Search

The Laue lens is an emerging technology that uses crystal diffraction to concentrate gamma-rays. The Darwin model describes diffraction in mosaic crystals at high energies (>50 keV) and is the basis for assessing the throughput of a Laue lens. While the Darwin model has been used successfully to describe binary alloys, it has shown inconsistencies when applied to diffraction with pure-metal crystals. This paper presents a systematic study of diffraction in pure-metal silver mosaic crystals and the performance of the Darwin model when applied to the data. Two cut silver crystals were tested at the Institute Laue–Langevin facility at three gamma-ray energies and at three different crystallographic orientations. The rocking curves were fitted using the Darwin model. A single Gaussian for the distribution of crystallite orientations leads to poor quality fits. A crystallite distribution that is the sum of two Gaussians gives improved fits. However, for each crystal, the Darwin model gives mosaicities and crystallite sizes that are not consistent with one another as a function either of energy or of crystallographic orientation, despite these being physical properties of the materials. Without an adequate model to describe diffraction in pure-metal crystals, either a Laue lens design must severely limit its catalogue of usable materials or each crystal in the design must be tested at the exact energy at which it is to be used. A more general theory to describe hard X-ray diffraction using pure-metal mosaic crystals, possibly involving a distribution of crystallite sizes, is therefore required.

A germanium crystal was bent through a grid of superficial grooves, manufactured on the sample surface. The resulting diffraction planes were bent thanks to quasi-mosaicity, which is an effect of mechanical anisotropy in crystals. High integrated diffraction efficiency was achieved in symmetric Laue geometry with a monochromatic X-ray beam set at 150 and 300 keV. It is demonstrated that the sample is capable of efficiently focusing X-rays. Such crystals can be used as optical components to focalize X- and γ-rays in a high-resolution Laue lens.

Crystals are the elementary constituents of Laue lenses, an emerging technology which could allow the realization of a space-borne telescope 10–100 times more sensitive than existing ones, in the 100 keV–1.5 MeV energy range. This paper addresses the development of efficient crystals for the realization of a Laue lens. In the theoretical part, 35 candidate crystals, both pure and two-component crystals, are considered. Their peak reflectivity at 100, 500 keV and 1 MeV is calculated assuming they are mosaic crystals. It is found that, by careful selection of crystals, it is possible to achieve a reflectivity above 30% over the whole energy range, and even up to 40% in the lower part of the energy range. In the experimental part, three different materials (Si1−xGexwith a gradient of composition, mosaic Cu and Au) have been measured at both ESRF and ILL using highly monochromatic beams ranging from 300 to 816 keV. The aim was to check their homogeneity, quality and angular spread (mosaicity). These crystals have shown outstanding performance, such as reflectivity up to 31% at ∼600 keV (Au) or 60% at 300 keV (SiGe) and angular spread as low as 15 arcsec for Cu, fulfilling very well the requirements for a Laue lens application. An unexpected finding is that there are important discrepancies with Darwin's model when a crystal is measured using various energies.

The diffraction properties of a crystalline silicon plate of which one face was mechanically indented have been studied. This treatment induced a permanent curvature in the sample, which allowed a diffraction efficiency of 88% for 150 keV photons,i.e.a reflectivity of 64% including the absorption. This efficiency is constant over 14 arcseconds and is very close to the theoretical expectation, meaning that the curvature is highly homogeneous. The technique enables the fabrication, in a very reproducible fashion, of crystals for the realization of an astronomical hard X-ray concentrator (Laue lens).

A method that employs simultaneous neutron and γ-ray diffraction techniques is proposed for accurate measurements of strains with or without an applied electric field (0–100 kV m−1). Instead of an absolute determination of the position of the neutron Bragg peak, a relative measurement of the neutron and γ peak positions is used and allows a determination of the lattice-parameter changes with an accuracy of within 2 × 10−6. This technique avoids systematic errors originating in sample rotations by taking advantage of a reference directly related to the crystal lattice. The method appears to be particularly important for crystals that exhibit structural phase transitions, as demonstrated by the measurement of the strain Uxx in KH2PO4 crystals as a function of temperature and applied electric field. In this case the errors would amount to ±20%. The results demonstrate the need to re-examine the theory previously proposed by Uesu, Tanaka & Kobayashi [Ferroelectrics (1974), 7, 247–249] to interpret the electromechanical properties of KH2PO4.