Détecteur de rayons X | EPFL Graph Search

Les détecteurs de rayons X sont des dispositifs capables de détecter la présence de rayons X. La technologie de détection des rayons X a fortement progressé depuis leur découverte, passant du simple film photographique à des dispositifs électroniques pouvant donner le flux de rayons X et leur énergie. Principe Les rayons X sont des rayonnements ionisants : ils éjectent des électrons de la matière par effet photoélectrique ou effet Compton. C'est ce phénomène qui est utilisé pour la détection. Tout d'abord, en interagissant avec certaines substances, la création d'ions par les rayons X peut amorcer des réactions chimiques. Ce phénomène est utilisé par les films photographiques. L'ionisation peut aussi provoquer une émission lumineuse par fluorescence, comme dans le cas des écrans fluorescents et des scintillateurs. La présence de charge dans la matière, qu'elle soit gazeuse ou solide, génère un courant électrique qui peut être détecté ; c'est le principe de la plupart des

X-ray imaging detector for radiological applications adapted to the context and requirements of low- and middle-income countries

Sébastien Emmanuel Blanc, Hubert Blanchard, Mario Andres Chavarria Varon, Matthias Hüser, Romain Sahli, René Salathé, Klaus Benedikt Schönenberger, Jean-Philippe Thiran

This paper describes the development of a novel medical x-ray imaging system adapted to the needs and constraints of low- and middle-income countries. The developed system is based on an indirect conversion chain: a scintillator plate produces visible light when excited by the x rays, and then, a calibrated multi-camera architecture converts the visible light from the scintillator into a set of digital images. The partial images are then unwarped, enhanced, and stitched through parallel field programmable gate array processing units and specialized software. All the detector components were carefully selected focusing on optimizing the system’s image quality, robustness, cost-effectiveness, and capability to work in harsh tropical environments. With this aim, different customized and commercial components were characterized. The resulting detector can generate high quality medical diagnostic images with detective quantum efficiency levels up to 60% (@2.34 μGy), even under harsh environments, i.e., 60 °C and 98% humidity.

2022

A Far-Infrared Polarimeter Diagnostic for the Measurement of the Poloidal Magnetic Field in the TCV Tokamak

Information about the poloidal field inside the plasma and about the current density profile is essential for the understanding of the behavior of tokamak plasmas. The poloidal field outside the plasma edge is easily measured by a set of magnetic coils, whereas measurements of the magnetic field distribution inside the plasma requires dedicated and often complex diagnostic systems. One of the methods is polarimetry using beams of electromagnetic waves traversing the plasma. It makes use of the fact that the magnetic field modifies the refractive index of the plasma and as a consequence affects the state of polarization of the passing waves. Specifically, Faraday rotation refers to the case of a linearly polarized wave propagating parallel to a magnetic field. In this case, the rotation of the polarization vector is proportional to the product of electron density and parallel magnetic field component. This thesis is devoted to the design of a specific polarimeter system and its implementation on the TCV tokamak. The conceptual design took into account the particular requirements of the TCV experiments and its experimental program with particular interest in measurements of current density profiles in low-density, EC-heated plasmas. Aiming at optimum performance of the polarimeter in this parameter range, the FIR laser wavelength of 432.5 µm was chosen. Since TCV is already equipped with an interferometer (at 214 µm) for measurements of the line-integrated density along 14 chords, the polarimeter was built as a separate instrument. The number of 10 spatial channels to cover the plasma diameter was found as a compromise between desired spatial resolution, access constraints and cost. The measurement of the Faraday rotation angle is based on a method suggested by Dodel and Kunz [1], which uses optical beams with rotating linear polarization. In this case, the Faraday rotation angle is retrieved from a phase measurement comparing the modulated signals from the probe detectors with that from a reference detector and to first order is insensitive to amplitude variations of the signals. Waveguide detectors based on Schottky barrier diodes are used as detectors for their sensitivity and large electrical bandwidth. Signal processing and analysis is performed in two branches : a) using specifically built analog electronic phase detectors followed by slow (250 kHz) ADCs b) using fast (5 MHz) ADCs and numeric signal processing on a mainframe computer. The thesis covers the description of the design and the various components of the polarimeter, presents initial tests and an evaluation of its performance based on simulations using real plasma configurations of TCV. The analysis of specific tests with the system installed on TCV revealed the presence of perturbations leading to parasitic contributions to the measured phase angles. The results of first measurements of the Faraday rotation angle for different plasma conditions on TCV are presented. Comparing the measured Faraday rotation angles with the results of calculations showed qualitative agreement, in particular the effects of an increase in electron density and of the reversal of the current direction were clearly seen. The system is also capable to detect a radial displacement of the magnetic axis of the plasma, as was demonstrated by comparing measurements from two specific plasma configurations. However, the absolute values still show significant deviations from the expected ones based on calculations. The discrepancies increase with increasing Faraday angle and depend on the direction of the plasma current. At high electron densities beam refraction becomes a problem and may lead to significant errors in the measurements and eventually to complete loss of the signal in several channels. In its present status, the polarimeter cannot yet provide results that are suitable to reconstruct the poloidal field or the current density profile of the plasma. Tentative explanations for this problem are given, but further specific tests are necessary to confirm them. Even in its present state, the polarimeter may be used to detect transient relative changes in the profiles of electron density and current. This was demonstrated by a series of experiments in plasmas with sawtooth activity. Comparing the signals from the polarimeter with those from other multi-chord diagnostics (interferometer and soft X-ray detectors) clearly revealed correlations but also specific differences. These experimental studies showed that the Far-Infrared Polarimetry still requires further improvements, but has potential to become a valuable diagnostic capable of directly measuring the current density profile in the TCV tokamak.

EPFL2011

Structural properties of organometalic halide perovskite and their application in photodetectors

Anastasiia Glushkova

Hybrid organometallic halide perovskites have been intensively investigated in the past years as highly efficient light harvesters for various optoelectronic applications for both sensing and emitting light. However, many open questions remain regarding the crystallization process, environmental impact and implementation of the material into electrical circuits. The central theme of this PhD dissertation is the structural characterization of hybrid organicinorganic perovskites and device fabrication based on these materials. The focus is on methylammoniumlead iodide, CH3NH3PbI3 as one of the most efficient photovoltaic materials. External factors such as temperature, pressure, humidity, etc. affect the materials properties. Thus, it is crucial to study their influence for further applications. In this thesis, it will be shown, that under high pressure (20 GPa) inert gases as Ar and Ne form high-pressure-induced compounds with CH3NH3PbI3. For Ne pressure transmitting media such high-pressure transformation is reversible and the Ne-incorporated compoundis even stable at ambient conditions after decompression. We show that when applying repeated phase transitions during thermal cycling around both 330 and 160 K, CH3NH3PbI3 does not return to the initial state. Instead, at 330 K, the crystal stabilizes an incommensurately modulated tetragonal phase with successive transitions. On the other hand, performing thermal cycling around 160 K generates an increase in the concentration of domains of different phases. In addition, we studied the influence of thermal treatment on the crystal structure in the absence of any phase transition. For that reason, the lower dimensional compound of NH3CH2CH2NH3PbI4 was synthesized and characterized before and after annealing process. We observe and discuss a correlation between photoconductivity and increased disorder. In this work a novel approach of aerosol jet printing deposition of CH3NH3PbI3 has been developed. Making use of intermediate phases of the crystallization process, this deposition method enables the creation of 3D structures of organic-inorganic perovskites on various surfaces. This technique was successfully used in the fabrication of heterostructures based on CH3NH3PbI3 and graphene. Due to large trap-assisted photogain, these heterostructures are very promising for photoconductors. Taking into account the strong X-ray stopping power of the high atomic number Pb and I, devices based on CH3NH3PbI3/graphene heterostructures are excellent for X-ray detection. Such X-ray detectors demonstrate a record high sensitivity value of 2.2 x10^8 uC/(Gy cm^2).

EPFL2020

A Far-Infrared Polarimeter Diagnostic for the Measurement of the Poloidal Magnetic Field in the TCV Tokamak

EPFL2011

Structural properties of organometalic halide perovskite and their application in photodetectors

Anastasiia Glushkova

EPFL2020