Crystals with curved diffracting planes for hard X-ray optics

Crystals 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)

Effects of the gravivector and graviscalar fields in N=2,8 supergravity

The available tests of the equivalence principle constrain the mass of the Higgs-like boson appearing in extended supergravity theories. We determine the constraints imposed by high precision experiments on the antigravity fields (gravivector and graviscalar) arising from $N=2,8$ supergravity.Comment: 9 pages, LaTe

Experimental study on grooved Si and Ge crystals for Laue lens application

An experimental study on the method of indentations for bent crystals to realize a hard X-ray Laue lens has been done. We tested the diffraction properties of indented Si and Ge crystalline plates at European Synchrotron Radiation Facility (Grenoble, France). The samples were analyzed by diffraction of their (111) planes with hard X-rays from 150 to 600 keV. Crystals have shown significantly high diffraction efficiency, i.e. a Si crystal has exhibited up to 80% at 300 keV. A Ge crystal has confirmed the observation for a Si one, though the diffraction efficiency was about 60%. In both cases rocking curves showed flat-toped rectangular shapes, which demonstrates that the method of indentations evenly bends the crystals. Moreover, measured angular spread was always very close to the morphological curvature of the sample under investigation, showing that this method offers high reproducibility and, thus, easy control of diffraction properties of the crystals

Improving the performance of printable carbon electrodes by femtosecond laser treatment

Low-cost carbon-conductive films were screen-printed on a Plexiglas® substrate, and then, after a standard annealing procedure, subjected to femtosecond (fs) laser treatments at different values of total accumulated laser fluence ΦA. Four-point probe measurements showed that, if ΦA > 0.3 kJ/cm2, the sheet resistance of laser-treated films can be reduced down to about 15 Ω/sq, which is a value more than 20% lower than that measured on as-annealed untreated films. Furthermore, as pointed out by a comprehensive Raman spectroscopy analysis, it was found that sheet resistance decreases linearly with ΦA, due to a progressively higher degree of crystallinity and stacking order of the graphitic phase. Results therefore highlight that fs-laser treatment can be profitably used as an additional process for improving the performance of printable carbon electrodes, which have been recently proposed as a valid alternative to metal electrodes for stable and up-scalable perovskite solar cells

Aluminum (Oxy)nitride thin films grown by fs-PLD as electron emitters for thermionic applications

Thin films based on aluminum nitride were obtained by fs-laser assisted Pulsed Laser Deposition (fs-PLD) at room temperature on tantalum substrates for studying the electron emission performance in the temperature range 700- 1600 °C, so to investigate the possibility of their exploitation as thermionic cathodes. Results of structural, chemical and morphological analyses show the growth of nanostructured thin films with a significant oxygen contamination, forming a mixture of crystalline aluminum nitride and aluminum oxide as well as metallic aluminum inclusions. Despite the considerable presence of oxygen, the developed cathodes demonstrate to possess promising thermionic emission characteristics, with a work function of 3.15 eV, a valuable Richardson constant of 20.25 A/(cm²K²), and a highly thermo-electronic stability up to operating temperatures of 1600 °C

Medical Applications of Tissue-Equivalent, Organic-Based Flexible Direct X-Ray Detectors

The aim of this study is to assess direct X-ray detectors based on organic thin films, fabricated onto flexible plastic substrates, and operating at ultra-low bias (<1 V), for different medical applications. With this purpose, flexible fully organic pixelated X-ray detectors have been tested at the imaging beamline SYRMEP (SYnchrotron Radiation for MEdical Physics) at the Italian synchrotron Elettra, Trieste. The detectors' performance has been assessed for potential employment both as reliable wearable personal dosimeters for patients and as flexible X-ray medical imaging systems. A spatial resolution of 1.4 lp mm−1 with a contrast of 0.37 has been evaluated. Finally, we validate the detector using X-ray doses and energies typically employed for actual medical radiography, and using X-ray beam pulses provided by a commercial dental radiography system, recording a sensitivity of 1.6 × 105 μC Gy−1 cm−3 with a linear response with increasing of the dose rates and a reliable signal to 100 ms X-rays pulses

Position and frequency shifts induced by massive modes of the gravitational wave background in alternative gravity

Alternative theories of gravity predict the presence of massive scalar, vector, and tensor gravitational wave modes in addition to the standard massless spin~2 graviton of general relativity. The deflection and frequency shift effects on light from distant sources propagating through a stochastic background of gravitational waves, containing such modes, differ from their counterparts in general relativity. Such effects are considered as a possible signature for alternative gravity in attempts to detect deviations from Einstein's gravity by astrophysical means.Comment: 9 pages, 1 figur

Singularities in scalar-tensor gravity

The analysis of certain singularities in scalar-tensor gravity contained in a recent paper is completed, and situations are pointed out in which these singularities cannot occur.Comment: 6 pages, LaTe

LIPSS Applied to Wide Bandgap Semiconductors and Dielectrics: Assessment and Future Perspectives

With the aim of presenting the processes governing the Laser-Induced Periodic Surface Structures (LIPSS), its main theoretical models have been reported. More emphasis is given to those suitable for clarifying the experimental structures observed on the surface of wide bandgap semiconductors (WBS) and dielectric materials. The role played by radiation surface electromagnetic waves as well as Surface Plasmon Polaritons in determining both Low and High Spatial Frequency LIPSS is briefly discussed, together with some experimental evidence. Non-conventional techniques for LIPSS formation are concisely introduced to point out the high technical possibility of enhancing the homogeneity of surface structures as well as tuning the electronic properties driven by point defects induced in WBS. Among these, double- or multiple-fs-pulse irradiations are shown to be suitable for providing further insight into the LIPSS process together with fine control on the formed surface structures. Modifications occurring by LIPSS on surfaces of WBS and dielectrics display high potentialities for their cross-cutting technological features and wide applications in which the main surface and electronic properties can be engineered. By these assessments, the employment of such nanostructured materials in innovative devices could be envisaged