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Crystals with curved diffracting planes for hard X-ray optics
No full textCrystals with curved diffraction planes (CDP crystals) are an emerging technology in hard X-ray optics. Through them, it is possible to condition hard X-ray beams with high energy efficiency and flexibility. Optical instruments for visible light are not effective to manipulate high-energy radiation. In fact, the phenomena of reflection and refraction on which they are based become unimportant for electromagnetic waves with high energy. For this reason, optics of hard X-rays and gamma rays has been up to now based direct view instruments.
In contrast, diffraction phenomena can be important even at high energies. Therefore, it is possible to condition hard X-ray beams by Bragg diffraction. Bragg diffraction is called Laue diffraction when the electromagnetic wave goes through the crystal. Due to the length of penetration of high-energy photons, this configuration appears to be preferable than diffraction in reflection geometry (Bragg diffraction). Recently, optical systems have been developed based on perfect crystals or mosaic crystals. However, instruments based on these materials have constructive difficulties, poor reproducibility, and finally a limited efficiency.
Crystals with curved lattice planes have an efficiency in the manipulation of X-rays that approaches the unity. Through the curvature of their plans, they offer a continuum of possible angles of diffraction to incident radiation. Thus, the curvature of planes allows to influence photons in a wide range of energies. It is also possible to apply more than one curvature to a single crystal, deforming the symmetry, thus turning it into a focusing element.
The curved crystals have elective application in the construction of lenses for hard X-rays (Laue lenses), composed by a set of crystal with diffraction planes oriented towards a common point, so as to diffract the incident radiation towards the focus of the lens. This tool is especially useful in the field of astrophysics and medical physics. The observation of the sky over energy of 70 keV is still left to direct view instruments. These have a low angular resolution, a low signal to noise ratio, and a high weight, which is an important parameter because the observation of X-rays is only possible out of the atmosphere. The cosmic sources of X-rays produce a very low flux of photons, so the signal to noise ratio of the instrument becomes a key factor for the investigation of these sources. Today, only the spectra of the most intense sources are known at energies above 70 keV.
Another discipline that can benefit from the use of CDPs crystals is nuclear medicine. Indeed, it would be possible to increase the sensitivity and accuracy of instruments for nuclear medicine by using focusing optics based on curved crystals. The spatial resolution of the instrument would be increased to a sub-millimeter precision, with which it would be possible to accurately locate tumors and other phenomena of interest, and to provide more information on the same. Due to the high reflectivity of the CDPs crystals, even the number of photons that can be collected on the detector would be increased, with a consequent improvement in the sensitivity of the instrument, and a decrease in the dose of radio-drug to be injected into the patient.
The formalism developed so far under the dynamical theory of diffraction allows to describe effectively crystals with flat diffraction planes, or crystals with diffraction planes strongly deformed. Various applications require CDPs with low curvatures, in particular when crystals with high atomic number are used. In this case, the formalism of dynamical theory is not applicable in a simple way. To know the performance of the CDPs crystals in these areas of application, it is necessary the development of a treatment dedicated to the purpose. The theoretical work has touched the
development of the dynamic theory of diffraction in order to cover its weaknesses for CDPs with low curvature.
The work of technological development and construction of hardware for scientific research focused on the development and production of innovative crystals for the construction of hard X-ray lenses. The construction of Laue lenses has been simplified with the introduction of focusing crystals. The thesis work was partially carried out within the LAUE Project, funded by ASI, whose purpose was the construction of a large-area Laue lenses. The work done during the doctoral thesis has led to a rethinking of the project in order to use focusing crystals and thus to increase the resolution and sensitivity of the lens.
The work on crystals for Laue lenses did not end with the LAUE Project, but it continued with a series of technological innovations with the aim to increase the maximum resolution and sensitivity achievable by Laue lenses. These innovations include the introduction of crystals with multiple curvatures and focusing crystals, the study of novel methods with which to produce crystals with CDPs, the construction of stacks of multi-crystals, the use of curved asymmetric diffraction planes. The experimental tests were performed at the facilities of ESRF (European Synchrotron Radiation Facility, Grenoble, France), ILL (Institut Laue-Langevin, Grenoble, France) and LARIX (LARge Italian X-ray facility, Ferrara, Italy).
Simulations were also performed to estimate the possible interferences that a Laue lens could suffer in orbit because of the interaction with cosmic rays and the consequent production of parametric X-rays. This source of X-rays may interfere with measurements of celestial X-ray sources. The work has led to the exclusion of this possibility, since the emission of parametric X-rays would be less than the sensitivity of the lens itself.
Finally, the technologies developed to produce CDPs crystals were used for the construction of a crystalline undulator built as a prototype of X-ray source with high-energy and high flux. Lattice planes of a crystals where shaped in an undulated structure. Characterization of these lattice planes showed near-ideal results. High-energy charged particles channeled through these planes would produce coherent X-rays. The prototype will be part of an experiment at the SLAC accelerator (San Francisco, USA)
Calculation of diffraction efficiency for curved crystals with arbitrary curvature radius
No full textA model is proposed to calculate the diffraction efficiency of X-rays in Laue geometry for curved crystals with an arbitrary value of the curvature radius. The model generalizes the results based on the dynamical theory of diffraction, which are valid only for crystals with a radius of curvature lower than the critical curvature. The model is proposed for any kind of crystal, and its efficiency tends to one-half in the limit of a thick flat crystal. On the basis of this model, it was possible to reconsider the results of recently observed diffraction efficiency for curved crystals. Finally, the model sets an upper limit for diffraction efficiency of low-curvature curved crystals, this latter being useful in applications such as the construction of a hard X-ray Laue lens
High diffraction efficiency with hard X-rays through a thick silicon crystal bent by carbon fiber deposition
No full textThe diffraction capability of two crystalline silicon plates bent by carbon fiber deposition has been studied. The performed treatment induced a permanent curvature in the samples, resulting in an increase of the diffraction efficiency. The obtained efficiencies are constant over a wide angular range and close to the theoretical expectations, meaning that the curvatures were homogeneous. Most importantly, the bending technique allowed the manufacture of bent samples up to 5 mm thick and with a radius of curvature down to 30 m. With such a technique, the fabrication of crystals for the realization of a hard X-ray concentrator (Laue lens) for astrophysical purposes is enabled
LOGOS-INFN
No full textLa rivelazione di raggi X e
è attualmente un argomento di grande interesse per la comunità scientica, in particolare nel campo dell'astrosica e della medicina nucleare. In entrambi i settori, però, la rivelazione di fotoni alle energie di interesse (centinaia di keV) risulta avere bassa efficienza e risoluzione, in quanto non si dispone di ottiche focalizzanti e i rivelatori sono esposti direttamente alla radiazione, mentre la ricostruzione dell'immagine è basata su collimatori e maschere codicate. Lo scopo principale di questo progetto è lo sviluppo di tecnologie abilitanti per la realizzazione di un elemento focalizzante per raggi X altamente performante e da utilizzare nell'ambito della realizzazione di una lente di Laue. La comunità scientica è alla ricerca di tecniche per la realizzazione di ottiche focalizzanti per raggi X duri e
, energia da 100 a 1000 keV. Tali ottiche, dette lenti di Laue, consistono in una serie di cristalli disposti in anelli concentrici, in modo tale da poter dirangere quanta più radiazione possibile verso un punto comune, il fuoco della lente, in cui viene posizionato un rivelatore. La possibilità di realizzare una lente di Laue altamente eciente e risolvente è legata principalmente alla scelta degli elementi ottici attivi, ossia i cristalli dirangenti. In questi ultimi anni sono stati proposti diversi tipi di cristalli per la realizzazione di tale lente, ma nessuna di queste proposte ha consentito il raggiungimento degli obiettivi di risoluzione ed efficienza necessari ai ni applicativi. Il presente progetto si pregge di rispondere a questa necessità della comunità scientica attraverso la realizzazione di cristalli di silicio e di germanio intrinsecamenti curvi sfruttando l'eetto di quasi-mosaicità di questi materiali. Solo grazie alle proprietà di anisotropia meccanica di questi monocristalli, scegliendo opportune orientazioni cristallograche e sagomature, a seguito di una piegatura meccanica si ottiene anche una curvatura dei piani cristallini posti lungo lo spessore. Cristalli così ottenuti possono essere usati per dirangere radiazione elettromagnetica di elevata energia con elevata efficienza. Tramite un opportuno assemblaggio di cristalli è prevista la possibilità di realizzare una lente di Laue atta a focalizzare radiazione elettromagnetica avente energia dell'ordine delle centinaia di keV con elevata efficienza e potere risolvente
Mechanical properties of crystals bent through indentations for the realization of a Laue lens for concentration of hard x-rays
No full textPermanent and reproducible deformation of silicon mono-crystals has been experimentally attained by mechanical grooving of one of its surfaces. The method produces uniform deformation that can be controlled by proper choice of the parameters of grooving. The strain in silicon plates was modeled analytically in terms of a superficial compression occurring in the material between the grooves, and the strain propagates deep into the bulk. The superficial grooved region acts as it were like a tensile film, allowing a spherical and homogeneous curvature of the whole crystal plate. Thereby, the classic Stoney’s formula was reconsidered and adapted for explanation of the experimental behavior. Minimum radius of curvature experimentally achieved was about 3 meters. Curved crystals are the basis for many applications, e.g. for x-ray optics to manipulate high energy photons for the construction of very sensitive instruments for x-ray astronomy
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