A genetic algorithm to design Laue lenses with optimal performance for focusing hard X- and gamma-rays

In order to focus hard X- and gamma-rays it is possible to make use of a Laue lens as a concentrator. With this optical tool it would be possible to improve the detection of radiation for several applications, spanning from the observation of the most violent phenomena in the sky to nuclear medicine applications, for diagnostic and therapeutic purposes. A code named LaueGen, based on a genetic algorithm and aimed to designing optimized Laue lenses, has been implemented. The genetic algorithm was selected because the optimization of a Laue lens is a complex and discretized problem. The output of the code consists in the design of a Laue lens composed of diffracting crystals selected and arranged in such a way to maximize the performance of the lens. The code allows one to manage crystals of any material and crystallographic orientation. The program is structured in such a way that the user can control all the initial parameters of the lens. As a result, LaueGen is highly versatile and can be used for the design of very small lens, e.g. for nuclear medicine, to very large lens, e.g. for satellite-borne astrophysical missions.Comment: 18 pages, 4 figure

Curved crystals as optical elements for focusing X- and γ rays in a Laue lens

Curved crystals as optical elements for focusing X- and γ rays in a Laue lens Abstract This thesis is devoted to achieve a method to realize bent crystals in order to diffract hard X- and γ rays with high-efficiency. A simple and economical method that would lead to the production of accurate and homogeneous bent samples is presented. In fact, a homogeneous curvature is a necessary condition for the diffraction of the radiation with high efficiency and resolution. Furthermore, an appropriate physical model is given, to foresee the curvature of the samples as a function of the production parameters. Several silicon and germanium samples were bent and pre-characterized at the Sensor and Semiconductor Laboratory (SSL) of Ferrara, Italy. The focusing capabilities of the crystals were tested with monochromatic and polychromatic X- and γ rays at the European Synchrotron Radiation Facility (ESRF) and at the Institut Laue-Langevin (ILL) in Grenoble, France. Investigations and experimental validations of diffraction with unusual crystal configurations are described. Furthermore, two proposals of Laue lens are given. Such proposals were theoretical obtained with LaueGen, a genetic algorithm written for this purpose. Finally, the performance of these simulated Laue lenses are shown and compared with the data available in the literature. The work of thesis was carried out within the Laue project, which is a project financed by the Italian Space Agency (ASI). The final aim of the Laue project was the realization of a prototype of Laue lens composed of germanium and gallium arsenide bent crystals. In particular, the realization and the precharacterization of the germanium samples were performed during this Ph.D. period

Enhancement of the Inelastic Nuclear Interaction Rate in Crystals via Antichanneling

The interaction rate of a charged particle beam with the atomic nuclei of a target varies significantly if the target has a crystalline structure. In particular, under specific orientations of the target with respect to the incident beam, the probability of inelastic interaction with nuclei can be enhanced with respect to the unaligned case. This effect, which can be named antichanneling, can be advantageously used in the cases where the interaction between beam and target has to be maximized. Here we propose to use antichanneling to increase the radioisotope production yield via cyclotron. A dedicated set of experimental measurements was carried out at the INFN Legnaro Laboratories with the AN2000 and CN accelerators to prove the existence of the antichanneling effect. The variation of the interaction yield at hundreds of keV to MeV energies was observed by means of sapphire and indium phosphide crystals, achieving an enhancement of the interaction rate up to 73% and 25%, respectively. Such a result may pave the way to the development of a novel type of nozzle for the existing cyclotrons, which can exploit crystalline materials as targets for radioisotope production, especially to enhance the production rate for expensive prime materials with minor upgrades of the current instrumentation

Next-generation ultra-compact calorimeters based on oriented crystals

Calorimeters based on oriented crystals provide unparalleled compactness and resolution in measuring the energy of electromagnetic particles. Recent experiments performed at CERN and DESY beamlines by the AXIAL/ELIOT experiments demonstrated a significant reduction in the radiation length inside tungsten and PbWO4, the latter being the scintillator used for the CMS ECAL, observed when the incident particle trajectory is aligned with a lattice axis within ∼1∘. This remarkable effect, being observed over the wide energy range from a few GeV to 1 TeV or higher, paves the way for the development of innovative calorimeters based on oriented crystals, featuring a design significantly more compact than currently achievable while rivaling the current state of the art in terms of energy resolution in the range of interest for present and future forward detectors (such as the KLEVER Small Angle Calorimeter at CERN SPS) and source-pointing space-borne γ-ray telescopes

Laue lens for radiotherapy applications through a focused hard x-ray beam: A feasibility study on requirements and tolerances

Focusing a hard x-ray beam would represent an innovative technique for tumour treatment, since such a beam may deliver a dose to a tumour located at a given depth under the skin, sparing the surrounding healthy cells. A detailed study of a focusing system for hard x-ray aimed at radiotherapy is presented here. Such a focusing system, named Laue lens, exploits x-ray diffraction and consists of a series of crystals disposed as concentric rings capable of concentrating a flux of x-rays towards a focusing point. A feasibility study regarding the positioning tolerances of the crystalline optical elements has been carried out. It is shown that a Laue lens can effectively be used in the context of radiotherapy for tumour treatments provided that the mounting errors are below certain values, which are reachable in the modern micromechanics. An extended survey based on an analytical approach and on simulations is presented for precisely estimating all the contributions of each mounting error, analysing their effect on the focal spot of the Laue lens. Finally, a simulation for evaluating the released dose in a water phantom is shown

Laue lens for astrophysics: Extensive comparison between mosaic, curved, and quasi-mosaic crystals

With the aim of concentrating hard X- and γ-rays coming from celestial sources in the 100–1000 keV energy range, the concept of Laue lens was introduced more than 50 years ago. Crystals are the core of a Laue lens, since they focus the incoming X-rays through Bragg diffraction. For concrete applications, crystals characterized by high diffraction reflectivity are needed along with high-resolution focusing of diffracted photons. Here, an extensive comparison of the types of crystals proposed so far is presented. In order to quantify the focusing capability of a Laue lens based on these crystals, a simulation of a single-ring Laue lens based on the considered optical elements is presented. Finally, the breakthrough in the panorama of diffracting crystals is discussed

Quasi-mosaicity as a tool for focusing hard x-rays

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. © 2012 SPIE

The 'quasi-mosaic' effect in crystals and its applications in modern physics

‘Quasi-mosaicity’ is an effect of anisotropy in crystals that permits one to obtain a curvature of internal crystallographic planes that would be flat otherwise. The term ‘quasi-mosaicity’ was introduced by O. Sumbaev in 1957. The concept of ‘quasi-mosaicity’ was then retrieved about ten years ago and was applied to steering of charged-particle beams at the Super Proton Synchrotron at CERN. Beams were deviated by exploiting channeling and volume reflection phenomena in curved crystals that show the ‘quasi-mosaic’ effect. More recently, a crystal of this kind was installed in the Large Hadron Collider at CERN for beam collimation by the UA9 collaboration. Since 2011, another important application involving the ‘quasi-mosaic’ effect has been the focalization of hard X-rays and soft -rays. In particular, the possibility of obtaining both high diffraction efficiency and the focalization of a diffracted beam has been proved, which cannot be obtained using traditional diffracting crystals. A comprehensive survey of the physical properties of ‘quasi-mosaicity’ is reported here. Finally, experimental demonstrations for adjustable values of the ‘quasi-mosaic’ curvature are provided

Proposal for a Laue lens relying on hybrid quasi-mosaic curved crystals

A promising method of concentrating X- and soft γ-rays from celestial sources is a Laue lens. A new scheme for this lens, relying on diffraction in curved Si and Ge crystals, is introduced here. The proposed Laue lens is based on high-efficiency diffraction of curved (111) or (22\hbox{$\overline{4}$}) crystalline planes, which are bent through quasi-mosaic effect. While diffraction in curved (111) quasi-mosaic crystals is well known and has recently been proposed for a Laue lens, diffraction by quasi-mosaic (22\hbox{$\overline{4}$}) planes is suggested and demonstrated here through experimental work carried out at the Institut Laue-Langevin (ILL, Grenoble, France) at DIGRA, a facility specifically built for characterizing instrumentation in Astrophysics. Results show that the diffraction efficiency in the (22\hbox{$\overline{4}$}) quasi-mosaic sample is amplified by more than one order of magnitude with respect to an equivalent crystal without quasi-mosaic effect. The lens has been designed in such a way as to maximize and smoothen its sensitivity, thanks to a custom-made code based on a genetic algorithm