High diffraction efficiency, broadband, diffraction crystals for use in crystal diffraction lenses

Abstract

A major goal of the MAX program is to detect and measure gamma rays produced following the nuclear reactions that take place in a supernova explosion. To detect a reasonable number of supernovae, sensitivities of the order of a few times 10-7 γ cm-2sec-1 are needed -much better than possible with current instruments. The approach in the MAX program is to use a crystal diffraction lens to collect photons over a large area and concentrate them on a small well-shielded detector, greatly improving the signal to noise ratio. The crystals need to have both high diffraction efficiency and a relatively broad energy bandwidth. With mosaic crystals there is a trade-off between bandwidth and diffraction efficiency -one can have either high efficiency or large bandwidth, but not both without losing too much intensity through atomic absorption. A recent breakthrough in our understanding of crystal diffraction for high-energy gamma rays has made it possible to develop crystals that have both high diffraction efficiency and a relatively broad energy bandwidth. These crystals have near perfect crystal structure, but the crystalline planes are slightly curved. Such curved planes can be obtained in 3 different ways, by using mixed crystals with a composition gradient, by applying a thermal gradient, and by mechanically bending a near perfect crystal. A series of experiments have been performed on all three types of crystals using high-energy x-ray beams from the Advanced Photon Source at the Argonne National Laboratory. Experiments performed at 3 energies, 93 keV, 123 keV and 153 keV, with both the thermal gradient Si crystals and with the mechanically bent Si crystals, demonstrated that one can achieve diffraction efficiencies approaching 100% with moderate energy bandwidths ( E/E = 1.4%) and low atomic absorption (transmission = 0.65), in excellent agreement with theory. The use of this type of diffraction crystal is expected to increase the sensitivity of gamma ray telescopes by a factor of 5 over that possible with normal mosaic crystals.

Monte Carlo study of detector concepts for the MAX Laue lens gamma-ray telescope

“…Despite their paramount interest, observations of gamma-ray lines have historically been plagued by intense and complex instrumental backgrounds against which the much smaller signals from celestial sources need to be discerned (e.g. Weidenspointner et al, 2005). Recently, a novel experimental technique, promising to improve the sensitivity of existing gamma-ray telescopes by one to two orders of magnitude, has been demonstrated: the Laue lens von Ballmoos et al, 2004).…”

“…Weidenspointner et al, 2005). Recently, a novel experimental technique, promising to improve the sensitivity of existing gamma-ray telescopes by one to two orders of magnitude, has been demonstrated: the Laue lens von Ballmoos et al, 2004).…”

“…A Laue lens takes advantage of Bragg diffraction in crystals to concentrate incident gamma rays onto a detector (Lund, 1992;Smither et al, 1995;Halloin et al, 2004;von Ballmoos et al, 2004). In this approach it is possible to employ a large photon collecting area together with a small detector volume, which results in a greatly increased signal-to-background ratio and hence a greatly improved sensitivity.…”

“…MAX is a Laue lens telescope that has been proposed to the French Space Agency CNES in response to an announcement of opportunity for a formation flight demonstration mission (Barrière et al, 2006;von Ballmoos et al, 2006). The MAX gamma-ray Laue lens consists of Ge and Cu crystals and has a focal length of about 86 m. MAX is designed to concentrate gamma rays in two 100 keV wide energy bands centered on the two lines which constitute the prime astrophysical interest of the mission: the 511 keV positron annihilation line, and the broadened 847 keV line from the decay of 56 Co copiously produced in Type Ia supernovae.…”

“…Abrosimov et al, 2006;Courtois et al, 2006;Smither et al, 2006). However, to achieve the best possible performance of MAX, it is also necessary to optimize the detector used to collect the source photons concentrated by the lens.…”