Quasi-mosaicity is a mechanical property driven by anisotropy in diamond-like structure crystals such as Si and Ge. QM crystals were recently proposed as optical components of a Laue lens for focusing hard X-rays (with energy larger than 70 keV). In contrast to a Laue lens based on conventional crystals (e.g. mosaic crystals), usage of QM crystals allows focusing the incident beam in a spot smaller than the dimension of the diffracting crystal. Focusing of photons in a small spot would allow an unprecedented resolution and sensitivity, together with a wide-passband response. In astrophysics, a Laue lens implementing QM crystals should allow observations of cosmic phenomena producing X-ray emissions with high sensitivity. As another, a Laue lens would be useful for imaging in nuclear medicine, leading to a lower radioactive dose imparted to the patient because of no need for tomography scanning. Quasi-mosaicity was also used for bending Si crystals in order to steer high-energy particles via coherent effects in crystals, viz. planar channeling and volume reflection. Channeled or reflected light particles are also useful as sources of gamma ray beams with intense flux, which can be either monochromatic or polychromatic.
A series of quasi-mosaic curved crystals has been fabricated for the Laue project to build up a prototype of a Laue lens. Curvature has been imparted to the samples thanks to the grooving method, which makes it possible to realize self-standing curved plates. Samples are 10 × 30 × 2 mm3 Ge crystals characterized by two curvatures of different crystallographic planes. The primary curvature allows the focalization of the diffracted X-rays on a focal spot smaller than the crystal dimension. The secondary curvature, i.e. quasi-mosaic curvature, amplifies the diffraction efficiency by more than one order of magnitude with respect to an equivalent crystal without quasi-mosaic curvature.
We will describe the LAUE project, supported by the Italian Space Agency, whose aim is to demonstrate the capability to build a focusing optics in the hard X-/soft gamma-ray domain (80{600 keV). To show the lens feasibility, the assembling of a Laue lens petal prototype with 20 m focal length is ongoing. Indeed, a feasibility study, within the LAUE project, has demonstrated that a Laue lens made of petals is feasible. Our goal is a lens in the 80-600 keV energy band. In addition to a detailed description of the new LARIX facility, in which the lens is being assembled, we will report the results of the project obtained so far.
We present the status of LAUE, a project supported by the Italian Space Agency (ASI), and devoted to develop
Laue lenses with long focal length (from 10–15 meters up to 100 meters), for hard X–/soft gamma–ray astronomy
(80-600 keV). Thanks to their focusing capability, the design goal is to improve the sensitivity of the current
instrumention in the above energy band by 2 orders of magnitude, down to a few times 10−8 photons/(cm2 s keV).
A stacking of plate-like curved crystals is proposed as an optical element for realization of a highly efficient Laue lens in
astrophysics. Si mono-crystal plates have been bent by surface grooving and positioned one over the other to form a stack.
Reciprocal alignment of the curved diffracting planes in the stack has been investigated by hard x-ray diffractometry using
a polychromatic and divergent beam. The stack exhibited a single and well-defined focal spot under x-ray diffraction,
highlighting that the plates are sufficiently aligned to behave as they were a single crystal. The curvature of the plates
in the stack is self-standing and can be highly controlled by adjusting the experimental parameters of grooving. Thanks
to the stacking, it would be possible to realize optical elements with arbitrarily large size. This achievement opens up
important implications toward the realization of satellite-borne experiments in astrophysics or instruments for nuclear
medicine with superior resolution. Surface grooving is easy, cheap, highly reproducible and has been established for Si
and Ge, highlighting very high diffraction efficiency over a broad range of energies up to 700 keV, peaking 95% at 150
keV for Si.
For fabrication of crystals with curved diffracting planes, several techniques have been worked out. Amongst curved
crystals, special interest is given to those that are being bent due to internal forces. Surface grooving is proposed as an
efficient method to reproducibly obtain self-standing bent crystals. Silicon or germanium plates can be plastically deformed
by grooving one of their major surfaces with very good control of the curvature. We present a systematic experimental
study and a model based on the theory of elasticity. The technique enables the fabrication, in a very reproducible fashion,
of curved crystals for the realization of an high-efficiency hard X-ray concentrator (Laue lens).
Quasi-mosaicity can be used to fabricate self-standing curved crystals with two curvatures of different crystalline planes.
Indeed, a primary curvature imparted to a crystal results in quasi-mosaic curvature of a different plane direction. We show
that, since the size of the focal spot of the photons diffracted by a crystal can be controlled by the quasi-mosaic curvature,
quasi-mosaic crystals allow focusing with very high resolution. A Laue lens, exploiting quasi-mosaic effect, has been
simulated and main results are shown. Self-standing quasi-mosaic crystals can be fabricated through several techniques,
such as film deposition or surface grooving method.
We present an experimental study on the method of surface grooving for bending crystals for the realization of a
hard x-ray Laue lens. Bent Si and Ge crystalline plates were analyzed by x-ray diffraction of their (111) planes
at the European Synchrotron Radiation Facility. Crystals diffracted photons from 150 to 700 keV with efficiency
peaking 95% at 150 keV for Si. Measured rocking curves of the samples showed flat-topped profiles with their
FWHM equal to the crystal bending, i.e., the method of surface grooving proved to evenly bend the crystals,
their energy passband being very well controlled. Surface grooving technique has been found to offer both high
reproducibility and easy control of diffraction properties of the crystals. Besides, this method is cheap, simple
and compatible with mass production, making it a reliable technique for fabrication of a Laue lens, where serial
production of crystals should be envisaged. A Laue lens made of crystals bent by surface grooves can lead to
significant detection improvement in astrophysical applications.
Quasi-mosaicity is an effect of secondary bending within a crystal driven by crystalline anisotropy. This effect
can be used to fabricate a series of curved crystals for the realization of a Laue lens. We highlight that crystals
bent by quasi-mosaic effect envisage very high resolution focusing with respect to mosaic crystals. Under same
conditions for energy passband and flux of incident photons, a Laue lens based on quasi-mosaic crystals would
increase the signal-to-noise ratio by more than one order of magnitude as for the same lens with mosaic crystals.
A proposal for the realization of a self-standing quasi-mosaic crystal is given. Here permanent bending of a
crystal is achieved as a result of superficial indentations.
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