Dissertations / Theses on the topic 'Radiation detector applications'

The Clouds and the Earth's Radiant Energy System (CERES) is a program sponsored by the National Aeronautics and Space Administration (NASA) aimed at evaluating the global energy balance. Current scanning radiometers used for CERES consist of thin-film thermistor bolometers viewing the Earth through a Cassegrain telescope. The Thermal Radiation Group, a laboratory in the Department of Mechanical Engineering at Virginia Polytechnic Institute and State University, is currently studying a new sensor concept to replace the current bolometer: a thermopile thermal radiation detector. This next-generation detector would consist of a thermal sensor array made of thermocouple junction pairs, or thermopiles. The objective of the current research is to perform a thermal analysis of the thermopile. Numerical thermal models are particularly suited to solve problems for which temperature is the dominant mechanism of the operation of the device (through the thermoelectric effect), as well as for complex geometries composed of numerous different materials. Feasibility and design specifications are studied by developing a dynamic electrothermal model of the thermopile using the finite element method. A commercial finite element-modeling package, ALGOR, is used.
Master of Science

The Thermal Radiation Group, a laboratory in the Department of Mechanical Engineering at Virginia Polytechnic Institute and State University, is currently working to develop a new technology for thermal radiation detectors. The Group is also studying the viability of replacing current Earth Radiation Budget radiometers with this new concept. This next-generation detector consists of a thermopile linear array thermal radiation detector. The principal objective of this research is to develop an optical model for the detector and its cavity. The model based on the Monte-Carlo ray-trace (MCRT) method, permits parametric studies to optimize the design of the detector cavity and the specification of surface optical properties. The model is realized as a FORTRAN program which permits the calculation of quantities related to the cross-talk among pixels of the detector and radiation exchange among surfaces of the cavity. An important capability of the tool is that it provides estimates of the discrete Green's function that permits partial correction for optical cross-talk among pixels of the array.
Master of Science

The ability of the electronic portal imaging device (EPID) to acquire a large two-dimensional array of digitized x-ray data in real time is extremely attractive for dosimetric measurements. To evaluate the potential use of an EPID for portal dose measurement in Wellington Blood and Cancer Centre, some dosimetric characteristics of the Varian's PortalVisionTM aS500 were investigated. PortalVisionTM incorporates an amorphous silicon detector (aSi). Some potential applications of EPID in linac QA were also explored. The EPID's performance for linearity with MU and dose rate was verified and it was found to be proportional over the entire measured range. Short term repeatability was found to be excellent. An investigation of calibration method to improve dosimetric accuracy demonstrated two methods of avoiding detector saturation. Firstly, acquiring flood field with the use of additional buildup and secondly, increasing the source to detector distance for calibration. A study of EPIDs behaviour under conditions of varying dose rate which commonly arise in EDW treatment techniques was carried out. The EPID exhibited a field size dependence in addition to a 8% discrepancy on the `hot edge' of EDW profiles. Further investigation into the field size dependence and the discrepancy at hot edge is required. EPIDs ability to acquire asymmetric field profile was also investigated. The profiles acquired using EPID deviated in shape and magnitude by upto 16% from the ion chamber profiles. Some potential applications of EPID to perform QA of linac beam properties, its ability to perform optical and mechanical linac QA have been explored. The EPID's capability to give constant output, flatness, symmetry, wedge angle and wedge factors with high level of accuracy and reproducibility was demonstrated. EPID was also found to be objective, efficient and feasible for performing optical linac QA. The use of EPID for linac QA could be simplified by improving the available software analysis tools thus making it more efficient.

Modelisation des reponses des detecteurs a semiconducteurs lorsque ceux-ci sont utilises en mode de comptage en vue de leur application a la dosimetrie gamma et neutron (thermique et rapide). A l'aide de trois programmesm de monte carlo, les valeurs optimales des parametres necessaires pour la mise au point des dosimetres individuels sont obtenues

Developing radiation delivery systems for optimisation of absorbed dose to the target without normal tissue toxicity requires advanced calculations for transport of radiation. In this thesis absorbed dose and fluence in different radiotherapy applications were calculated by using Monte Carlo (MC) simulations.

In paper I-III external neutron activation of gadolinium (Gd) for intravascular brachytherapy (GdNCB) and tumour therapy (GdNCT) was investigated. MC codes MCNP and GEANT4 were compared. MCNP was chosen for neutron capture reaction calculations. Gd neutron capture reaction includes both very short range (Auger electrons) and long range (IC electrons and gamma) products. In GdNCB the high-energetic gamma gives an almost flat absorbed dose delivery pattern, up to 4 mm around the stent. Dose distribution at the edges and inside the stent may prevent stent edge and in-stent restenosis. For GdNCT the absorbed dose from prompt gamma will dominate over the dose from IC and Auger electrons in an in vivo situation. The absorbed dose from IC electrons will enhance the total absorbed dose in the tumours and contribute to the cell killing.

In paper IV a model for calculation of inter-cluster cross-fire radiation dose from β-emitting radionuclides in a breast cancer model was developed. GEANT4 was used for obtaining absorbed dose. The dose internally in cells binding the isotope (self-dose) increased with decreasing β-energy except for the radionuclides with substantial amounts of conversion electrons and Auger electrons. An effective therapy approach may be a combination of radionuclides where the high self-dose from nuclides with low β-energy should be combined with the inter-cell cluster cross-fire dose from high energy β-particles.

In paper V MC simulations using correlated sampling together with importance sampling were used to calculate spectra perturbations in detector volumes caused by the detector silicon chip and its encapsulation. Penelope and EGSnrc were used and yielded similar results. The low energy part of the electron spectrum increased but to a less extent if the silicon detector was encapsulated in low z-materials.

The work in this thesis is focused on characterisation and evaluation of two classes of science grade imaging radiation detectors. The first class is Monolithic Active Pixel Sensors (MAPS). The advances in CMOS fabrication technologies over the last four decades allowed MAPS to compete with Charge-Coupled Devices (CCD) in many applications. The technology also provides relatively inexpensive ways to tailor design to suit specific application needs. It is important to understand performance capabilities of new sensor designs through characterisation and optimisation of readout parameters. In this work three MAPSs were characterised. The first one - HEPAPS4 - designed for charged particle detection, with the potential technology application in the vertex detector for the International Linear Collider. The noise of the sensor was measured to be 35±5 e, which agrees well with simulated data. The dark current was found to be 175 pA/cm2. The SNR performance for minimum ionising particles detection was demonstrated to be 40. The sensor was also evaluated for indirect detection of thermal and fast neutrons using lithium and polyethylene converters. The technology performed well in such an application with an estimated fast neutron detection efficiency of ~0.01%. The second sensor characterised – Vanilla MAPS – was designed to evaluate new techniques for fast readout, small noise and reduced image lag. The system was capable to readout 150 full frames (520x520 pixels) per second; the sensor showed 14±4 e noise and decreased image lag. The dark current was found to be ~50 pA/cm2. The back-thinned version of the sensor demonstrated dramatic improvement in quantum efficiency from 0% to 20% at 220 nm. The third device is parametric sensor eLeNA. It features 14 test structure designed to evaluated noise reduction architectures. The most promising structures showed temporal noise values as low as 6 e and 20 e fixed pattern noise. Medipix as an example of the second class of imaging detectors - hybrid pixel detectors - was evaluated in two applications. It was used as the core element of the ATLAS radiation background monitoring system. The sensors were covered with neutron converters, which extended the number of radiation types that can be detected. X-ray calibration was performed, showing excellent tolerance of all 18 devices characterised. Detection efficiencies were estimated to be ~1% for thermal and ~0.1% for fast neutrons. The second application of Medipix was mass spectrometry. The detector was place in the focal plane of a prototype mass spectrometer. 2D representation of data allowed focusing correction of the ion beam. The system was capable to detect ions in the range of 5-25 keV. The detector characterisation with broad range of ions (from Cu to Pb) showed very good abundance agreement with table data.

Doutoramento em Física
Este trabalho descreve o desenvolvimento e aplicação de sistemas baseados em detetores gasosos microestruturados, para imagiologia de fluorescência de raios-X por dispersão em energia (EDXRF). A técnica de imagiologia por fluorescência de raios-X assume-se como uma técnica poderosa, não-destrutiva, em análises da distribuição espacial de elementos em materiais. Os sistemas para imagiologia de EDXRF desenvolvidos são constituídos por: um tubo de raios-X, usado para excitar os elementos da amostra; um detetor gasoso microestruturado; e uma lente pinhole que foca a radiação de fluorescência no plano do detetor formando assim a imagem e permitindo a sua ampliação. Por outro lado é estudada a influência do diâmetro da abertura do pinhole bem como do fator de ampliação obtido para a imagem, na resolução em posição do sistema. Foram usados dois conceitos diferentes de detetores gasosos microestruturados. O primeiro, baseado na microestrutura designada por 2D-Micro-Hole & Strip Plate (2D-MHSP) com uma área ativa de 3 3 cm2, enquanto que o segundo, baseado na estrutura 2D-Thick-COBRA (2D-THCOBRA) apresenta uma área ativa de deteção de 10 10 cm2. Estes detetores de raios-X de baixo custo têm a particularidade de funcionar em regime de fotão único permitindo a determinação da energia e posição de interação de cada fotão que chega ao detetor. Deste modo permitem detetar a energia dos fotões X de fluorescência, bem como obter imagens 2D da distribuição desses fotões X para o intervalo de energias desejado. São por isso adequados a aplicações de imagiologia de EDXRF. Os detetores desenvolvidos mostraram resoluções em energia de 17% e 22% para fotões incidentes com uma energia de 5.9 keV, respectivamente para o detetor 2D-MHSP e 2D-THCOBRA e resoluções em posição adequadas para um vasto número de aplicações. Ao longo deste trabalho é detalhado o desenvolvimento, o estudo das características e do desempenho de cada um dos detetores, e sua influência na performance final de cada sistema proposto. Numa fase mais avançada apresentam-se os resultados correspondentes à aplicação dos dois sistemas a diversas amostras, incluindo algumas do nosso património cultural e também uma amostra biológica.
The present document describes the development and application of two energy dispersive X-ray fluorescence (EDXRF) imaging systems based on micropattern gaseous detectors. The X-ray fluorescence imaging technique is assumed as a powerful technique, non-destructive, on analysis of the spatial distribution of elements in materials. The imaging systems developed for EDXRF imaging consist of: an X-ray tube, acting as the external source to excite the sample; a micropattern gas detector; and a pinhole which directs the fluorescence radiation to the detector. On the other hand, the influence of the pinhole aperture diameter and the magnification factor obtained for the image in the position resolution of the system, is studied. Two different concepts of micropattern gaseous detectors were used. The first one is based on the 2D Micro-Hole & Strip Plate (2D-MHSP) microstructure with an active area of 3 3 cm2, while the second is based on the 2D Thick- COBRA (2D-THCOBRA) with a sensitive detection area of about 10 10 cm2. These X-ray detectors are single photon counting detectors, allowing the determination of the interaction position and energy of each photon reaching the detector. Therefore they can work as low cost energy dispersive detectors as well as obtain 2D images of the distribution of X photons for a range of energies required, which makes them suitable for EDXRF imaging applications. The detectors have shown an energy resolution of about 17% and 22% for 5.9 keV X-ray photons, for the 2D-MHSP detector and 2D THCOBRA, respectively, and apropriate spatial resolutions for a wide range of applications. Throughout this work, the development and study of the characteristics and the performance of each of the detectors used as well as their influence in the final performance of each proposed system is shown in detail. Results concerning the application of the two systems to several samples, including some cultural heritage samples and also a biological sample are shown.

Through this thesis we wanted to present a branch of radiotherapy that uses proton beams to destroy tumors, namely proton therapy. This technique, although relatively new (1946), is rapidly spreading thanks to the advantage of being able to precisely locate the release of the therapeutic dose of radiation. After a brief presentation of the discovery of ionizing radiations’ history and their possible applications, we focused on the study of the protons’ behavior when they interact with matter, going to show why they are so advantageous, by studying different quantities such as stopping power, flow rate, flow rate variation, multiple coulomb scattering and proton RBE. In fact, proton therapy represents a new and important therapeutic approach that allows a large part of healthy tissue to absorb less dose than in conventional therapies that use photons or electrons. The most interesting aspect of this thesis, and still with a broad future perspective, concerns the different types of detectors used in this therapy, which play a fundamental role in the progress of nuclear medicine, leading to ever better methods of prevention, diagnosis and treatment of illnesses. The future goal of this therapy is to develop new detectors, that are more equivalent to human tissues, both in behavior and detections, in order to obtain always better performances.

The application of radiation detector materials, capable of operating at room temperature, has been studied. The advantage of such materials is that they do not need to be cooled to reduce thermally generated leakage currents. Therefore, systems utilising these detectors can be smaller and more portable than ones containing HpGe and Si(Li) detectors which need to be cooled using liquid nitrogen to reduce leakage current. Three materials, silicon, cadmium telluride and mercuric iodide, have been investigated. The silicon photodiode showed very good energy resolution down to photon energies of 13.4keV, but its low detection efficiency restricts its use to energies below 200keV for direct detection. Cadmium telluride produced energy spectra with worse energy resolution, due mainly to charge collection, leakage current and fabrication. But, cadmium telluride was able to produce spectra up to 661.6keV with reasonable efficiency and resolution. Mercuric iodide produced spectra with energy resolution on a par with the silicon photodiode but with a higher detection efficiency. Mercuric iodide crystals have been produced at Surrey using the polymer assisted transport growth method. A large crystal with dimensions exceeding 1cm3 has also been produced. Due to problems with application of electrical contacts no useful spectra were obtained with these devices. The importance of the polymer in the starting material was investigated. The addition of polyethylene was found to favour the growth of platelets. The size of the platelets was also found to increase with increasing proportion of polymer in the stalling mix. A computer simulation enabling the performance of semiconductor radiation detector materials to be predicted is also presented. The model follows processes from photon interactions in the detector through to charge collection. Energy spectra may be produced from the output of this model and energy resolution and detection efficiency may be analysed as a function of detector thickness, applied bias and photon energy. This model could be used to optimise performance of radiation detectors.

The Thermal Radiation Group, a laboratory in the department of Mechanical Engineering at Virginia Polytechnic Institute and State University, is currently working towards the development of a new technology for cavity-based radiometers. The radiometer consists of a 256-element linear-array thermopile detector mounted on the wall of a mirrored wedge-shaped cavity. The objective of this research is to provide analytical and experimental characterization of the proposed radiometer. A dynamic end-to-end opto-electrothermal model is developed to simulate the performance of the radiometer. Experimental results for prototype thermopile detectors are included. Also presented is the concept of the discrete Green's function to characterize the optical scattering of radiant energy in the cavity, along with a data-processing algorithm to correct for the scattering. Finally, a parametric study of the sensitivity of the discrete Green's function to uncertainties in the surface properties of the cavity is presented.
Master of Science

[EN] This thesis focuses on the development of a simulator of a gamma radiation portable detector. The aim is to determine the feasibility of such a software tool in a virtual reality (VR) based application, with the purpose of using it in training tasks in the framework of nuclear safeguards and security activities. The work starts with the definition of the series of technical requirements which are necessary to achieve a working prototype of an application of the kind aforementioned. In order to achieve these requirements, a series of incremental prototypes of a VR based simulator are devised, implemented and tested. Each of these prototype versions tries to improve on its predecessor by introducing new concepts aimed at better satisfying the requirements set. The thesis is structured in several main chapters which divide the bulk of the PhD work in independent sections. Therefore, first the problem is introduced and then the current state of the art analysed. Next the first solutions are explained and following these introductory chapters the main contribution of the author is found in the development chapter. This chapter explains the ideas and methods created in a chronological manner, taking the reader through the steps the author took in the same order as he did. Logically, the next chapter deals with the testing of these methods in order to assess their validity and last a conclusion chapter evaluates if the objectives set at the beginning of the thesis have been met according to the results obtained.
[ES] Esta tesis trata el desarrollo de un simulador de un detector portátil de radiación gamma. El objetivo es determinar si una herramienta software basada en técnicas de realidad virtual como esta es factible. Con la intención de utilizarla en tareas de entrenamiento de personal en el sector de la salvaguarda y seguridad nuclear. El trabajo empieza con la definición de la serie de requisitos técnicos que son necesarios para conseguir un prototipo funcional de una aplicación como la anteriormente descrita. Para conseguir cumplir estos requisitos, se han desarrollado, implementado y probado una serie de prototipos incrementales de un simulador basado en realidad virtual. Cada uno de estos prototipos intenta mejorar a su predecesor introduciendo nuevos conceptos con el objetivo de satisfacer mejor los requisitos planteados. La tesis está estructurada en varios capítulos principales que dividen el grueso del trabajo en secciones independientes. Por lo tanto, en primer lugar se introduce el problema y luego se analiza el estado del arte. A continuación se explican las primeras soluciones probadas seguidas del capítulo de desarrollo que contiene la contribución principal del autor. Este capítulo explica las ideas y métodos creado en orden cronológico, llevando al lector por los mismos pasos que el autor dio. De manera lógica, el siguiente capítulo trata con el testeo de estos métodos para evaluar su validez y un último capítulo de conclusión analiza si se cumplieron los objetivos propuesto al inicio de la tesis según los resultados obtenidos.
[CAT] Esta tesi tracta el desenrotllament d'un simulador d'un detector portàtil de radiació gamma. L'objectiu és determinar si una ferramenta software basada en tècniques de realitat virtual com esta és factible, amb la intenció d'utilitzar-la en tasques d'entrenament de personal en el sector de la salvaguarda i seguretat nuclear. El treball comença amb la definició de la sèrie de requisits tècnics que són necessaris per a aconseguir un prototip funcional d'una aplicació com l'anteriorment descrita. Per a aconseguir complir estos requisits, s'han desenrotllat, implementat i provat una sèrie de prototips incrementals d'un simulador basat en realitat virtual. Cada un d'estos prototips intenta millorar el seu predecessor introduint nous conceptes amb l'objectiu de satisfer millor els requisits plantejats. La tesi està estructurada en diversos capítols principals que dividixen el total del treball en seccions independents. Per tant, en primer lloc s'introduïx el problema i després s'analitza l'estat de l'art. A continuació s'expliquen les primeres solucions provades, seguides del capítol de desenrotllament, que conté la contribució principal de l'autor. Este capítol explica les idees i mètodes creats en orde cronològic, portant al lector pels mateixos passos que l'autor va fer. De manera lògica, el següent capítol tracta el testeig d'estos mètodes per a avaluar la seua validesa i un últim capítol de conclusió analitza si es van complir els objectius proposats a l'inici de la tesi segons els resultats obtinguts.
Moltó Caracena, T. (2016). Simulation of Portable Gamma Radiation Detectors for Virtual Reality based Training applications [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/61301
TESIS

In this diploma thesis, we present a thorough investigation of the Vavilov-Cerenkov (VC) effect and its application to the detection of ultrahigh energyneutrinos. Neutrinos have no net charge and cannot emit VC radiation(VCR) but when they interact with a stationary medium such radiation isproduced by the Askar'yan effect. It has been proposed to use the lunarregolith as a detector material for such neutrino interactions, but in orderto do so, the VCR emitted from neutrino interactions has to be filtered outfrom that of other energetic particles impinging upon and penetrating intothe lunar surface. Since the principal difference between these emissions isdependent on the depth at which they occur, any boundary effects dependenton the distance from the point of emission to the surface is of interest.Therefore we examine the behavior of VCR near dielectric boundaries withextra care, both analytically and numerically. In order to keep the physicalpicture as clear as possible the analytical derivations are conducted in thetemporal domain. The boundary problem is studied both with respect tothe Maxwell boundary conditions and the method of images. In order toverify the analytic results and to make further investigations, a numericalMaxwell solver for the general VCR problem was constructed. The analyticand numeric results are then compared and proven to be equivalent. TheMaxwell solver is used to study the boundary problem and it is shown thatthe VCR emitted in a medium near the surface does not depend on the distanceto the surface in the same way as the opposite problem with a chargeabove the medium. In the case of a charge distribution traveling partiallyon both sides of the boundary some frequency dependent effects are shown.

Silicon photon multipliers, or SiPMs, are single photon detectors that have grown increasing interest in the last decade as an alternative to photomultiplier tubes in many field of physics, engineering and medicine. Compared to PMTs, SiPMs are more compact, rugged and operate at much lower bias voltage, in the order of tens of volts. Moreover they are insensitive to magnetic field and can achieve a very high radiopurity SiPM detectors work on the principle of a diode operated above the breakdown voltage, in Geiger mode. In this condition, the electric field in the depletion region is high enough that the electron-hole pairs, generated by a single photon absorption through photoelectric effect, create secondary charges by impact ionization in a potentially diverging avalanche effect that can be exploited to generate a macroscopical current at the output of the diode. Thanks to this effect, the SiPM is capable of counting the number of impinging photons down to single photon level. Noise sources in the SiPM include dark counts and correlated noise. Dark counts are counts happening when an electron-hole pair is generated in the active volume of the device in absence of photon absorptions. These events are caused either by thermal generation, diffusion from the neutral region or by tunnel effect. Correlated noise events, or counts, on the other hand, are generated when a primary firing cell retriggers after a certain time or cause the triggering of another cell. All these noise sources introduce errors in the photon count by adding fake events to the output signal of the detector.Traditional SiPM application is 511 keV gamma-ray detection in PET machines, using scintillator LYSO crystals to convert a single gamma ray into a flash of visible photons. An application based on the same principle was studied in this thesis by coupling FBK RGB-HD SiPMs with CsI:Tl crystals in order to detect lower energy X and gamma-rays. This setup has proven to be effective in the detection of radiation with energy as low as 5.9 keV with a resolution of 38.3%, which is the minimum value of energy resolution measured with SiPMs coupled to scintillator crystals at such low energy. At the same time it was observed that large area detectors provided a dynamic range wide enough to simultaneously detect radiation ranging from 6.4 keV to 122 keV with minimal saturation. In another activity of this thesis it was developed a simulation software that reproduces the behaviour of a SiPM under different light conditions by taking into account the detector efficiency, the dead time and the recharge behaviour of its cells and theoretical modelizations of the noise parameters that affect the measurement. From a given light profile the simulation generates a waveform that reproduces the one measured during the operation of real SiPMs. This waveform was then analysed using FBK software developed for SiPM characterization and the results showed an excellent agreement between the simulated detector and a reference SiPM. This software will become a useful tool for the design of SiPMs for future experiments because it will allow to tune the properties of the detectors to specific applications and it will reduce the need of layout and process split to find the optimal configuration of the detector parameters.Among all FBK technologies, this work was focused on the position-sensitive LG-SiPM. Unlike standard SiPMs, which have a single output, the LG-SiPM employs a more complex structure that splits the current signal into four output channels with ratios depending on the position of the impinging light on its surface. Center of mass calculations are used to reconstruct the position of the firing cell with precision down to some tens of microns while maintaining the fast time response of SiPMs. An application of the LG-SiPM was studied in the framework of the ARIADNE experiment in collaboration with the university of Liverpool. In this work the LG-SiPM was used to detect scintillation light coming from ionization tracks generated by alpha particles inside a CF4 TPC chamber. The ionized electrons where drifted through the action of a high electric field in the TPC towards a THGEM where they created light with timing depending on the distance of each track segment from the scintillator. The LG detector was able to reconstruct the 3D track particle inside the chamber with an error below 8 mm RMS inside the 40 l chamber and, at the same time, to reconstruct the energy released by the particle as function of time and calculate the total energy of the interacting particle and its linear energy transfer. These results open a novel approach for the TPC position reconstruction that combines the low number of readout channels needed for the LG detector to its time-continuous response which allows to reconstruct the tree-dimensional track of a particle inside the chamber.During the experiment it emerged the presence of an artifact that drifted all the reconstructed tracks towards the centre of the detection area, at the end of the signal. This effect was studied by creating a second simulation software that recreates the electrical behaviour of the LG-SiPM equivalent circuit when one or more cells trigger. It was simulated the output of the circuit with different light conditions and different values of the circuit elements and it was observed that the presence of the artifact was related to low intensity currents flowing through the net of the LG-SiPM metal tracks and quenching resistors. Several simulations were run in order to identify the optimal configuration of parameters for the reduction of this unwanted effect and to implement improvements in future LG-SiPM productions.Another application of the LG-SiPM in the field of radiation detection is the position reconstruction of the scintillation light emitted by gamma-rays in a monolithic crystal. Using a thin CsI:Tl crystal and lowering the detector temperature it was possible to distinguish different positions of interaction on the surface of the detector with an error below 1 mm FWHM. This technology can be effective for the creation of monolithic, position sensitive X and gamma-ray detector with good energy resolution for low energy spectroscopy or medical imaging devices.

A pulse height tally response expansion (PHRE) method is developed for detectors. By expanding the incident flux at the detector window/surface, a set of response functions is constructed via Monte Carlo estimators for pulse height tallies. B-spline functions are selected to perform the expansion of the response functions as well as for the expansion of the incident flux in photon energy. The method is verified for several incident flux spectra on a CsI(Na) detector. Results are compared to the solutions generated using direct Monte Carlo calculations. It is found that the method is several orders faster than MCNP5 while maintaining paralleled accuracy.

Des detecteurs de rayonnement a base de diodes p-i-n en silicium amorphe hydrogene (a-si:h) et de couches de diamant polycristallin ont ete elabores par la technique de depot chimique en phase vapeur assiste par plasma. Les echantillons ont ete caracterises afin de determiner les proprietes physiques, optiques et electriques et d'optimiser les conditions de depot du point de vue de la detection de rayonnements. Les detecteurs ont ete testes sous rayonnements de maniere a determiner leurs performances en termes de sensibilite et de rapport signal sur bruit. Les mecanismes de collection des charges ont ete analyses. Dans les diodes p-i-n en a-si:h, l'effet du profil de champ electrique, du transport dispersif des trous et de la recombinaison des charges a ete etudie. Dans le diamant polycristallin, des mecanismes de recombinaison dans le volume des cristallites et au niveau des joints de grains ont ete mis en evidence. L'interaction de rayonnements , et dans les detecteurs a ete etudiee par simulation numerique avec un code de calcul monte carlo et par des mesures experimentales. Le faible pouvoir d'arret permet de detecter de maniere selective des particules chargees en presence d'un bruit de fond du au rayonnement. L'utilisation de diodes a-si:h minces de grande surface permet donc d'ameliorer la sensibilite de detecteurs pour des mesures de contamination surfaciques necessitant une bonne discrimination entre particules chargees et photons. La reponse dosimetrique des detecteurs sous photons ou electrons a ete simulee par la methode de monte carlo et comparee a celle d'un detecteur classique. Des mesures sous faisceau de references ont ete effectuees afin de valider les resultats obtenus par simulation. Le diamant, dont le numero atomique est proche de celui des tissus humains, est un bon materiau equivalent tissu et peut etre utilise pour la mesure de la dose sans apporter de corrections dues a l'energie des photons incidents.

Thesis (MSc)--Stellenbosch University, 2005.
Some digitised pages may appear illegible due to the condition of the original hard copy.
ENGLISH ABSTRACT: Airborne gamma-ray spectrometry is used for exploration and environmental monitoring over large areas. Such datasets of 7-ray are acquired using large 16l NaI(T1) detector crystals mounted in an aircraft. The response of these detectors to naturally occurring radioactive isotopes is a function of detector volume, efficiency and resolution. This response is modelled using the Monte Carlo method. A photon-electron transport computer code, NIPET, is written to calculate the intrinsic efficiency and photofraction of these large prismatic crystals. The intrinsic efficiency indicates the number of incident photons that interact in some way with the detector resulting in a deposition of energy in the crystal. The photofraction, on the other hand, indicates how many of these photons that impinge on the detector are actually recorded with their full initial energy. This study presents explicity calculated photofraction values for 16l NaI(Tl) crystals. Comparison of these new values, calculated with this computer program, with that of published values for small 3" x 3" crystals shows good agreement. However, for large crystals, the calculated values differ from those in the literature by almost 15%. The study also finds that for the gamma-ray energy range between 0.2 - 3.0 MeV, these detectors have an intrinsic efficiency above 90%, whereas the photofraction values range from 80% for low energy photons to above 40% for high energy photons in the case of large crystals.
AFRIKAANSE OPSOMMING: Lug gamma-straal spektroskopie is nuttig vir eksplorasie en omgewings ondersoeke oor groot areas. Metings word tipies gedoen met 16l NaI(T1) detektors wat monteer is in ’n vliegtuig. Die respons van hierdie detektors ten opsigte van natuurlike radioaktiewe isotope is ’n funksie van detektorvolume, doeltreffendheid en oplosvermoë. Hierdie respons word gemodeleer deur gebruik te maak van ’n Monte Carlo tegniek. ’n Rekenaarprogram, NIPET, is geskryf om die intrinsieke doeltreffendheid en fotofraksie vir hierdie groot prismatiese kristalle te bereken. Die intrinsieke doeltreffendheid dui die breukdeel van die invallende fotone aan wat ’n interaksie ondergaan binne die detektor kristal en sodoende energie deponeer. Die fotofraksie, daarenteen, dui die breukdeel invallende fotone aan wat absorbeer word deur die kristal met hul voile invallende energie. In hierdie tesis word eksplisiete fotofraksiewaardes vir 16l NaI(T1) kristale bereken. Goeie ooreenstemming word gevind tussen fotofraksie waardes bereken met hierdie program en die waardes gepubliseer in die literatuur vir klein 3" x 3" kristalle. Vir groter kristalle is daar egter swakker ooreenstemming. Berekende fotofraksie waardes verskil met 15% van die waardes in die literatuur. Daar word ook bevind dat vir gamma-energie tussen 0.2 - 3.0 MeV, die intrinsieke doeltreffendheid vir hierdie tipe detektors bo 90% is en dat fotofraksiewaardes wissel tussen 80% vir lae energie tot bo 40% vir hoë foton energie.

The development of a replacement to the conventional film based X-ray imaging technique is required for many reasons. One possible route for this is the use of a large area film of a suitable semiconductor overlaid on an amorphous silicon readout array. A suitable semiconductor exists in cadmium telluride and its tertiary alloy cadmium zinc telluride. In this thesis the spectroscopic characteristics of commercially available CZT X- and gamma-radiation detectors are established. The electronic, optical, electro-optic, structural and compositional properties of these detectors are then investigated. The attained data is used to infer a greater understanding for the carrier transport in a CZT radiation detector following the interaction of a high energy photon. Following this a method used to fabricate large area films of CdTe on a commercial scale is described. This is cathodic electrodeposition from an aqueous electrolyte. The theory and experimental arrangement for this technique are described in detail with preliminary results from the fabricated films presented. Attention is then turned to the CdS/CdTe films that are produced commercially for the photovoltaic industry. In this case the crystalline nature, surface topography and optical properties are investigated. A conclusion examines the progress that has been made towards the development of a large area flat panel digital imaging technique.

In der vorliegenden Arbeit wird der Einsatz von Positronenemittern zur Bestimmung der Dichte eines zweiphasigen Mediums beschrieben. Zur Messung wird die Dichteabhängigkeit der Positronenreichweite ausgenutzt. Die Realisierbarkeit des Verfahrens wird zunächst in einer Computersimulation geprüft. Danach werden Experimente an einem hochauflösenden PET-Scanner vorgestellt. Bei diesen Experimenten wird die Dichteabhängigkeit der Reichweite von Positronen anhand von Modellmedien aus festen Schaumstoffen untersucht. Dabei werden die in der Computersimulation erzielten Ergebnisse bestätigt. Aus den experimentellen Ergebnissen wird ein Modell zur Beschreibung der Abnahme der Koinzidenzrate in Abhängigkeit von der Entfernung des Detektors von der Positronenquelle und der Mediendichte entwikkelt. Auf der Basis des Modells erfolgt die Konzipierung einer für die Dichtemessung optimierten Detektoranordnung. Die Funktionsweise dieses Detektorsystems wird in Form technischer Unterlagen und experimenteller Ergebnisse beschrieben. Mit dem optimierten Detektorsystem werden die Untersuchungen an den Modellmedien nochmals durchgeführt, wobei die bisherigen Ergebnisse bestätigt werden. Das beschriebene Meßverfahren dient der Bestimmung der mittleren Dichte in einem überwiegend gasförmigen Medium, wobei eine Mittelwertbildung über das Meßvolumen erfolgt. Das Verfahren kombiniert die Vorteile bekannter densitometrischer Meßmethoden auf der Basis einer Schwächung von Strahlung, indem die hohe Dichtesensitivität der Positronenstrahlung mit der relativ geringen Absorption der energiereichen Annihilationsquanten kombiniert wird.

In der vorliegenden Arbeit wird der Einsatz von Positronenemittern zur Bestimmung der Dichte eines zweiphasigen Mediums beschrieben. Zur Messung wird die Dichteabhängigkeit der Positronenreichweite ausgenutzt. Die Realisierbarkeit des Verfahrens wird zunächst in einer Computersimulation geprüft. Danach werden Experimente an einem hochauflösenden PET-Scanner vorgestellt. Bei diesen Experimenten wird die Dichteabhängigkeit der Reichweite von Positronen anhand von Modellmedien aus festen Schaumstoffen untersucht. Dabei werden die in der Computersimulation erzielten Ergebnisse bestätigt. Aus den experimentellen Ergebnissen wird ein Modell zur Beschreibung der Abnahme der Koinzidenzrate in Abhängigkeit von der Entfernung des Detektors von der Positronenquelle und der Mediendichte entwikkelt. Auf der Basis des Modells erfolgt die Konzipierung einer für die Dichtemessung optimierten Detektoranordnung. Die Funktionsweise dieses Detektorsystems wird in Form technischer Unterlagen und experimenteller Ergebnisse beschrieben. Mit dem optimierten Detektorsystem werden die Untersuchungen an den Modellmedien nochmals durchgeführt, wobei die bisherigen Ergebnisse bestätigt werden. Das beschriebene Meßverfahren dient der Bestimmung der mittleren Dichte in einem überwiegend gasförmigen Medium, wobei eine Mittelwertbildung über das Meßvolumen erfolgt. Das Verfahren kombiniert die Vorteile bekannter densitometrischer Meßmethoden auf der Basis einer Schwächung von Strahlung, indem die hohe Dichtesensitivität der Positronenstrahlung mit der relativ geringen Absorption der energiereichen Annihilationsquanten kombiniert wird.

Les travaux de cette thèse consistent à développer des outils de spectroscopie et d'imagerie térahertz pour des applications médicales. L'objectif est de déterminer le potentiel et l'efficacité de la spectroscopie térahertz et de l'imagerie dans la détection des régions cancéreuses et la distinction entre les tissus malades et sains pour le cancer du sein chez les femmes. La spectroscopie térahertz est une technique sans contact, non ionisante pour obtenir des résultats rapides, comparée à l'analyse clinique standard. Les études expérimentales sont divisées en deux sections principales :Section I :Cette partie se concentre sur la spectroscopie en utilisant un rayonnement THz. La maîtrise de cette technique permet de travailler en mode réflexion ou transmission avec des fréquences dans la bande passante térahertz. Plusieurs types de matériaux ont été utilisés comme fantômes pour la calibration de l'expérience : des solides (silice, téflon, saphir et verre), des liquides (méthanol, eau et alcool) et des tissus biologiques (cancer, fibres et gras), ainsi qu'un mélange (eau-méthanol). Les indices de réfraction, les coefficients d'absorption et les fonctions diélectriques complexes ont d'abord été mesurés et extraits puis fittés avec un modèle de Debye. Les tissus biologiques sont apparus hétérogènes en épaisseur et avec des surfaces qui peuvent être irrégulières, ce qui rend difficile l'extraction d'informations précises, en raison d’artefacts induits. Les signaux ont été traités en suivant un protocole rigoureux : Les mesures sont effectuées sur un support parfaitement caractérisé en transmission pour réduire les incertitudes sur la phase lors des mesures en réflexion. Les signaux THz réfléchis aux interfaces entre l'air / échantillon, air / fenêtre, eau / fenêtre et fenêtre / fenêtre sont utilisés comme signal de base pour estimer et améliorer le rapport signal-bruit dans les mesures de spectroscopie. L'avantage de cette méthode est sa précision, sa simplicité et sa facilité d'application pour un système de réflexion avec un angle d'incidence. La mesure des indices de réfraction et des coefficients d'absorption des échantillons avec des tissus tumoraux et sains a révélé que les régions tumorales présentent des différences significatives par rapport au tissu normal lors de l’interaction tissu-rayonnement térahertz.Section II :La deuxième partie de cette étude porte sur l'imagerie THz pour la détection du cancer du sein, à la fois dans les modes de transmission et de réflexion. Plusieurs types d'échantillons ont été étudiés. Les coupes utilisées comprenaient des tissus inclus en paraffine, des tissus frais sortis du bloc opératoire, fixés au formol et des blocs. Pour cela le spectromètre a été déplacé à l'hôpital. Plus de 50 échantillons ont été ainsi inspectés. TroisIVméthodes de traitement d'image ont été utilisées : le découpage, l'automatisation et le tri d'images manuel. De plus, les images obtenues dans le domaine temporel et dans le domaine fréquentiel ont été analysées pour décrire et identifier les différentes régions du tissu mammaire étudiées et déterminer le contraste entre le tissu sain et le tissu malade. La quantité d'eau différentielle présente dans les tissus malades peut être l'une des origines de contraste. En effet, le tissu cancéreux possède une teneur en eau plus élevée que celle des fibres ou des tissus adipeux normaux, ce qui permet de discriminer les régions cancéreuses, fibreuses et graisseuses sur les images THz
The work of this thesis consists in developing terahertz spectroscopy and imaging tools for medical applications. The goal is to determine the potential and effectiveness of terahertz spectroscopy and imaging in the detection of cancer regions and the distinction between diseased and healthy tissue for breast cancer in women. Terahertz spectroimaging is a non-contact, non-ionizing technique for rapid results compared to standard clinical analysis. Experimental studies are divided into two main sections:Section IThis part focuses on THz spectroscopy using THz radiation. The mastery of this technique makes it possible to work in reflection or transmission mode with frequencies in the terahertz bandwidth. Several types of materials have been used as ghosts for the calibration of the experiment: solids (silica, teflon, sapphire and glass), liquids (methanol, water and alcohol) and biological tissues (cancer, fiber and fat), as well as a mixture (water-methanol). The refractive indices, the absorption coefficients and the complex dielectric functions were first measured and extracted and then fitted with a Debye model. Biological tissues have appeared heterogeneous in thickness and with surfaces that may be irregular, making it difficult to extract accurate information because of induced artifacts. The signals have been processed according to a rigorous protocol: The measurements are carried out on a perfectly characterised substratet in transmission to reduce the uncertainties on the phase during the measurements in reflection. The THz signals reflected at the interfaces between the air / sample, air / window, water / window and window / window are used as a basic signal to estimate and improve the signal-to-noise ratio in the spectroscopy measurements. The advantage of this method is its accuracy, simplicity and ease of application for a reflection system with an angle of incidence. Measurement of refractive indices and absorption coefficients of samples with tumor and healthy tissue revealed that the tumor regions showed significant differences from normal tissue during terahertz tissue-radiation interaction.Section II:The second part of this study focuses on THz imaging for breast cancer detection in both transmission and reflection modes. Several types of samples have been studied. Sections used included paraffin-embedded tissue, fresh tissues removed from the OR, formalin-fixed, and blocks. For this the spectrometer has been moved to the hospital. More than 50 samples were inspected. Three image processing methods were used: cutting, automation and manual image sorting. In addition, time domain and frequency domain images were analyzed to describe and identify the different regions of mammary tissue studied and to determine the contrast between healthy tissue and diseased tissue. The amount of differential water present in diseased tissue can be one of the sources of contrast. In fact, the cancerous tissue has a higher water content than that of normal fibers or adipose tissue, which makes it possible to discriminate the cancerous, fibrous and fatty regions on the THz images

This work describes the construction of an inductively-coupled plasma tandem quadrupole mass spectrometer where a quadrupole ion storage trap acts as a second sector and collision cell to achieve neutralization or collisional dissociation of interfering species before mass analysis. Because most elements exist as singly-charged ions in an inductively-coupled plasma (ICP) plume, the ICP can be used as an ionization source for mass analysis (ICP/MS). By reducing the sample to elemental ions before mass analysis, ICP/MS spectra tend to be simple compared with those obtained by ICP-optical emission spectrometry (ICP-OES) where elements may have hundreds to thousands of emission lines and spectral overlaps can be severe. This is especially troublesome in the analysis of rare earth elements which have the largest numbers of active emission lines when excited in an ICP. In addition, detection limits by ICP/MS are often up to 3 orders of magnitude lower than by ICP-OES. ICP/MS analysis is not immune from isobaric and isotopic interferences or matrix effects. For most analyses, an acid digestion precedes aspiration as an aqueous solution into an argon plasma gas. This can lead to large amounts of Ar+ etc., which may interfere to varying degrees with analytes of interest. Oxides, argides and hydrides of matrix ions or other analytes may also form and interfere. These same processes can also split peak areas between the atomic form of an analyte and the molecular. In isotope ratio studies where precise measurements on more than one isotope per analyte are needed, these effects may be compounded. Isobaric interferences normally require high resolution mass analysis to resolve if they cannot be separated prior to sample introduction. However, the interface between a high vacuum, high resolution sector or ion cyclotron resonance mass spectrometer and an atmospheric pressure plasma is non-trivial and such instruments are expensive. The focus of this work is new approach which uses a collision cell where weakly-bound molecular species can be dissociated and ions with relatively high electron affinities, such as Ar+, can be neutralized through charge exchange reactions.

This thesis concerns the development and characterisation of X-ray imaging systems based on single photon processing. “Colour” X-ray imaging opens up new perspectives within the fields of medical X-ray diagnosis and also in industrial X-ray quality control. The difference in absorption for different “colours” can be used to discern materials in the object. For instance, this information might be used to identify diseases such as brittle-bone disease. The “colour” of the X-rays can be identified if the detector system can process each X-ray photon individually. Such a detector system is called a “single photon processing” system or, less precise, a “photon counting system”. With modern technology it is possible to construct photon counting detector systems that can resolve details to a level of approximately 50 µm. However with such small pixels a problem will occur. In a semiconductor detector each absorbed X-ray photon creates a cloud of charge which contributes to the image. For high photon energies the size of the charge cloud is comparable to 50 µm and might be distributed between several pixels in the image. Charge sharing is a key problem since, not only is the resolution degenerated, but it also destroys the “colour” information in the image. This thesis presents characterisation and simulations to provide a detailed understanding of the physical processes concerning charge sharing in detectors from the MEDIPIX collaboration. Charge summing schemes utilising pixel to pixel communications are proposed. Charge sharing can also be suppressed by introducing 3D-detector structures. In the next generation of the MEDIPIX system, Medipix3, charge summing will be implemented. This system, equipped with a 3D-silicon detector, or a thin planar high-Z detector of good quality, has the potential to become a commercial product for medical imaging. This would be beneficial to the public health within the entire European Union.
Denna avhandling berör utveckling och karaktärisering av fotonräknande röntgensystem. ”Färgröntgen” öppnar nya perspektiv för medicinsk röntgendiagnostik och även för materialröntgen inom industrin. Skillnaden i absorption av olika ”färger” kan användas för att särskilja olika material i ett objekt. Färginformationen kan till exempel användas i sjukvården för att identifiera benskörhet. Färgen på röntgenfotonen kan identifieras om detektorsystemet kan detektera varje foton individuellt. Sådana detektorsystem kallas ”fotonräknande” system. Med modern teknik är det möjligt att konstruera fotonräknande detektorsystem som kan urskilja detaljer ner till en upplösning på circa 50 µm. Med så små pixlar kommer ett problem att uppstå. I en halvledardetektor ger varje absorberad foton upphov till ett laddningsmoln som bidrar till den erhållna bilden. För höga fotonenergier är storleken på laddningsmolnet jämförbar med 50 µm och molnet kan därför fördelas över flera pixlar i bilden. Laddningsdelning är ett centralt problem delvis på grund av att bildens upplösning försämras, men framför allt för att färginformationen i bilden förstörs. Denna avhandling presenterar karaktärisering och simulering för att ge en mer detaljerad förståelse för fysikaliska processer som bidrar till laddningsdelning i detektorer från MEDIPIX-projekter. Designstrategier för summering av laddning genom kommunikation från pixel till pixel föreslås. Laddningsdelning kan också begränsas genom att introducera detektorkonstruktioner i 3D-struktur. I nästa generation av MEDIPIX-systemet, Medipix3, kommer summering av laddning att vara implementerat. Detta system, utrustat med en 3D-detektor i kisel, eller en tunn plan detektor av högabsorberande material med god kvalitet, har potentialen att kunna kommersialiseras för medicinska röntgensystem. Detta skulle bidra till bättre folkhälsa inom hela Europeiska Unionen.

Grace a son faisceau de photons de haute energie, l'experience graal va pouvoir mesurer pour la premiere fois des observables de simple et double polarisation liees a la polarisation du faisceau pour des reactions de photoproduction d'etrangete. Apres un expose des modeles phenomenologiques decrivant le mecanisme de telles reactions et de la definition des observables de polarisation, cette these presente en detail le faisceau de gammas produit par retrodiffusion compton et le detecteur de particules de l'experience graal. La simulation monte carlo permet d'etudier la reponse detaillee du detecteur aux evenements photonucleaires. Des procedures de reconstruction de traces, de reduction de bruit de fond et d'analyse finale ont ete developpees, testees et optimisees. Parmi deux procedes qui suppriment la presque totalite du bruit de fond, celui garantissant la meilleure efficacite a ete choisi. La reproduction precise des valeurs d'une observable de simple polarisation et de deux observables de double polarisation pour un ensemble de donnees correspondant a 420000 reactions gamma(p,lambda)k prouve que des telles quantites peuvent etre mesurees avec exactitude par l'experience graal

Une architecture de capteurs à pixels CMOS permettant la désertion du volume sensible par polarisation via la face avant du circuit est étudiée à travers la caractérisation en laboratoire d’un capteur prototype. Les performances de collection de charge confirment la désertion d‘une grande partie de l’épaisseur sensible. De plus, le bruit de lecture restant modeste, le capteur présente une excellente résolution en énergie pour les photons en dessous de 20 keV à des températures positives. Ces résultats soulignent l’intérêt de cette architecture pour la spectroscopie des rayons X mous et pour la trajectométrie des particules chargées en milieu très radiatif. La profondeur sur laquelle le capteur est déserté est prédite par un modèle analytique simplifié et par des calculs par éléments finis. Une méthode d’évaluation de cette profondeur par mesure indirecte est proposée. Les mesures corroborent les prédictions concernant un substrat fin, très résistif, qui est intégralement déserté et un substrat moins résistif et mesurant 40 micromètres, qui est partiellement déserté sur 18 micromètres mais détecte correctement sur la totalité de l’épaisseur. Deux développements de capteurs destinés à l’imagerie X et à la neuro-imagerie intracérébrale sur des rats éveillés et libres de leurs mouvements sont présentés
An architecture of CMOS pixel sensor allowing the depletion of the sensitive volume through frontside biasing is studied through the characterization in laboratory of a prototype. The charge collection performances confirm the depletion of a large part of the sensitive thickness. In addition, with a modest noise level, the sensor features an excellent energy resolution for photons below 20 keV at positive temperatures. These results demonstrate that such sensors are suited for soft X-ray spectroscopy and for charged particle tracking in highly radiative environment. A simplified analytical model and finite elements calculus are used to predict the depletion depth reached. An indirect measurement method to evaluate this depth is proposed. Measurements confirm predictions for a thin highly resistive epitaxial layer, which is fully depleted, and a 40micrometers thick bulk less resistive substrate, for which depletion reached 18 micrometers but which still offers correct detection over its full depth. Two sensor designs dedicated to X-ray imaging and in-brain neuroimaging on awake and freely moving rats are presented

Le projet de physique nucleaire graal installe a grenoble aupres du synchrotron europeen (esrf), fournit un faisceau de photons etiquetes de haute energie (jusqu'a 1,5 gev). Ce faisceau, produit par retrodiffusion compton, est facilement polarisable et il permet de sonder d'une maniere originale la structure du nucleon. L'ensemble de detection associe comprend entre autres, des chambres a fils proportionnelles et des hodoscopes de scintillateurs. Un ensemble de six asic (application specific integrated circuit) a ete developpe, il assure le traitement des signaux des detecteurs et le conditionnement des donnees jusqu'au systeme d'acquisition. Cette electronique integree permet l'implantation des circuits directement sur les detecteurs. Les avantages evidents qui en decoulent sont une meilleure qualite des signaux et une fiabilite accrue, toutes deux dues a la reduction de la longueur et du nombre des connexions. Le wire processor (wp), asic concu et teste durant cette these, permet le traitement des signaux issus des fils de chambres ou des photomultiplicateurs. Le circuit comprend deux voies identiques qui permettent: l'amplification, la discrimination d'amplitude, la generation d'un retard programmable et l'ecriture dans une memoire deux etats, si la mise en coincidence avec un signal exterieur de validation a eue lieu. La mesure de l'efficacite d'une chambre a fils a permis de montrer le bon fonctionnement du wp, de l'electronique de conditionnement des donnees, du systeme d'acquisition et de la chambre elle meme

Thesis (M.S.)--Worcester Polytechnic Institute.
Keywords: basis functions; perfectly matched layers; PML; neck model; parallel plate resonator; finite element; circulator; glottal waveform; multi-transmission line; dielectric properties of human tissues; radiation currents; weighted residuals; non-acoustic sensor. Includes bibliographical references (p. 104-107).

Elaboration de monocristaux hg1::(2)-alpha en vue de leur utilisation comme detecteurs de rayonnements. La purification du materiau de base, le controle du transport de matiere, le controle de la sursaturation a l'interieur des ampoules de croissance, la monogermination et l'etude de la morphologie du cristal sont traites en relation avec les proprietes de detection. Une approche theorique par simulation monte-carlo de la reponse d'un detecteur hgi::(2) a ete faite

Sewer and oil pipeline spillage issues have become major causes of pollution in urban and rural areas usually caused by blockages in the water storage and drainage system, and oil spillage of underground oil pipelines. An effective way of avoiding this problem will be by deploying some mechanism to monitor these installations at each point in time and reporting unusual liquid activity to the relevant authorities for prompt action to avoid a flooding or spillage occurrence. This research work presents a low cost energy efficient liquid level monitoring technique using Radio Frequency Identification Technology. Passive UHF RFID tags have been designed, modelled and optimized. A simple rectangular tag, the P-shaped tag and S-shaped tag with UHF band frequency of operation (850-950 MHz) has been designed and modelled. Detailed parametric analysis of the rectangular tag is made and the optimised design results analysed and presented in HFSS and Matlab. The optimised rectangular tag designs are then deployed as level sensors in a gully pot. Identical tags were deployed to detect 4 distinct levels in alternate positions and a few inches in seperation distance within the gully pot height (Low, Mid, High and Ultra high). The radiation characteristic of tag sensors in deployment as modelled on HFSS is observed to show consistent performance with application requirements. An in-manhole chamber antenna for an underground communication system is analysed, designed, deployed and measured. The antenna covers dual-band impedance bandwidths (i.e. 824 to 960 MHz, and 1710 to 2170 MHz). The results show that the antenna prototype exhibits sufficient impedance bandwidth, suitable radiation characteristics, and adequate gains for the required underground wireless sensor applications. Finally, a Linearly Shifted Quadrifilar Helical Antenna (LSQHA) designed using Genetic Algorithm optimisation technique for adoption as an RFID reader antenna is proposed and investigated. The new antenna confirms coverage of the RFID bandwidth 860-960 MHz with acceptable power gain of 13.1 dBi.

La thérapie photodynamique anticancéreuse repose sur la production d'oxygène singulet lors d'une réaction photochimique entre un agent photosensibilisant fixé sur les cellules cancéreuses et une irradiation lumineuse dans le rouge. Pour avoir une efficacité thérapeutique, il est important de connaitre l'atténuation lumineuse dans les tissus et ainsi la profondeur de pénétration de la lumière afin de définir une dosimétrie optique précise. La connaissance de ce paramètre doit s'effectuer en continu et in situ. Les mesures préliminaires de la rétrodiffusion et de la transmission lumineuse dans des solutions biologiques ainsi que le développement d'une simulation numérique décrivant les deux phénomènes ont été des bases solides pour la réalisation d'un capteur à fibres optiques capable d'estimer la profondeur de pénétration de la lumière à partir de la rétrodiffusion de celle-ci dans ces milieux. Une instrumentation est associée au capteur pour permettre l'estimation de la profondeur de pénétration de la lumière dans les tissus en continu et in situ. Les résultats des mesures avec le capteur dans des milieux liquides et solides sont rapportés puis comparés aux valeurs de référence. Le dispositif réalisé peut être utilisé pour des mesures cliniques directes

Near field imaging using microwave in medical applications has gain much attention recently as various researches show its high ability and accuracy in illuminating object comparing to the well-known screening tools such as Magnetic Resonance Imaging (MRI), digital mammography, ultrasound etc. This has encourage and motivate scientists continue to exploit the potential of microwave imaging so that a better and more powerful sensing tools can be developed. This thesis documents the development of antenna design for microwave imaging application such as breast cancer detection. The application is similar to the concept of Ground Penetrating Radar (GPR) but operating at higher frequency band. In these systems a short pulse is transmitted from an antenna to the medium and the backscattered response is investigated for diagnose. In order to accommodate such a short pulse, a very wideband antenna with a minimal internal reflection is required. Printed monopole and planar metal plate antenna is implemented to achieve the necessary operating wide bandwidth. The development of new compact printed planar metal plate ultra wide bandwidth antenna is presented. A generalized parametric study is carried out using two well-known software packages to achieve optimum antenna performance. The Prototype antennas are tested and analysed experimentally, in which a reasonable agreement was achieved with the simulations. The antennas present an excellent relative wide bandwidth of 67% with acceptable range of power gain between 3.5 to 7 dBi. A new compact size air-dielectric microstrip patch-antenna designs proposed for breast cancer detection are presented. The antennas consist of a radiating patch mounted on two vertical plates, fed by coaxial cable. The antennas show a wide bandwidth that were verified by the simulations and also confirmed experimentally. The prototype antennas show excellent performance in terms the input impedance and radiation performance over the target range bandwidth from 4 GHz to 8 GHz. A mono-static model with a homogeneous dielectric box having similar properties to human tissue is used to study the interaction of the antenna with tissue. The numerical results in terms the matching required of new optimised antennas were promising. An experimental setup of sensor array for early-stage breast-cancer detection is developed. The arrangement of two elements separated by short distance that confined equivalent medium of breast tissues were modelled and implemented. The operation performances due to several orientations of the antennas locations were performed to determine the sensitivity limits with and without small size equivalent cancer cells model. In addition, a resistively loaded bow tie antenna, intended for applications in breast cancer detection, is adaptively modified through modelling and genetic optimisation is presented. The required wideband operating characteristic is achieved through manipulating the resistive loading of the antenna structure, the number of wires, and their angular separation within the equivalent wire assembly. The results show an acceptable impedance bandwidth of 100.75 %, with a VSWR < 2, over the interval from 3.3 GHz to 10.0 GHz. Feasibility studies were made on the antenna sensitivity for operation in a tissue equivalent dielectric medium. The simulated and measured results are all in close agreement.

Near field imaging using microwave in medical applications has gain much attention recently as various researches show its high ability and accuracy in illuminating object comparing to the well-known screening tools such as Magnetic Resonance Imaging (MRI), digital mammography, ultrasound etc. This has encourage and motivate scientists continue to exploit the potential of microwave imaging so that a better and more powerful sensing tools can be developed. This thesis documents the development of antenna design for microwave imaging application such as breast cancer detection. The application is similar to the concept of Ground Penetrating Radar (GPR) but operating at higher frequency band. In these systems a short pulse is transmitted from an antenna to the medium and the backscattered response is investigated for diagnose. In order to accommodate such a short pulse, a very wideband antenna with a minimal internal reflection is required. Printed monopole and planar metal plate antenna is implemented to achieve the necessary operating wide bandwidth. The development of new compact printed planar metal plate ultra wide bandwidth antenna is presented. A generalized parametric study is carried out using two well-known software packages to achieve optimum antenna performance. The Prototype antennas are tested and analysed experimentally, in which a reasonable agreement was achieved with the simulations. The antennas present an excellent relative wide bandwidth of 67% with acceptable range of power gain between 3.5 to 7 dBi. A new compact size air-dielectric microstrip patch-antenna designs proposed for breast cancer detection are presented. The antennas consist of a radiating patch mounted on two vertical plates, fed by coaxial cable. The antennas show a wide bandwidth that were verified by the simulations and also confirmed experimentally. The prototype antennas show excellent performance in terms the input impedance and radiation performance over the target range bandwidth from 4 GHz to 8 GHz. A mono-static model with a homogeneous dielectric box having similar properties to human tissue is used to study the interaction of the antenna with tissue. The numerical results in terms the matching required of new optimised antennas were promising. An experimental setup of sensor array for early-stage breast-cancer detection is developed. The arrangement of two elements separated by short distance that confined equivalent medium of breast tissues were modelled and implemented. The operation performances due to several orientations of the antennas locations were performed to determine the sensitivity limits with and without small size equivalent cancer cells model. In addition, a resistively loaded bow tie antenna, intended for applications in breast cancer detection, is adaptively modified through modelling and genetic optimisation is presented. The required wideband operating characteristic is achieved through manipulating the resistive loading of the antenna structure, the number of wires, and their angular separation within the equivalent wire assembly. The results show an acceptable impedance bandwidth of 100.75 %, with a VSWR < 2, over the interval from 3.3 GHz to 10.0 GHz. Feasibility studies were made on the antenna sensitivity for operation in a tissue equivalent dielectric medium. The simulated and measured results are all in close agreement.

Les microsystèmes sont le dernier développement de la microélectronique. Leur apparition ouvre des possibilités révolutionnaires dans plusieurs domaines d'application, dont l'exploitation de l'espace. L'utilisation des microsystèmes dans l'espace se heurte au problème de l'exposition à la radiation, notamment pour la partie électronique. Cet obstacle a été surmonte dans le passe par la mise en place de filières de fabrication résistantes (durcies) aux effets de la radiation. Le rétrécissement des budgets militaires a provoqué la disparition de la plupart des technologies de fabrication durcies, ce qui est en train de pousser les constructeurs vers l'emploi de technologies commerciales standard (COTS). L'objectif de cette thèse a été d'investiguer des techniques de conception pour le durcissement d'un microsystème fabrique par une technologie COTS. Le microsystème en question est un capteur de rayonnements infrarouges base sur des thermopiles en silicium, suspendues par une étape de micro-usinage en volume par la face avant. Les éléments pertinents des différents domaines de connaissance impliques sont passés en revue, avec une analyse des techniques de durcissement applicables à la construction de l'électronique de lecture en technologie CMOS. Un programme de caractérisation expérimentale a été réalisé, et il a permis d'établir le niveau de sensibilité de la technologie aux rayonnements et l'efficacité des techniques de durcissement développées. Les très bons résultats obtenus ont permis de passer à la réalisation de la chaine de lecture du capteur, qui a été fabriquée, caractérisée et qualifiée pour l'espace.

La puissance de la métrologie optique nécessite généralement des géométries simples avec un alignement précis et une phase optique bien contrôlée. Dans cette thèse, nous développons plutôt la notion d'interférométrie du chaos, en utilisant un champ optique ayant un désordre géométrique maximal et une phase aléatoire. Nous montrons que la stochasticité conduit à une très grande sensibilité interférométrique et ouvre la possibilité d'un large éventail de nouvelles mesures optiques et d'une nouvelle méthode que nous appelons la Cavity Amplified Speckle Spectroscopy (Spectroscopie de Speckle Amplifiée par Cavité).L'idée clef est d'injecter un laser monochromatique à très faible bande passante dans une cavité à haute réflectivité Lambertienne, qui agit comme un résonateur aléatoire à gain élevé. On obtient alors un billard Lambertien cohérent en 3D, rempli d'un champ aléatoire 3D statistiquement homogène dans l'espace et invariant par rotation. En tout point P, il peut être décrit comme la superposition cohérente d'un grand nombre d'ondes planes prises au hasard dans une distribution statistique unique qui combine indépendamment (1) une distribution à symétrie sphérique du vecteur d'onde sur une sphère ||k||=k0, avec (2) une distribution uniforme de la phase sur [0,2pi], et (3) un état de polarisation uniformément distribué sur la sphère de Poincarré. Le motif de speckle 3D aléatoire qui en résulte reste constant dans le temps tant que la diffusion de la longueur d'onde du laser peut être négligée. A plus long terme, cependant, il se comporte ergodiquement. Ce travail représente la première réalisation expérimentale de la notion de champ aléatoire 3D proposée par Berry, et il se rapporte également aux recherches sur l'enchevêtrement classique de la lumière. Les concepts de résonateur aléatoire à gain élevé, ou billard lambertien cohérent, correspondent à un nouveau domaine de l’optique, qui n'obéit ni à l'équation d'onde ni à l'équation de diffusion, et devrait conduire à de nouvelles recherches théoriques et expérimentales.En pratique, avec une diffusion suffisamment lente de la phase d'entrée et un bruit de photons suffisamment faible, l'intensité du champ de speckle ne fluctue et ne devient ergodique que si la géométrie de la cavité n'est pas constante, ou si elle contient un milieu avec une distribution de longueurs de chemins optiques ou une polarisation non constante. En utilisant des spectres de décorrélation d'intensité obtenus entre 100 MHz et 0,01 Hz à partir de grains de speckle individuels, nous démontrons la possibilité de mesurer les variations picométriques de la géométrie de la cavité et de détecter sous l’ angstrom le mouvement de diffuseurs en solution. Cette interférométrie volumique du chaos peut également être utilisée pour amplifier des signaux de diffusion auparavant indétectables, et nous montrons un setup miniaturisé de diffusion de la lumière fonctionnant avec des volumes de l’ordre du microlitre et des systèmes quasi-transparents. Un brevet a été déposé pour toute une série d'applications, notamment la détection des vibrations sismiques et acoustiques, la caractérisation du bruit de phase des lasers et la mesure d'échantillons très dilués et peu diffusants
The power of optical metrology generally requires simple geometries with precise alignment and well controlled optical phases. In the present thesis, we develop instead the notion of chaos interferometry, using an optical field with maximal geometric disorder and phase randomness. We show that stochasticity leads to a very high interferometric sensitivity and opens up the possibility for a wide range of new optical measurements and a new method we call Cavity Amplified Speckle Spectroscopy.The key idea is to inject a very small bandwidth monochromatic laser into a cavity with high albedo Lambertian reflectivity, which acts as a high-gain random resonator. A 3D coherent Lambertian billiard is obtained, filled with a 3D random field that is statistically uniform in space and invariant by rotation. At any given point P, it can be described as the coherent superposition of a large number of plane waves randomly taken from a unique statistical distribution that independently combines (1) a spherically symmetric distribution of the wave vector on a sphere ||k||=k0, with (2) a uniform distribution of the phase on [0,2pi], and (3) a uniformly distributed polarization state on the Poincarré sphere. The resulting random 3D speckle pattern remains constant with time as long as the diffusion of the laser’s wavelength can be neglected. At longer times however, it behaves ergodically. This work represents the first experimental realization of the notion of a 3D random field proposed by Berry, and it also relates to the investigations on classical light entanglement. The concepts of high-gain random resonator, or coherent Lambertian billiard, correspond to a new kind of field in optics, that obeys neither the wave equation nor the diffusion equation, and should lead to new theoretical and experimental investigations.Practically, with a slow enough diffusion of the input phase and a small enough photon-number noise, the speckle intensity field fluctuates and becomes ergodic only if the geometry of the cavity is not constant, or if it contains a medium with a non-constant optical path length distribution or polarization. Using intensity decorrelation spectra obtained between 100 MHz and 0.01 Hz from single speckles, we show the possibility to measure picometer variations of the cavity geometry and to detect sub-angstrom motion of scatterers in solutions. Chaos interferometry can also be used to amplify previously undetectable scattering signals, and we show a miniaturized light scattering setup working with microliter volumes and quasi-transparent systems. A patent was filled for a range of applications including seismic and acoustic vibration sensing, laser phase noise characterization and measurements of highly diluted and poorly scattering samples

As radiation specialists, one of our primary objectives in the Navy is protecting people and the environment from the effects of ionizing and non-ionizing radiation. Focusing on radiological dispersal devices (RDD) will provide increased personnel protection as well as optimize emergency response assets for the general public. An attack involving an RDD has been of particular concern because it is intended to spread contamination over a wide area and cause massive panic within the general population. A rapid method of triage will be necessary to segregate the unexposed and slightly exposed from those needing immediate medical treatment. Because of the aerosol dispersal of the radioactive material, inhalation of the radioactive material may be the primary exposure route. The primary radionuclides likely to be used in a RDD attack are Co-60, Cs-137, Ir-192, Sr-90 and Am-241. Through the use of a MAX phantom along with a few Simulink MATLAB programs, a good anthropomorphic phantom was created for use in MCNPX simulations that would provide organ doses from internally deposited radionuclides. Ludlum model 44-9 and 44-2 detectors were used to verify the simulated dose from the MCNPX code. Based on the results, acute dose rate limits were developed for emergency response personnel that would assist in patient triage.

The outcomes for both (i) radiation therapy and (ii) preclinical small animal radio- biology studies are dependent on the delivery of a known quantity of radiation to a specific and intentional location. Adverse effects can result from these procedures if the dose to the target is too high or low, and can also result from an incorrect spatial distribution in which nearby normal healthy tissue can be undesirably damaged by poor radiation delivery techniques. Thus, in mice and humans alike, the spatial dose distributions from radiation sources should be well characterized in terms of the absolute dose quantity, and with pin-point accuracy. When dealing with the steep spatial dose gradients consequential to either (i) high dose rate (HDR) brachytherapy or (ii) within the small organs and tissue inhomogeneities of mice, obtaining accurate and highly precise dose results can be very challenging, considering commercially available radiation detection tools, such as ion chambers, are often too large for in-vivo use.

In this dissertation two tools are developed and applied for both clinical and preclinical radiation measurement. The first tool is a novel radiation detector for acquiring physical measurements, fabricated from an inorganic nano-crystalline scintillator that has been fixed on an optical fiber terminus. This dosimeter allows for the measurement of point doses to sub-millimeter resolution, and has the ability to be placed in-vivo in humans and small animals. Real-time data is displayed to the user to provide instant quality assurance and dose-rate information. The second tool utilizes an open source Monte Carlo particle transport code, and was applied for small animal dosimetry studies to calculate organ doses and recommend new techniques of dose prescription in mice, as well as to characterize dose to the murine bone marrow compartment with micron-scale resolution.

Hardware design changes were implemented to reduce the overall fiber diameter to <0.9 mm for the nano-crystalline scintillator based fiber optic detector (NanoFOD) system. Lower limits of device sensitivity were found to be approximately 0.05 cGy/s. Herein, this detector was demonstrated to perform quality assurance of clinical 192Ir HDR brachytherapy procedures, providing comparable dose measurements as thermo-luminescent dosimeters and accuracy within 20% of the treatment planning software (TPS) for 27 treatments conducted, with an inter-quartile range ratio to the TPS dose value of (1.02-0.94=0.08). After removing contaminant signals (Cerenkov and diode background), calibration of the detector enabled accurate dose measurements for vaginal applicator brachytherapy procedures. For 192Ir use, energy response changed by a factor of 2.25 over the SDD values of 3 to 9 cm; however a cap made of 0.2 mm thickness silver reduced energy dependence to a factor of 1.25 over the same SDD range, but had the consequence of reducing overall sensitivity by 33%.

For preclinical measurements, dose accuracy of the NanoFOD was within 1.3% of MOSFET measured dose values in a cylindrical mouse phantom at 225 kV for x-ray irradiation at angles of 0, 90, 180, and 270˝. The NanoFOD exhibited small changes in angular sensitivity, with a coefficient of variation (COV) of 3.6% at 120 kV and 1% at 225 kV. When the NanoFOD was placed alongside a MOSFET in the liver of a sacrificed mouse and treatment was delivered at 225 kV with 0.3 mm Cu filter, the dose difference was only 1.09% with use of the 4x4 cm collimator, and -0.03% with no collimation. Additionally, the NanoFOD utilized a scintillator of 11 µm thickness to measure small x-ray fields for microbeam radiation therapy (MRT) applications, and achieved 2.7% dose accuracy of the microbeam peak in comparison to radiochromic film. Modest differences between the full-width at half maximum measured lateral dimension of the MRT system were observed between the NanoFOD (420 µm) and radiochromic film (320 µm), but these differences have been explained mostly as an artifact due to the geometry used and volumetric effects in the scintillator material. Characterization of the energy dependence for the yttrium-oxide based scintillator material was performed in the range of 40-320 kV (2 mm Al filtration), and the maximum device sensitivity was achieved at 100 kV. Tissue maximum ratio data measurements were carried out on a small animal x-ray irradiator system at 320 kV and demonstrated an average difference of 0.9% as compared to a MOSFET dosimeter in the range of 2.5 to 33 cm depth in tissue equivalent plastic blocks. Irradiation of the NanoFOD fiber and scintillator material on a 137Cs gamma irradiator to 1600 Gy did not produce any measurable change in light output, suggesting that the NanoFOD system may be re-used without the need for replacement or recalibration over its lifetime.

For small animal irradiator systems, researchers can deliver a given dose to a target organ by controlling exposure time. Currently, researchers calculate this exposure time by dividing the total dose that they wish to deliver by a single provided dose rate value. This method is independent of the target organ. Studies conducted here used Monte Carlo particle transport codes to justify a new method of dose prescription in mice, that considers organ specific doses. Monte Carlo simulations were performed in the Geant4 Application for Tomographic Emission (GATE) toolkit using a MOBY mouse whole-body phantom. The non-homogeneous phantom was comprised of 256x256x800 voxels of size 0.145x0.145x0.145 mm3. Differences of up to 20-30% in dose to soft-tissue target organs was demonstrated, and methods for alleviating these errors were suggested during whole body radiation of mice by utilizing organ specific and x-ray tube filter specific dose rates for all irradiations.

Monte Carlo analysis was used on 1 µm resolution CT images of a mouse femur and a mouse vertebra to calculate the dose gradients within the bone marrow (BM) compartment of mice based on different radiation beam qualities relevant to x-ray and isotope type irradiators. Results and findings indicated that soft x-ray beams (160 kV at 0.62 mm Cu HVL and 320 kV at 1 mm Cu HVL) lead to substantially higher dose to BM within close proximity to mineral bone (within about 60 µm) as compared to hard x-ray beams (320 kV at 4 mm Cu HVL) and isotope based gamma irradiators (137Cs). The average dose increases to the BM in the vertebra for these four aforementioned radiation beam qualities were found to be 31%, 17%, 8%, and 1%, respectively. Both in-vitro and in-vivo experimental studies confirmed these simulation results, demonstrating that the 320 kV, 1 mm Cu HVL beam caused statistically significant increased killing to the BM cells at 6 Gy dose levels in comparison to both the 320 kV, 4 mm Cu HVL and the 662 keV, 137Cs beams.

Dissertation

The demand for wide bandgap semiconductors for radiation detector applications has significantly increased in recent years due to an ever-growing need for safeguard measures and medical imaging systems amongst other applications. The need for these devices to be portable and efficient, and to operate at room temperature is important for practical applications. For radiation detectors, the semiconductor materials are mainly required to have an optimal energy gap, high average atomic number, good electrical resistivity and charge transport properties as well as purity and homogeneity. Cadmium zinc telluride (CZT) distinctly stands out among the other choices of semiconductor materials for radiation detector applications, due to its attractive material properties and the room temperature operation possibility. A tremendous amount of research is being conducted to improve CZT technology and its implementation into more commercial systems. Applications of CZT detector technology in national security, high energy physics, nuclear spectroscopy, and medical imaging systems are of special interests. However, CZT devices still face challenges that need to be understood and overcome in order to have more efficient radiation detector systems. One such challenge lies in the understanding of the surfaces of CZT detectors and surface recombination effects on charge transport, charge collection efficiency, and detector performance. Another common issue is the degradation of CZT detectors due to the presence of defects which can act as traps for the charge carriers and cause incomplete charge collection from the detectors. Thus, a major challenge is that, the commercial CZT crystals have large concentrations of defects and impurities that need to be characterized, and their effects on the detector performance should be studied. Photoluminescence (PL) spectroscopy is a sensitive, non-contact and non-destructive method, suitable to characterize lower concentrations of point defects, such as substitutional impurities (donors, acceptors) and native defects in CZT crystals. A PL spectrum provides information regarding the defect nature of the crystal by determining the presence and the type of vacancies, interstitials, and impurities in the lattice. The main objective of this thesis is to address the presence of the defects in CZT crystals, identify their types, and study their roles in the performance of x-ray radiation detectors using PL spectroscopy. Additionally, using PL method and different excitation sources including UV excitation, this thesis studies the surface of CZT samples and investigates the PL signature of the surface oxide of the samples, in an effort to optimize the surface processing and thereby improve CZT detector performance.
Graduate

La tomodensitométrie par rayons-X (CT) est une modalité d’imagerie produisant une carte tridimensionnelle du coefficient d’atténuation des rayons-X d’un objet. En radiothérapie, le CT fournit de l’information anatomique et quantitative sur le patient afin de permettre la planification du traitement et le calcul de la dose de radiation à livrer. Le CT a plusieurs problèmes, notamment (1) une limitation au niveau de l’exactitude des paramètres physiques quantitatifs extraits du patient, et (2) une sensibilité aux biais causés par des artéfacts de durcissement du faisceau. Enfin, (3) dans le cas où le CT est fait en présence d’un agent de contraste pour améliorer la planification du traitement, il est nécessaire d’effectuer un deuxième CT sans agent de contraste à des fins de calcul de dose, ce qui augmente la dose au patient. Ces trois problèmes limitent l’efficacité du CT pour certaines modalités de traitement qui sont plus sensibles aux incertitudes comme la protonthérapie. Le CT spectral regroupe un ensemble de méthodes pour produire plusieurs cartes d’atténuation des rayons-X moyennées sur différentes plages énergétiques. L’information supplémentaire, pondérée en énergie qui est obtenue permet une meilleure caractérisation des matériaux analysés. Le potentiel de l’une de ces modalités spectrales, le CT bi-énergie (DECT), est déjà bien démontré en radiothérapie, alors qu’une approche en plein essor, le CT spectral à comptage de photons (SPCCT), promet davantage d’information spectrale à l’aide de détecteurs discriminateurs en énergie. Par contre, le SPCCT souffre d’un bruit plus important et d’un conditionnement réduit. Cette thèse investigue la question suivante : y a-t-il un bénéfice à utiliser plus d’information résolue en énergie, mais de qualité réduite pour la radiothérapie ? La question est étudiée dans le contexte des trois problèmes ci-haut. Tout d’abord, un estimateur maximum a posteriori (MAP) est introduit au niveau de la caractérisation des tissus post-reconstruction afin de débruiter les données du CT spectral. L’approche est validée expérimentalement sur un DECT. Le niveau de bruit du pouvoir d’arrêt des protons diminue en moyenne d’un facteur 3.2 à l’aide de l’estimateur MAP. Celui-ci permet également de conserver généralement le caractère quantitatif des paramètres physiques estimés, le pouvoir d’arrêt variant en moyenne de 0.9% par rapport à l’approche conventionnelle. Ensuite, l’estimateur MAP est adapté au contexte de l’imagerie avec agent de contraste. Les résultats numériques démontrent un bénéfice clair à utiliser le SPCCT pour l’imagerie virtuellement sans contraste par rapport au DECT, avec une réduction de l’erreur RMS sur le pouvoir d’arrêt des protons de 2.7 à 1.4%. Troisièmement, les outils développés ci-haut sont validés expérimentalement sur un micro-SPCCT de la compagnie MARS Bioimaging, dont le détecteur à comptage de photons est le Medipix 3, qui est utilisé pour le suivi de particules au CERN. De légers bénéfices au niveau de l’estimation des propriétés physiques à l’aide du SPCCT par rapport au DECT sont obtenus pour des matériaux substituts à des tissus humains. Finalement, une nouvelle paramétrisation du coefficient d’atténuation pour l’imagerie pré-reconstruction est proposée, dans le but ultime de corriger les artéfacts de durcissement du faisceau. La paramétrisation proposée élimine les biais au niveau de l’exactitude de la caractérisation des tissus humains par rapport aux paramétrisations existantes. Cependant, aucun avantage n’a été obtenu à l’aide du SPCCT par rapport au DECT, ce qui suggère qu’il est nécessaire d’incorporer l’estimation MAP dans l’imagerie pré-reconstruction via une approche de reconstruction itérative.
X-ray computed tomography (CT) is an imaging modality that produces a tridimensional map of the attenuation of X-rays by the scanned object. In radiation therapy, CT provides anatomical and quantitative information on the patient that is required for treatment planning. However, CT has some issues, notably (1) a limited accuracy in the estimation of quantitative physical parameters of the patient, and (2) a sensitivity to biases caused by beam hardening artifacts. Finally, (3) in the case where contrast-enhanced CT is performed to help treatment planning, a second scan with no contrast agent is required for dose calculation purposes, which increases the overall dose to the patient. Those 3 problems limit the efficiency of CT for some treatment modalities more sensitive to uncertainties, such as proton therapy. Spectral CT regroups a set of methods that allows the production of multiple X-ray attenuation maps evaluated over various energy windows. The additional energy-weighted information that is obtained allows better material characterization. The potential of one spectral CT modality, dual-energy CT (DECT), is already well demonstrated for radiation therapy, while an upcoming method, spectral photon counting CT (SPCCT), promises more spectral information with the help of energy discriminating detectors. Unfortunately, SPCCT suffers from increased noise and poor conditioning. This thesis thus investigates the following question: is there a benefit to using more, but lower quality energy-resolved information for radiotherapy? The question is studied in the context of the three problems discussed earlier. First, a maximum a posteriori (MAP) estimator is introduced for post-reconstruction tissue characterization for denoising purposes in spectral CT. The estimator is validated experimentally using a commercial DECT. The noise level on the proton stopping power is reduced, on average, by a factor of 3.2 with the MAP estimator. The estimator also generally con- serves the quantitative accuracy of estimated physical parameters. For instance, the stopping power varies on average by 0.9% with respect to the conventional approach. Then, the MAP estimation framework is adapted to the context of contrast-enhanced imaging. Numerical results show clear benefits when using SPCCT for virtual non-contrast imaging compared to DECT, with a reduction of the RMS error on the proton stopping power from 2.7 to 1.4%. Third, the developed tools are validated experimentally on a micro-SPCCT from MARS Bioimaging, which uses the Medipix 3 chip as a photon counting detector. Small benefits in the accuracy of physical parameters of tissue substitutes materials are obtained. Finally, a new parametrization of the attenuation coefficient for pre-reconstruction imaging is pro- posed, whose ultimate aim is to correct beam hardening artifacts. In a simulation study, the proposed parametrization eliminates all biases in the estimated physical parameters of human tissues, which is an improvement upon existing parametrizations. However, no ad- vantage has been obtained with SPCCT compared to DECT, which suggests the need to incorporate MAP estimation in the pre-reconstruction framework using an iterative reconstruction approach.

Tese de doutoramento em Engenharia Física, na especialidade de Instrumentação, apresentada à Faculdade de Ciências e Tecnologia da Universidade de Coimbra
Desde cedo que a comunidade científica compreendeu que gases nobres em liquido são excelentes meios de detecção de radiação, combinando a sua elevada densidade, elevado grau de homogeneidade e de elevado rendimento de cintilação. Para além destas características inerentes, estes têm a potencialidade de fornecer ambos sinais de ionização – criando electrões livres – e cintilação em resposta à interacção com radiação ionizante e, tendo em vista a sua aplicação em experiências de eventos raros relacionados com física de neutrinos ou matéria–escura, a capacidade de autoblindagem garante a exclusão de eventos induzidos por radiação de fundo. O facto de não absorverem a sua própria luz, emergente dos eventos de cintilação, permite a expansão deste tipo de detectores até grandes volumes, sendo que as colaborações mais recentes propõem detectores com dezenas de toneladas de xénon em estado liquido. As experiências actuais que usam gases nobres em estado líquido empregam xénon ou árgon numa só fase (estado líquido) ou em dupla-fase (estado líquido + gasoso) e as suas aplicações abrangem desde as já referidas experiências de procura de eventos raros, passando por imagiologia médica tais como detectores de radiação gama para PET ou câmaras Compton “3- γ” em combinação com PET, passando também por aplicações de segurança como sistemas de inspecção para detecção de material físsil e, finalmente, em câmaras Compton para aplicações de astrofísica. Em ambas as configurações a leitura dos sinais de cintilação é geralmente feita através de um grande número de dispendiosos fotomultiplicadores de vácuo agrupados. A presente tese de doutoramento é dedicada aos fotomultiplicadores gasosos de grande área para aplicações criogénicas desenvolvidos no contexto do programa doutoral, tendo em vista a sua eventual aplicação como um dispositivo complementar aos métodos existentes de detecção de cintilação, para aplicação em futuras experiências de grande escala. Esta pesquisa foi direccionada tendo em vista o desenvolvimento de eficientes fotomultiplicadores gasosos de grande área, potencialmente mais económicos por unidade de área, baseados em “Thick Gas-Electron Multipliers” (THGEMs). Combinando fotocátodos de alta eficiência com multiplicadores gasosos de electrões capazes de atingir elevado ganho em carga obteve-se assim um dispositivo com elevada sensibilidade para a detecção de fotões únicos, com a possibilidade de discriminação em posição com resolução espacial inferior a um milímetro e com resolução temporal da ordem de poucos nano segundos. Contrariamente ao que sucede com a tecnologia de vácuo actualmente, com este dispositivo a localização em posição de fotões em grandes áreas é feita num único dispositivo integrando electrónica habitualmente utilizada em experiências de rastreamento de partículas. Neste trabalho o fotomultiplicador gasoso desenvolvido consiste numa cadeia de THGEMs combinados com um fotocátodo de iodeto de césio (CsI) sensível ao ultravioleta enquanto que os testes criogénicos foram realizados na Time Projection Chamber (TPC) de dupla fase de xénon líquido recentemente desenvolvida no Weizmann Institute of Science (WILiX). Relativamente ao GPM desenvolvido foram medidos ganhos máximos em carga de ~8×10^5 em misturas de Ne/CH4(5%) e de ~3×10^5 em misturas de Ne/CH4(20%), a uma pressão de 0.7bar à temperatura de ~180K, para fotões únicos. Foi obtida uma probabilidade de descarga com uma mistura de Ne/CH4(5%) a um ganho em carga de 1×10^5 e com sinais de cintilação secundária S2, induzidos por partículas alfa, a uma taxa de 40Hz de cerca 10^-6. Foram medidos desvios no ganho em carga de cerca de 7–15% durante um período de dois meses, operando em modo selado com 0.7bar de Ne/CH4(20%) a uma temperatura de ~190K, não se registando mudanças significativas tanto para fotões únicos induzidos por uma lâmpada UV como para sinais de cintilação primária S1 induzidos por partículas alfa, indicando a estabilidade da mistura em modo selado assim como para a estabilidade da eficiência quântica do fotocátodo de iodeto de césio. Para além disso foi obtido um valor de 1.2 – 1.3ns para a resolução temporal do GPM em resposta a sinais de cintilação, induzindo cerca de 170–200 fotoelectrões iniciais no GPM. Para a leitura em posição de eventos o GPM foi equipado com um ânodo segmentado em “pads” individuais, cujo desenho foi precedido por extensas simulações em GEANT4 que forneceram dados importantes para a optimização da geometria dos ânodos e qual a resolução espacial esperada para diferentes configurações. Das simulações pôde-se determinar que a resolução espacial esperada do GPM será ~5mm para sinais de cintilação devido a 10 electrões numa TPC de xénon liquido de dupla fase. Nos testes realizados determinou-se que para ~1.3×10^5 fotoelectrões iniciais a posição calculada apresenta um desvio menor que um milímetro da posição real. Os estudos iniciados em Coimbra combinando multiplicadores gasosos de electrões com regiões de indução micrométricas (GEM–MIGAS) fomentaram o desenvolvimento de um análogo com THGEMs e regiões de indução sub–milimétricas. Estes estudos envolveram simulações electrostáticas de forma a compreender a relação entre os campos eléctricos no interior de THGEMs e da região de indução enquanto que o trabalho experimental demonstrou que com misturas de Ne-CH4, sendo mais eficientes em absorver a radiação UV das avalanches, permitem atingir ganhos mais elevados em condições estáveis. Foram também testadas misturas à base de hélio uma vez que podem potencialmente apresentar uma boa alternativa para misturas à base de néon, devido ao ganho em carga elevado, aplicando tensões de operação mais reduzidas, semelhante eficiência de extracção de fotoelectrões e custos mais reduzidos. Em modo de corrente, um detector com um estágio apresentou ganhos superiores a 10^5.
Liquid noble gases are known to be excellent detection media due their characteristics of high density, high homogeneity and high scintillation yield. They provide both ionization and scintillation signals when transversed by ionizing particles and efficient background suppression due to self-shielding. They are transparent to their own scintillation light and allow expansion to large detector masses. Current noble liquid detectors employ either liquid argon or liquid xenon, in single-phase (liquid only) or double-phase (liquid and gas) configurations and the present application ranges from detection of rare scattering events like Dark Matter search or neutrino physics, to medical imaging like in gamma detectors for PET and LXe Compton Telescope for “3-γ imaging” in combination with PET, gamma/neutron imaging detectors for radionuclide security inspections and Compton Cameras for applications in astrophysics. Both configurations rely on measuring the scintillation light emitted from the liquid-phase or from the liquid and gas-phases with costly large arrays of vacuum photomultipliers. This Ph.D. thesis is dedicated to the large-area cryogenic gas-avalanche photomultipliers (GPMs) prototypes developed within the Ph.D. program, envisioning their application as a complimentary scintillation detection method for current and future large scale experiments. The research and development efforts aimed for a potentially economic and efficient large-area GPM based on Thick Gas-Electron Multipliers – THGEMs – combining a high efficiency photocathode with a high-gain gas-avalanche electron multiplier, providing high single-photon sensitivity and the possibility for localization of the photons with sub-mm spatial resolution and few-ns temporal resolution. Unlike current vacuum devices, photon localization over large areas can be made in a single device, using integrated electronics developed for particle tracking. The GPM consists on a cascade of THGEMs combined with a cesium iodide UV-photocathode and the cryogenic tests were performed coupling it to a double-phase liquid xenon detector (a Time Projection Chamber; TPC), in the recently developed Weizmann Institute Liquid Xenon – WILiX – cryogenic system. Moreover, for the successful use of a cesium iodide photocathode, techniques for the production, characterization and transportation were developed and implemented which allowed systematically reproducing photocathodes whose quantum efficiency ranged from 24% to 30% for a wavelength of 175nm, corresponding to the liquid xenon scintillation light, and assembling them to the GPM successfully. Within the Ph.D. thesis it is shown that the maximum gain obtained at 0.7bar and 180K was ~8×105 for Ne/CH4(5%) and ~3×105 for Ne/CH4(20%), for single-photons. With Ne/CH4(5%) at a gain of 1×105 and alpha particle-induced S2 signals at a rate of 40Hz, the discharge probability was found to be of the order of 10-6. Over a period of two months, operating in sealed mode at 0.7bar of Ne/CH4(20%) at a temperature of ~190K, gain measurements were consistent within 7–15%, showing no significant change both for the UV–lamp induced signals and alpha-induced S1 signals, indicating that there were no significant changes in either the gas composition or the CsI quantum efficiency. In terms of the time resolution of the GPM – defined as the temporal spread, or jitter, of the GPM response pulse to a scintillation signal – it was found that for scintillation signals producing ~170 – 200 photoelectrons the resolution was on the nanosecond scale, approaching ~1.2 – 1.3ns at a gain of 3×105. For position sensitive capability the GPM was equipped with position sensitive anodes. Their design was preceded by extensive and CPU–time intensive GEANT4 simulations and analysis that provided valuable information on the expected spatial resolution for different conditions. For the selected pixel size and geometry one can expect a position resolution of ~5mm for scintillation signals due to 10electrons in a double–phase LXe TPC. The tests performed with a second GPM showed that for ~1.3×105 initial photoelectrons the calculated centers–of–gravity are in very good agreement (sub-millimeter) to the actual UV source positions. Furthermore, following the promising studies performed in Coimbra with Gas Electron Multipliers with a micro-induction gap amplifying structure – GEM–MIGAS – an analogous configuration consisting of a THGEM coupled to a submillimetric induction gap was investigated to eventually obtain a GPM configuration capable of reaching higher gains with lower biasing voltages. The investigation combined extensive simulation work showing an interdependence of hole/induction region electric fields while experimental results showed that Ne-CH4 mixtures, having a more effective UV quenching than Ne-CF4 mixtures, allowed achieving higher charge gains in stable operating conditions. Helium based mixtures were also tested in terms of charge–gain and photoelectron extraction efficiency, since they can present a good alternative to Ne-based mixtures for the potential higher gains, with lower applied voltages, similar photoelectron extraction efficiency and lower costs. A Single-THGEM detector was operated in He/CF4 and He/CH4 mixtures reaching effective charge-gains well above 105, measured in current mode, applying relatively low voltages, when compared to Ar mixtures.

碩士
國立清華大學
工程與系統科學系
93
Recently, the semiconductor devices processing has been rapidly developed. The detectors using semiconductor devices have been used for radiation monitors. The radiation detector for high dose range is very expensive and needs export from abroad. Consequently, the purposes of this thesis are to find the radiation detectors using the commercial semiconductor devices and study electric characteristics of those devices. This article focuses on applications of the high range radiation monitoring during accident of nuclear power plant. Therefore, electric characteristics of the semiconductor devices were studied through the experiments for achieving cheaper radiation detectors and monitors. This dissertation comprises three studies. It is successfully found in the first study that the Power Metal-Oxide-Semiconductor-Field-Effect-Transistor (MOSFET) devices can be used for the high range dosimeters. Due to the interface trap charges increased by radiation interaction, we can measure the change of pumping current in corresponding with interface charges induced by radiation exposure dose. We also find that the Power MOSFETs are suitable to be used in a high temperature and high radiation dose environment of the Nuclear Power Plant. The second study is radiation measurement of pulse mode operation. Normally, PIN photodiode is used to be a pulse mode radiation detector by means of charges generation in its P-I-N structure under radiation interaction. We find that the Power MOSFET device exists a parasitic n+/ p+/ n-/ n+ device from the source to the drain, which is similar to the P-I-N structure. To be used in high radiation dose environment, we find that the Power MOSFET devices have more advantage than the PIN photodiodes. The third study is radiation measurement of current mode operation. It is successfully found that the Solar Cell devices can be used for the high range radiation measurement. As the PN junction of the device is exposed by radiation, the electron-hole pairs are generated by energy deposited from gamma-rays and they are easy to escape from energy band. Thus, a large amount of electron-hole pairs generated by high radiation can produce a measurable current. The experimental result of current to exposure dose rate is 5~8E-11 A/(rad/hr) approximately. We also develop a current amplifier to build a radiation monitor. The radiation measurement calibration has been performed on the monitor, which was proved to have a very good linearity in logarithm scale. Hence, Solar Cell detector and monitor can be used for the high range radiation measurement during accident of nuclear power plant.