Ionizing Radiation Detection Using Microstructured Optical Fiber

Ionizing radiation detecting microstructured optical fibers are fabricated, modeled and experimentally measured for X-ray detection in the 10-40 keV energy range. These fibers operate by containing a scintillator material which emits visible light when exposed to ionizing radiation. An X-ray source characterized with a CdTe spectrometer is used to quantify the X-ray detection efficiency of the fibers. The solid state CdTe detector is considered 100% efficient in this energy range. A liquid filled microstructured optical fiber (MOF) is presented where numerical analysis and experimental observation leads to a geometric theory of photon transmission using total internal reflection. The model relates the quantity and energy of absorbed X-rays to transmitted and measured visible light photons. Experimental measurement of MOF photon counts show good quantitative agreement with calculated theoretical values. This work is extended to a solid organic scintillator, anthracene, which shows improved light output due to its material properties. A detailed description of the experimental approach used to fabricate anthracene MOF is presented. The fabrication technique uses a modified Bridgman-Stockbarger crystal growth technique to grow anthracene single crystals inside MOF. The anthracene grown in the MOF is characterized using spectrophotometry, Raman spectroscopy, and X-ray diffraction. These results show the anthracene grown is a high purity crystal with a structure similar to anthracene grown from the liquid, vapor and melt techniques. The X-ray measurement technique uses the same approach as that for liquid filled MOF for efficiency comparison. A specific fiber configuration associated with the crystal growth allows an order of magnitude improvement in X-ray detection efficiency. The effect of thin film external coatings on the measured efficiency is presented and related to the fiber optics. Lastly, inorganic alkali halide scintillator materials of CsI(Tl), CsI(Na), and NaI(Tl) are grown as single crystals inside the MOF. These alkali halide fibers show an improvement in X-ray detection efficiency comparable with the CdTe detector and can be more efficient, dependent upon the photon counter efficiency and fiber configuration. The fiber configuration for this improved efficiency is described as the same for the higher efficiency anthracene MOF

Fundamental and Applied Aspects of X-Ray Spectrometry: Detector Influence and Photoelectric Effect Cross-Sections

The first part of this work reports the elementary theory of the atomic photoeffect presented in a form that is suited for practical numerical calculation. A detailed derivation of subshell cross sections for both excitation and ionization, comprising the angular distributions of emitted photoelectrons, is presented taking into account the effect of the polarization of the photons. The theoretical formulas have been implemented in a computer program PHOTACS that calculates tables of excitation and ionization cross sections for any element and subshell. Numerical calculations are practicable for excitations to final states with the principal quantum number up to about 20 and for ionization by photons with energy up to about 2 MeV. The effect of the finite width of atomic energy levels is accounted for by convolving the calculated subshell cross section with a Lorentzian profile. The second part of this work reports unfolding strategies for correcting a radiation measurement from the effects of the detector-pulse handling circuitry system. These strategies comprise the correction from the effects of pulse pile-up (PPU) and the detector response function (DRF). A first principles balance equation for second order PPU is derived and solved for the particular case of rectangular pulse shape. A Monte Carlo (MC) strategy is then implemented in the code MCPPU (Multi-shape pulse pile-up correction) allowing handling more general cases. Regarding the DRF, computed with deterministic or MC codes, it is presented the new tool RESOLUTION which introduces in the computed DRF the effects of energy resolution and incomplete charge collection. In the end the computer program UMESTRAT (Unfolding Maximum Entropy STRATegy) is presented in an updated version which include a new constrain to the total number of photons of the spectrum, which can be easily determined by inverting the diagonal efficiency matrix

Artifact Correction and Real-Time Scatter Estimation for X-Ray Computed Tomography in Industrial Metrology

Artifacts often limit the application of computed tomography (CT) in industrial metrology. In order to correct these artifacts, the so-called simulation-based artifact correction (SBAC) was developed in this thesis. For this purpose, analytical and Monte Carlo (MC) based models were set up to simulate the CT measurement process for a given component as accurately and efficiently as possible. Calculating the difference between this simulation and an ideal one yields an estimate of the present artifacts that can be used to correct the corresponding CT measurement. The potential of this approach was demonstrated for the correction of the most common CT artifacts, i.e. beam hardening, x-ray scattering, off-focal radiation, partial volume effects, and cone-beam artifacts. In any case, the SBAC provided CT reconstructions that showed almost no artifacts and whose quality was clearly superior to state-of-the-art reference approaches. In this context, the problem of long runtimes of scatter simulations was solved by another novel approach, the so-called deep scatter estimation (DSE). The DSE uses a deep convolutional neural network which was trained to map the acquired projection data to given MC scatter estimates. Once the DSE network is trained, it can be used to process unknown data in real-time. In different simulation studies and measurements, it could be shown that DSE generalizes to various acquisition conditions and components while providing scatter distributions that differ by less than 2 % from MC simulations. Thus, the two developed approaches make an important contribution to correct CT artifacts efficiently and to extend the applicability of CT in the field of industrial metrology

Electron Cyclotron Heating and Suprathermal Electron Dynamics in the TCV Tokamak

This thesis is concerned with the physics of suprathermal electrons in thermonuclear, magnetically confined plasmas. Under a variety of conditions, in laboratory as well as space plasmas, the electron velocity distribution function is not in thermodynamic equilibrium owing to internal or external drives. Accordingly, the distribution function departs from the equilibrium Maxwellian, and in particular generally develops a high-energy tail. In tokamak plasmas, this occurs especially as a result of injection of high-power electromagnetic waves, used for heating and current drive, as well as a result of internal magnetohydrodynamic (MHD) instabilities. The physics of these phenomena is intimately tied to the properties and dynamics of this suprathermal electron population. This motivates the development of instrumental apparatus to measure its properties as well as of numerical codes to simulate their dynamics. Both aspects are reflected in this thesis work, which features advanced instrumental development and experimental measurements as well as numerical modeling. The instrumental development consisted of the complete design of a spectroscopic and tomographic system of four multi-detector hard X-ray (HXR) cameras for the TCV tokamak. The goal is to measure bremsstrahlung emission from suprathermal electrons with energies in the 10-300 keV range, with the ultimate aim of providing the first full tomographic reconstruction at these energies in a noncircular plasma. In particular, suprathermal electrons are generated in TCV by a high-power electron cyclotron heating (ECH) system and are also observed in the presence of MHD events, such as sawtooth oscillations and disruptive instabilities. This diagnostic employs state-of-the-art solid-state detectors and is optimized for the tight space requirements of the TCV ports. It features a novel collimator concept that combines compactness and flexibility as well as full digital acquisition of the photon pulses, greatly enhancing its potential for full spectral analysis in high-fluency scenarios. Additional flexibility is afforded by the possibility to rotate the orientation of two of the cameras, permitting the crucial comparison of radiation emitted perpendicular and parallel to the primary magnetic field. The design of the HXR system was optimized through an extensive iterative simulation process with the aid of tomographic reconstruction codes as well as quasilinear Fokker-Planck modeling of ECH-driven TCV plasmas. In parallel, the selection of the detectors for this system was performed through comprehensive laboratory testing of several candidate detectors available on the market. While the design was completed in the course of the thesis work, commissioning of the system has only commenced recently with one of the four cameras installed on TCV. The first preliminary results, discussed in the last part of this thesis, include basic parameter scans of ECH wave-plasma interaction and the investigation of the dynamic response of suprathermal electrons to modulated ECH. In addition, the cameras possess the novel ability to discriminate against very high-energy γ-ray radiation that cannot be collimated and must thus be excluded from spatial distribution analysis. A basic study of the conditions for γ-ray suppression was conducted in preparation for future experiments. The Fokker-Planck modeling tool used in this diagnostic development was acquired through a collaboration with CEA-Cadarache, initially with the primary motivation of studying the simultaneous plasma heating by 2nd and 3rd harmonic electron cyclotron waves that is uniquely possible on TCV. This motivated a dedicated study, both theoretical and experimental, of one particular instance of this combined heating, which became a second primary subject of this thesis work. The particular scenario studied here is one in which a single ECH frequency is resonant at both harmonics in the same plasma. The primary objective of this study was to determine whether a synergy effect existed, permitting an enhancement of the intrinsically weak 3rd harmonic absorption by the suprathermal electrons generated at the 2nd harmonic resonance. An associated question was whether this effect, if it existed, was experimentally measurable or was in fact observed in TCV. The simulations performed in the course of this study indeed predict the existence of such a synergy, although the answer to the second question was ultimately negative, at least within the current technical limitations. This study has proven nevertheless highly valuable in providing new insight into the complex velocity-space dynamics that govern ECH wave-particle interaction and suprathermal electron dynamics

Life of a Photon in X-ray Spectroscopy

This thesis summarizes the experimental work in which an ultrafastX-ray laser plasma source was combined with variousscalable direct detection schemes to test a novel approach forlab-based time-resolved X-ray absorption spectroscopy. A laserplasma source based on a water jet target was built and commissioned.X-ray and electron emissions of this source were characterizedwith various direct detection schemes. The proceduresfor spectral retrieval with direct detection CCD’s were optimizedwith regard to the laser plasma source. The novel approach of usinga single photon measuring cryogenic microcalorimeter arrayas a high-resolution (DE/E 2000 @ 6 keV) energy-dispersivedetector was investigated. The potentially very high quantumefficiency, large detection angle and straightforward scalabilitymake this device an interesting photon analyzer for low photonyield experiments. In this thesis a prototype version of this detectorwas built (in cooperation), implemented and commissionedinto the laser plasma setup. With this combination of a lab-basedbroad-band source and the free standing microcalorimeter spectrometerhigh resolution X-ray absorption spectra in transmissionmode were achieved. The thesis presents the first hard X-rayabsorption fine structure (XAFS) spectrum taken with this novelapproach and discusses further improvements and applications

Improving the Aesthetics and Performance of Perovskite Materials for Photovoltaics

Within the last decade, lead halide perovskite solar cells have rapidly evolved to the cusp of commercialisation. Current record device efficiencies have surpassed 25% however; a principal limitation of these materials is their instability on exposure to ambient conditions. Methylammonium lead tri-bromide (MAPbBr3) perovskite has shown superior stability over other lead halide perovskite materials, yet the efficiencies of MAPbBr3 devices are significantly lower with a record efficiency of 10.4%. This research investigates the treatment of MAPbBr3 perovskite solar cells with organic dyes of complementary absorbance in a bid to maximise the light harvesting, increase the photocurrent and improve the device efficiency. Initial investigations focused on developing an optimised build method capable of manufacturing MAPbBr3 devices which consistently achieve above 1% efficiency. The optical characterisation of six organic dyes revealed a red indoline dye, D205 and a blue squaraine, SQ2 (which both absorb strongly between 300-700 nm) would offer the best complementary absorbance to MAPbBr3 perovskite. On adding the dyes, the perovskite layer underwent an evident colour change highlighting the potential for coloured perovskite cells which could be beneficial for building-integrated applications. MAPbBr3 cells co-sensitised using a novel method (which sensitises the film after perovskite crystallisation) show improved efficiency (2.6% SQ2, 3.1% D205) over perovskite-only devices (2%) with a 10% photocurrent contribution from the dye. Whilst increases in the photocurrent are observed with co-sensitisation, increased device efficiencies are mainly derived from improvements in the fill factor. We also see lower series resistance and increased photoluminescence lifetime with co-sensitisation where control and co-sensitised MAPbBr3 thin-films produce average lifetimes of 0.44 ns and 0.80 ns, respectively. Further investigation has revealed the dye solvent, toluene, and the dye both help to improve device performance acting as both a treatment and a second sensitiser in the device by passivating defects and lowering recombination losses whilst providing additional photocurrent through increased absorbance. As a result, co-sensitised devices show slower recombination kinetics resulting in increased open-circuit voltage under lower light levels. These effects have proven beneficial for thicker co-sensitised devices (>0.7 µm) where they have often translated into large increases in device efficiency. In future, this may be beneficial for indoor or lower light level PV systems including within the rapidly expanding internet of things market

Novel processes for large area gallium nitride single crystal and nanowire growth.

III-nitrides (InN, GaN, AlN) are some of the most promising materials for making blue light emitting diodes (LED), blue laser diodes (LD) and high power, high temperature field effect transistors (FET). Current techniques produce GaN films with defect densities on the order of 107/cm2 or higher. The performance and life-time of the devices critically depend upon the defect densities and high power, high frequency devices require the defect densities to be lower than 104/cm2. So, the need for new processes to produce large size GaN crystals with defect densities less than 107/cm2 is immediate. In addition to large area single crystals (or wafers), the nanowires also present as an alternating platform for making devices. So, the processes for controlled synthesis, modifying and integrating sub 100 nm nanowires into electronic devices are of great interest. This thesis presents a new concept of ‘self-oriented growth\u27 of GaN platelets shaped crystals on molten gallium to produce near single crystal quality GaN films over large areas (\u3e 1 cm²). The process involves direct nitridation of Ga films using nitrogen plasma at low pressures (few mTorr). GaN flakes with areas over 25 mm² have been successfully obtained. Raman spectra of the resulting GaN crystals show no stress and low native donor concentration on the order of 1017/cm³. XRD texture analysis showed an overall c-axis tilt of 2.2o between GaN domains within the flake. The cross-sectional TEM micrographs showed that the resulting GaN films are free from dislocation crops inside the grains but showed diffraction contrast due to small mis-orientation between the grains. The twist and tilt angles between adjacent columnar grains were determined using convergent beam electron diffraction technique to be less than 8o and 1o, respectively. HRTEM micrographs of the grain boundaries showed sharp interfaces resulting with both twisted and perfect attachments. This thesis also presents direct synthesis approach for GaN nanowires with control on growth directions using modified nitridation conditions. The nitridation in the presence of hydrogen or ammonia resulted in oxide sheath free GaN nanowires as thin as 20 nm and long as 100 Ìm in \u3c0001\u3e direction. The nitridation using low Ga flux in a vapor transport set-up resulted in sub 100 nm GaN nanowires with \u3c10-10\u3e growth direction. The difference in the nucleation and growth mechanism allowed control on the nanowire directions. Homo-epitaxial experiments onto pre-synthesized GaN nanowires with the above two growth directions using the vapor transport of Ga and dissociated ammonia exhibited different morphologies, e.g. micro hexagonal columns for \u3c0001\u3e nanowires and micro belts for nanowires with \u3c10-10\u3e growth direction. The results further illustrate a new phenomenon of enhanced surface diffusion on nanowires in general but more pronounced for wires with \u3c0001\u3e growth direction. The results from homo-epitaxy experiments suggest that the \u3c10-10\u3e direction wires could be used as seeds for growing large area GaN crystals in vapor phase homo-epitaxy schemes

Sixteenth International Laser Radar Conference, part 2

Given here are extended abstracts of papers presented at the 16th International Laser Radar Conference, held in Cambridge, Massachusetts, July 20-24, 1992. Topics discussed include the Mt. Pinatubo volcanic dust laser observations, global change, ozone measurements, Earth mesospheric measurements, wind measurements, imaging, ranging, water vapor measurements, and laser devices and technology