated and the noise of the detector was considered proportional to the focal spot size. In particular, was chosen to enclose 90% of the diffracted photons (see Table 3 ) in the focal plane and the signal within this area. Thus, the background was taken proportional to the volume of the detector, which faced the incoming X-rays with a surface as large as the encircled area. Because of different focalization properties, the obtained S/N for the three cases were significantly different. As a result of the simulation, the quotient between the S/Ns for the three cases turned out to be as follows: Thus, QM crystals are also the most efficient in terms of the signal-to-noise ratio. The comparison worked out in this paper is based on a photon beam with energy centred at 200 keV. This is mainly because of the high diffraction efficiency of Ge in this energy range. The obtained results also hold for different energies. For low energy, such as 100 keV, Si crystals can be used. For photons with energy greater than 600 keV, the diffraction efficiency of Si and Ge crystals decreases. With the aim of focusing such high energy photons at high efficiency, crystals with high , such as silver, gold, lead, etc. ( Barrière et al. 2009 ), are needed. These materials cannot be fabricated as perfect crystals, but only as mosaic crystals. Thus, CDP and QM crystals would be not available if high material are used, and a comparison between these types of crystal would be impossible. In this case, it would be important to take into account that metal crystals have particular mechanical and chemical properties, which make them very difficult to cut, align, and carry out the gluing and polishing process. In this paper, a comparison between different types of diffracting crystals has been worked out. Actually, the feasibility of a Laue lens depends on several criteria that have to be taken into account. With regard to the crystals, important parameters are the availability of crystalline materials, reproducibility of the crystal manufacturing, and alignment and mounting of the crystals on the lens frame. Moreover, in the context of a space-borne Laue lens, additional criteria must be considered, such as payload, mechanical robustness to shocks and vibrations, and long-time reliability with respect to chemical and temperature cycling. A simulation for
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