LAUE Project Research Articles

Expected performances of a Laue lens made with bent crystals

In the context of the LAUE project devoted to build a Laue lens prototype for focusing celestial hard X-/soft gamma-rays, a Laue lens made of bent crystal tiles, with 20 m focal length, is simulated. The focusing energy passband is assumed to be 90--600 keV. The distortion of the image produced by the lens on the focal plane, due to effects of crystal tile misalignment and radial distortion of the crystal curvature, is investigated. The corresponding effective area of the lens, its point spread function and sensitivity are calculated and compared with those exhibited by a nominal Laue lens with no misalignment and/or distortion. Such analysis is crucial to estimate the optical properties of a real lens, in which the investigated shortcomings could be present.

The LAUE project: latest developments

We present the status of the LAUE project devoted to develop a technology for building long focal length Laue lenses for hard X-/soft gamma-ray astronomy (80-600 keV). The Laue lens will be composed of bent crystals of Gallium Arsenide (GaAs, 220) and Germanium (Ge, 111), and, for the first time, the focusing property of bent crystals will be exploited for this field of applications. At the present the goal of the project is the building of a 20 m focal length Laue lens petal capable of focusing X-ray in the energy range 90-300 keV.

Focusing effect of bent GaAs crystals for γ-ray Laue lenses: Monte Carlo and experimental results

We report on results of observation of the focusing effect from the planes (220) of Gallium Arsenide (GaAs) crystals. We have compared the experimental results with the simulations of the focusing capability of GaAs tiles through a developed Monte Carlo. The GaAs tiles were bent using a lapping process developed at the cnr/imem - Parma (Italy) in the framework of the laue project, funded by ASI, dedicated to build a broad band Laue lens prototype for astrophysical applications in the hard X-/soft gamma-ray energy range (80-600 keV). We present and discuss the results obtained from their characterization, mainly in terms of focusing capability. Bent crystals will significantly increase the signal to noise ratio of a telescope based on a Laue lens, consequently leading to an unprecedented enhancement of sensitivity with respect to the present non focusing instrumentation.

A simulation study on the focal plane detector of the LAUE project

The LAUE project, supported by the Italian Space Agency (ASI), is devoted to the development of a long focal length (even 20m or longer) Laue lens for gamma ray astronomy between 80 and 600keV. These lenses take advantage of Bragg diffraction to focus radiation onto a small spot drastically improving the signal to noise ratio as well as reducing the required size of the detector significantly. In this paper we present a Monte-Carlo simulation study with MEGALIB to optimize, for space applications, the detector size to achieve high detection efficiency, and to optimize the position resolution of the detector to reconstruct the Point Spread Function of the lens considered for the LAUE project. Then we will show simulations, using the SILVACO semiconductor simulation toolkit, on the optimized detector to estimate its capacitance per channel and depletion voltage. In all of the simulations, two materials were compared; a low density material (Silicon) and a high density material (Germanium).

Study and characterization of bent crystals for Laue lenses

We report the results obtained from the characterization of bent Silicon, Germanium and Gallium Arsenide crystals, which are being used for focusing photons in the 90-300 keV energy range, in the framework of the LAUE project. Such crystals represent the best choice for building Laue lenses thanks to their high reflectivity and to the narrow Point Spread Function (PSF) they can provide. The test aims to select the best material, to optimize the thickness and to estimate the curvature radius of the crystal tiles that will be used for building a 20 m focal length Laue lens petal.

X-ray diffraction efficiency of bent GaAs mosaic crystals for the Laue project

In a Laue lens made of single crystals oriented to diffract parallel x-rays at the lens focus, the energy and angular resolution are limited by the crystal size and by the crystal mosaicity. The use of extended crystals bent according to the lens curvature provides better focusing, with the resolution given essentially by the crystal mosaicity. With this approach, a crystal mosaicity as low as 15–25 arcsec, well below the mosaicity value of copper crystals, was found suitable for the new design of the Laue lens. The reflectivity and transmission profiles and the integrated intensity have been measured in flat and bent GaAs and Si crystals prepared by the method of surface damaging by using sandpaper of different grain size. The surface grinding induces a local lattice strain which produces a self-standing bent crystal. Bent crystals with radius of curvature lower than a critical value given by the extinction length behave as ideal mosaic crystals, maximizing the diffraction efficiency at high x-ray energies. It is found that the surface grinding does not affect the crystal diffraction efficiency, the damage thickness being limited to a few tens microns near the crystal surface.

Highly reproducible quasi-mosaic crystals as optical components for a Laue lens

The realization of a Laue lens for astronomical purposes involves the mass production of a series of crystalline tiles as optical components, allowing high-efficiency diffraction and high-resolution focusing of photons. Crystals with self-standing curved diffraction planes is a valid and promising solution. Exploiting the quasi-mosaic effect, it turns out to be possible to diffract radiation at higher resolution. In this paper we present the realization of 150 quasi-mosaic Ge samples, bent by grooving one of their largest surface. We show that grooving method is a viable technique to manufacture such crystals in a simple and very reproducible way, thus compatible with mass production. Realized samples present very homogenous curvature. Furthermore, with a specific chemical etch, it is possible to fine adjust one by one the radius of curvature of the grooved samples. Realized crystals was selected for the ASI’s Laue project, that involves the implementation of a prototype of a Laue lens for hard X- and soft γ-ray astronomy.

THE LAUE PROJECT FOR BROADBAND GAMMA-RAY FOCUSING LENSES

We present the LAUE project devoted to develop an advanced technology for building a high focal length Laue lens for hard X and soft gamma–ray astronomy (80-600 keV). The final goal of the project, supported by the Italian Space Agency, is to build and test a focusing optics that can improve the current sensitivity in the above energy band by 2 orders of magnitude.

The discovery of X-ray diffraction

Starting from the discovery of Roentgen’s X-ray (Rontgen 1895), I here reconstruct the various attempts carried out to find X-ray diffraction using very thin wedge-shaped slits. These experiments allowed A. Sommerfeld to achieve, at the beginning of 1912, a qualitative estimate of the wavelength of X-rays around 4 × 10−9 cm. This value was at the base, together with the adoption of the space-lattice model for the crystal, of Laue idea of using a crystal as a diffraction grating for X-ray. Then we will discuss the feasibility of the Laue project, as well as the realization of the experiment, and the first results of W. Friedrich and P. Knipping, with the theoretical interpretation given to them by Laue. Laue’s discovery of X-ray diffraction was immediately taken up by William Henry Bragg and re-interpreted by his son William Lawrence in terms of X-ray reflection by the planes of the crystal, thus becoming, in a few months, a formidable tool for the study of crystal structures, as well as for the precise determination of the wavelengths of X-rays (to Bragg father we owe the construction of the first X-ray ionization spectrometer). Particular attention will be given to the works of C. G. Barkla on X-ray absorption, which resulted in the discovery in 1906 of the Roentgen characteristic radiation of the elements. These works strengthened the hypothesis of the wave nature of X-rays, at the expense of the “corpuscular hypothesis” (supported notably by J. Stark and by W.H. Bragg), and represented a solid reference point for the project of Laue.