R and D progress on second-generation crystals for Laue lens applications

Abstract

The concept of a gamma-ray telescope based on a Laue lens offers the possibility to increase the sensitivity by more than an order of magnitude with respect to existing instruments. Laue lenses have been developed by our collaboration for several years : the main achievement of this R&D program was the CLAIRE lens prototype, which has successfully demonstrated the feasibility of the concept in astrophysical conditions. Since then, the endeavour has been oriented towards the development of efficient diffracting elements (crystal slabs) in order to increase both the effective area and the width of the energy bandpass focused, the aim being to step from a technological Laue lens to a scientifically exploitable lens. The latest mission concept featuring a gamma-ray lens is the European GammaRay Imager (GRI) which intends to make use of the Laue lens to cover energies from 200 keV to 1300 keV.Investigations of two promising materials, low mosaicity copper and gradient concentration silicongermanium are presented in this paper. The measurements have been performed during three runs: 6 + 4 days at the European Synchrotron Radiation Facility (Grenoble, France), on beamline ID15A, using a 500 keV monochromatic beam, and 14 days on the GAMS 4 instrument of the Institute Laue Langevin (Grenoble, France) featuring a highly monochromatic beam of 517 keV. Despite it was not perfectly homogeneous, the presented copper crystal has exhibited peak reflectivity of 25 % in accordance with theoretical predictions, and a mosaicity around 26 arcsec, the ideal range for the realization of a Laue lens such as GRI. Silicon-germanium featuring a constant gradient have been measured for the very first time at 500 keV. Two samples showed a quite homogeneous reflectivity reaching 26%, which is far from the 48 % already observed in experimental crystals but a very encouraging beginning. The measured results have been used to estimate the performance of the GRI Laue lens design.

Quasi-mosaicity as a powerful tool to investigate coherent effects

“…Here r is the radius of the ring, Ω P is the bending angle induced in the tile by primary curvature and E the mean energy diffracted by the ring, this latter depending on the crystal reflection planes, as given by the formula [27] = ℎ…”

“…For the simulation, we considered the crystal tiles to be disposed as concentric rings onto the spherical calotte of a Laue lens with a focal distance f. The energy passband of a crystal in a ring is given by the formula [27] Δ ≃ 2Ω P /…”

Bent crystal selection and assembling for the LAUE project

“…In the framework of the Laue project [1], [2], one of the main goals is the development of a technology for the production of bent crystals with the required characteristics, like proper curvature, high efficiency, low mosaicity. The advantage of curved crystals is their high diffraction efficiency and their capability of better concentrating the signal collected over a large area into a small focal spot [3], [4]. This challenging goal is being achieved, thanks to the cooperation of many institutions.…”

Laue lens: the challenge of focusing gamma rays

“…Then, detailed studies started to refine all parameters in order to design lens exploitable for astrophysics. This led to the MAX concept of mission that has been proposed to the CNES in 2005 [2], and more recently to the Gamma Ray Imager mission (GRI) that has been proposed to European Space Agency (ESA) in the frame of Cosmic Vision AO1 [3,4].…”

“…As we can see from Bragg's law (1), a perfect single crystal that presents only a single angle of incidence to the incoming radiations will behaves as a monochromator, diffracting a very narrow line. Instead, suitable crystals must have an angular spread in their diffracting planes such that neighbouring rings create bandpass-overlaps resulting in continuous energy coverage over a specified band (angular acceptance, energy bandpass and profile of the diffracted beam are directly related [4]). This limits the choice to crystals having either a mosaic structure or curved diffracting planes (a complete description of Laue lenses can be found in [5]).…”