Laue-telescope construction - a new approach

Abstract. The feasibility of focusing soft gamma rays (0.1-2 MeV) using Laue diffraction in high quality metal crystals have been studied in many laboratories for over 50 years [1-4]. The manufacture of suitable crystals does not anymore appear as the primary obstacle. But the overall efficiency of Laue lenses still appears marginal, the issue of the precise and stable mounting of thousands of crystal facets has turned out to be very complex, and the design of an efficient detector to be used at the focus of the lens is still largely unproven. A method to improve the flux collection efficiency of a Laue lens by almost a factor two will be described here. This is possible using double layers of crystals and relying on the fact that the efficiency of a single crystal layer is limited by the coherent primary extinction rather than by incoherent scattering or absorption. If the energy band passes of the two crystal layers do not overlap, each layer can diffract relatively undisturbed by the other. Two construction aspects concerning the mounting and alignment of the crystals are discussed in this work. Although such very technical issues may seem hardly worth discussing in a scientific paper, the mounting of the crystals and the verification of the critical crystal tilt have turned out to be quite difficult. To get around the problems we propose here some radical departures from the currently employed techniques. The technologies proposed can be readily developed and tested on small-scale experiments. The last aspect discussed is the problem of increasing the efficiency of detecting the diffracted photons at the telescope focus. In existing wide-field gamma-ray instruments the detection efficiency in the MeV region rarely exceeds 20%. It is disheartening to invest millions in building a complex lens knowing that more than three quarters of the photons focused by the lens will be lost in the detector. Fortunately, in the focus of a Laue lens it is possible to do better, and our conclusion is that with proper design we can approach or even exceed 50% detector efficiency.