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<p>Active Galactic Nuclei (AGNs) are accreting supermassive black holes at the center of galaxies, known for rich spectral features and multi-time scale variability in their electromagnetic emission. The origin of the variability in AGN light curves can be either intrinsic, meaning related processes that take place inside the AGN system, or extrinsic, i.e., from the propagation of light towards Earth. In this dissertation, I present my work focusing on AGN variability. The first two works focus on the variability of blazars, a subclass of AGN with their relativistic jets beaming towards the observer. The first work combines 3D relativistic magnetohydrodynamics (RMHD) simulations with radiation transfer and shows the kink instability within the blazar jet can cause quasi-periodic radiation signatures within a typical period of time scales from weeks to months. The second work combines 2D Particle-in-Cell (PIC) simulations with radiation transfer and shows that isolated and merging plasmoids due to magnetic reconnection in a blazar environment could produce rich radiation and polarization signatures. The last work explores an extrinsic origin for AGN variability: a scenario in which interstellar medium (ISM) within our galaxy can refract light coming from AGNs. It suggests that plasma structures in ISM with an axisymmetric geometry can account for extreme scattering events (ESEs) in AGN observations. Future research directions include studies of the kink instability in jets that propagate in different environments and simulations of magnetic reconnection in 3D which may reveal additional particle acceleration mechanisms, which may play important role in the resulting radiation and polarization signatures. </p>

Astrophysical phenomena such as self-annihilation of primordial black holes (PBHs), gamma ray bursts (GRBs), and supernovae are expected to produce single dispersed pulses detectable in the low end of the radio spectrum. Analysis of these pulses could provide valuable information about the sources, and the surrounding and intervening medium. The Eight-meter-wavelength Transient Array (ETA) is a radio telescope dedicated to the search for these pulses in an 18 MHz bandwidth centered at 38 MHz. ETA consists of 10 dual-polarized active dipoles providing an all-sky field of view. This thesis describes the results of a search campaign using ETA, namely, a Crab giant pulse (CGP) search. CGPs are already known to exist, and thus provide an excellent diagnostic for system performance. We found 11 CGP candidates in 14 hours of data. Although there has not been a single compelling detection (signal-to-noise ratio > 6), our analysis shows that at least a few of these candidates may be CGPs. We also explain the analysis preparation for PBH and GRB searches. Additionally, we describe the instrument and a software "toolchain" developed for the analysis of data that includes calibration, radio frequency interference (RFI) mitigation, and incoherent dedispersion. A dispersed pulse simulation code was developed and used to test the toolchain. Finally, improvements are suggested. / Master of Science

<p dir="ltr">Biopharmaceutical manufacturing efforts are gradually shifting toward single-use bioreactors (SUB) because of their lower cost and modular scalability. One challenge that hinders further adoption of SUBs is the lack of sensors that can be integrated into sterile equipment, while being cost-effective for disposal with the SUB at the end of its operational life.</p><p dir="ltr">In this dissertation I discuss the development of γ-irradiated thin-film sensors that provide accurate pH monitoring in sterile media, with a focus on the reference electrode (RE). Thin-film Ag/AgCl REs exhibit excellent stability across a pH range of 4.5–9.0 and are inert to various organic species, for up to 60 days post irradiation. Notably, γ-irradiation of a thin-film pH sensor (pH electrode plus RE) produces a logarithmic voltage decay that can be readily corrected by a single-point calibration function. These sensors can accurately report pH changes in sterile cell culture media over several weeks.</p><p dir="ltr">The scalable production of thin-film pH sensors can be achieved by optimizing a semi-automated fabrication process for membrane deposition. Improvements in process control are fortified by a comprehensive uncertainty analysis to determine the relative contributions of error from manufacturing, measurement, and data analysis. Identifying key sources of uncertainty enables further optimization of the fabrication process as well as sensor performance.</p><p dir="ltr">This dissertation also includes a discussion on novel approaches for reducing the conditioning time of thin-film REs, a current impediment for their rapid deployment. Studies include the effects of autoclaving on membrane permeability and tuning the porosity of prefabricated membranes, and their impacts on ion flux across a high-impedance barrier. Changes in permeability produces a trade-off between conditioning speed and potentiometric inertness of the RE, but also reveals insights for future efforts to improve electrode performance.</p>

This thesis covers the individual work of Michael Chin as part of the sponsored research project funded by the U.S. State Department in support of a computational design of a "Mobile Pit Verification System" (MPVS), a mobile “drive by” passive radiation detection system to be applied in special nuclear materials (SNM) storage facilities for validation and compliance purposes. The MPVS system is intended to enable a comprehensive, rapid verification and validation of stored nuclear weapon core physics packages containing SNM, or so-called “weapon pits,” in weapon materials and stockpile storage facilities. The MPVS platform is designed to move at a constant speed and accumulate a signal for each stored weapon pit container. The gamma detector was selected to be a 4 × 4 × 8 cubic inch CsI detector while the neutron detector array designed for the “Transport Simulation and Validation of a Synthetic Aperture SNM Detection System (“T-SADS”) project was used in conjunction with this work; T-SADS was a 3 year project funded by DOE-NNSA which was completed on May 2013. The computational design effort for this project was completed in April 2013, and leveraged novel computational radiation transport methods, algorithms, and SNM identification methods, including a synthetic aperture collection approach, and a new gamma ratio methodology for distinguishing between naturally occurring radiation materials and weapon class SNM materials. Both forward and adjoint transport methods were utilized to characterize the adjoint reaction rate as a function of inter-source spacing, collimation thickness, linear and angular field of view, source age, source type, source geometry, and mobile platform speed. The integrated count was then compared with background radiation and the associated probabilities of detection and false alarm were then computed. Publications resulting from this research were published in PHYSOR 2012, presented at the 53rd annual Proceedings of the INMM, and at the Mathematics & Computation 2013 Conference.

Thesis (PhD)--University of Stellenbosch, 2004. / ENGLISH ABSTRACT: Foreland basins commonly fill with sediment derived from the adjacent fold/thrust belt, providing a relatively simple source-to-basin configuration. However, that is not true for the early southwestern Karoo Basin, since the composition of the Ecca Group sedimentary rocks do not match the composition of the adjacent fold/thrust belt. The southwestern Karoo Basin is bordered to the west and south by the Cape Fold Belt (CFB) and provides the opportunity to study the linkage between its early structural evolution and deposition in the two spatially and temporally distinct Tanqua and Laingsburg depocentres. The CFB was formed when the early Palaeozoic passive continental margin, which formed a large section of the southern edge of Gondwana, evolved into an active convergent margin during the late Palaeozoic. Orogenesis resulted in a northwest-trending Cedarberg branch and an eastwest-trending Swartberg branch. The oroclinal bend between the two branches includes large-scale northeast-trending syntaxis structures, such as the Hex River and Baviaanshoek anticlinoria, which influenced the sedimentation path into the basin. Spectral gamma ray (SGR), mineralogical and geochemical studies of exposed rocks from the Tanqua and Laingsburg depocentres indicate a near uniform provenance for both, dominated by granitic and metamorphic material derived from a provenance seemingly far beyond the CFB. SGR data, combined with lithology, show that regional stratigraphic correlation is possible in the Skoorsteenberg, Kookfontein and Waterford Formations in the Tanqua depocentre. The same is true for the Laingsburg and Fort Brown Formations in the Laingsburg depocentre. There are no major changes in the SGR data set between the successive sandstone or shale units that could imply different origin, and no distinct signals in the SGR pattern of the shale intervals that could potentially correspond to maximum flooding surfaces. The Tanqua and Laingsburg depocentre sandstones are very fine- to lower mediumgrained, tightly packed, poorly to well sorted, and have undergone mechanical compaction and pressure solution. The mineralogical composition and texture of these sandstones suggest that they have undergone high-grade diagenesis to low-grade regional burial metamorphism to the lower greenschist facies (250 ± 50ºC; ~2 kbars). They are mineralogically and geochemically classified as lithic arenites and greywackes, and the Tanqua depocentre sandstones are slightly more mature than the Laingsburg depocentresandstones. REE patterns for the Tanqua and Laingsburg depocentre sandstones are similar, suggesting that both form part of the same evolutionary pattern and that the sediments have one common origin, i.e. a provenance predominantly composed of granitic material. Homogenous εNd-values for all sandstone samples of around –5 at the time of deposition indicate that there is little or no variation in provenance between the Tanqua and Laingsburg depocentre sediments. TCHUR model ages of 0.70 to 0.95 Ga, and TDM model ages of 1.19 to 1.49 Ga, resulted from a mixture of Archaean and Proterozoic material in unknown proportions. The most likely source terrane is thought to be the North Patagonian Massif. The latter show Nd isotopic compositions corresponding to an average εNd-value of -5 at 265 Ma. / AFRIKAANSE OPSOMMING: Voorlandkomme word oor die algemeen gevul met sediment afkomstig van die aanliggende plooigordel, wat lei tot ‘n redelik eenvoudige brongebied-tot-afsettingskom konfigurasie. Dit is egter nie van toepassing vir die vroeë suidwestelike Karookom nie, aangesien die samestelling van die Ecca Groep sedimentêre gesteentes nie ooreenstem met die samestelling van die aanliggende plooigordel nie. Die suidwestelike Karookom word aan die weste en suide begrens deur die Kaapse Plooigordel en bied die geleentheid om die verwantskap tussen die vroeë strukturele evolusie en afsetting in die twee ruimtelik en temporeel afsonderlike Tankwa en Laingsburg subkomme te bestudeer. Die Kaapse Plooigordel het gevorm toe die vroeë Palaeosoïkum kontinentale grens, wat ‘n groot deel van die suidelike grens van Gondwana was, ontwikkel het tot ‘n aktiewe konvergerende grens gedurende die laat Palaeosoïkum. Orogenese het gelei tot die vorming van ‘n noordwes-strekkende Sederberg tak en ‘n ooswes-strekkende Swartberg tak. Die oroklinale buig tussen die twee takke sluit grootskaalse noordoosstrekkende sintaksis strukture in, soos die Hex Rivier en Baviaanshoek antiklinoria, wat die sedimentasie rigtings na die kom beïnvloed het. Spektrale gammastraal (SGR), mineralogiese en geochemiese studies op die dagsome van die Tankwa en Laingsburg subkomme dui ‘n byna identiese brongebied aan vir beide, oorheers deur granitiese en metamorfe materiaal vanaf ‘n brongebied oënskynlik vêr vanaf die Kaapse Plooigordel. SGR data, gekombineer met litologie, dui aan dat dit moontlik is om regionale stratigrafiese korrelasies in the Skoorsteenberg, Kookfontein en Waterford Formasies in die Tankwa subkom te maak. Dieselfde geld vir die Laingsburg en Fort Brown Formasies in die Laingsburg subkom. Daar is geen groot veranderinge, wat ‘n verskil in oorsprong kan aandui, in the SGR datastel tussen die opeenvolgende sandsteen of skalie eenhede nie, en ook geenuitstaande tekens in the SGR patroon van die skalie-intervalle wat moontlik kan ooreenstem met ‘n maksimum vloedingsvlak nie. Die Tankwa en Laingsburg subkom sandsteenlae is baie fyn- tot laervlak mediumkorrelrig, dig gekompakteer, swak tot goed gesorteer, en het meganiese kompaksie en drukoplossing ondergaan. Die mineralogiese samestelling en tekstuur van hierdie sandsteenlae dui daarop dat hulle hoë-graadse diagenese tot lae-graadse regionale begrawingsmetamorfose tot laervlak groenskis fasies (250 ± 50ºC; ~2 kbars) ondergaan het. Hulle word mineralogies en geochemies geklassifiseer as litiese areniete en grouwakke. Die Tankwa subkom sandsteenlae is effens meer volwasse as die Laingsburg subkom sandsteenlae. Die lantanietgroep patroon vir die Tanqua en Laingsburg sandsteenlae is eenders, wat aandui dat beide deel gevorm het van dieselfde evolusionêre ontwikkeling en dat die sedimente een gesamentlike oorsprong gehad het, naamlik ‘n brongebied bestaande hoofsaaklik uit granitiese materiaal. Homogene εNd-waardes van ongeveer –5 by tye van afsetting vir al die sandsteen monsters dui daarop dat daar min of geen verandering in brongebied vir die Tankwa en Laingsburg subkom sedimente was nie. TCHUR model ouderdomme van 0.70 tot 0.95 Ga, en TDM model ouderdomme van 1.19 tot 1.49 Ga, is afkomstig van ‘n mengsel van Argeïese en Proterosoïese materiaal in onbekende hoeveelhede. Die mees waarskynlike brongebied is die Noord Patagoniese Gebergtes. Dit wys Nd isotopiese samestellings wat ooreenstem met ‘n gemiddelde εNd-waarde van –5 by 265 Ma.

Ion beam therapy is an innovative radiotherapy technique using mainly carbon ion and proton irradiations. Its aim is to improve the current treatment modalities. Because of the sharpness of the dose distributions, a control of the dose if possible in real time is highly desirable. A possibility is to detect the prompt gamma rays emitted subsequently to the nuclear fragmentations occurring during the treatment of the patient. In a first time two different Compton cameras (double and single scattering) have been optimised by means of Monte Carlo simulations. The response of the camera to a photon point source with a realistic energy spectrum was studied. Then, the response of the camera to the irradiation of a water phantom by a proton beam was simulated. It was first compared with measurement performed with small-size detectors. Then, using the previous measurements, we evaluated the counting rates expected in clinical conditions. In the current set-up of the camera, these counting rates are pretty high. Pile up and random coincidences will be problematic. Finally we demonstrate that the detection system is capable to detect a longitudinal shift in the Bragg peak of +or- 5 mm, even with the current reconstruction algorithm.

Le Large Area Telescope (LAT) à bord du satellite Fermi, lancé le 11 Juin 2008, est sensible au rayonnement gamma de20 MeV à plus de 300 GeV. 148 pulsars gamma à ce jour ont été détectés avec le Fermi-LAT, dont 117 sont détaillés dansle Deuxième Catalogue de Pulsars gamma de Fermi (2PC). Les pulsars forment la plus grande classe de sources au GeVdans la Voie Lactée. Les études de pulsars gamma apportent des contraintes importantes sur les modèles d’émission gamma etpermettent des percées dans notre compréhension des pulsars.Cette thèse présente des détections de sept pulsars gamma faibles déjà connus en radio, dont quatre jeunes ou d’âgesmoyens et trois millisecondes (MSP). L’analyse spectrale et la caractérisation des courbes de lumiére sont décriteset comparées à la population de 2PC. Ces détections ont l’intérêt particulier de sonder et d’étendre l’espace desparamètres de la population actuelle. Elles illustrent la distorsion inhérente dans 2PC et marquent la transition de lapremière phase (pulsars brillants) à la seconde phase (pulsars faibles) d’opération de Fermi. Le développement desmodèles théoriques et un recensement complet de la population galactique des étoiles à neutrons bénéficieront de plusde détections de pulsars faibles dans les années à venir.Certains pulsars gamma dans 2PC manifestant de grande efficacité gamma sont étudiés dans cette thèse. L’examen desdifférents paramètres qui influencent l’efficacité conduit à quelques candidats pour lesquels un plus grand momentd’inertie est nécessaire afin de résoudre le paradoxe de grande efficacité. Les observations de Fermi en rayons gamma ontl’air à apporter des contraintes indépendantes sur le moment d’inertie et les équations d’état des étoiles à neutrons. / The Large Area Telescope (LAT) on board the Fermi satellite, launched on June 11 2008, is sensitive to g raysfrom 20 MeV to over 300 GeV. gamma-ray pulsations from 148 pulsars to date have been detected with the Fermi-LAT, 117are detailed in the Second Fermi-LAT Gamma-ray Pulsar Catalog (2PC). Pulsars are by far the largest GeV sourceclassin the Milky Way. Studies of the increasing gamma-ray pulsar sample bring important constraints on theoretical gamma-rayemission models and enable breakthroughs in our understanding of pulsars.This thesis presents detections of seven faint gamma-ray pulsars that are already known at radio wavelengths, includingfour young or middle aged pulsars and three millisecond pulsars (MSPs). Spectral analysis and light curve characterizationsare detailed and compared to the 2PC sample. These detections have the special interest of allowing us toprobe and extend the parameter space of the current population. They illustrate the inherent bias in 2PC and mark thetransition from the early part (bright pulsars) to the later part (faint pulsars) of the Fermi mission. Theoretical modeldevelopments and a complete census of Galactic neutron star populations will benefit from more detections of weakpulsars in the years to come.Some high gamma-ray efficiency pulsars in 2PC are studied in this thesis. Investigations on different parameters whichinfluence the efficiency result in a few candidates for which a large moment of inertia is necessary to cure the highefficiency paradox. Fermi gamma-ray observations seem to bring independent constraints on the moment of inertia andequations of state of neutron stars

Le satellite Fermi a été lancé le 11 juin 2008, avec à son bord le Large Area Telescope (LAT). Le LAT est un télescope sensible au rayonnement ? de 20 MeV à plus de 300 GeV. Avant la mise en service du LAT, six pulsars jeunes et énergétiques étaient connus dans le domaine ? . Le nombre de détections de pulsars par le LAT prédit avant lancement était de plusieurs dizaines au moins. Le LAT permettait également l’étude des pulsars milliseconde (MSPs), jamais détectés avec certitude à très haute énergie jusqu’alors. Cette thèse aborde dans un premier temps la campagne de chronométrie des pulsars émetteurs radio et/ou X, candidats à la détection par le LAT, en collaboration avec les grands radiotélescopes et télescopes X. Cette campagne a permis la recherche de signaux ? pulsés avec une grande sensibilité. En outre, la plupart des MSPs galactiques ont été suivis dans le cadre de cette campagne, sans biais de sélection a priori sur cette population d’étoiles. Pour la première fois, des pulsations ont été détectées pour huit MSPs galactiques au-dessus de 100 MeV. Quelques bons candidats à une détection prochaine apparaissent. Une recherche similaire a été conduite pour des MSPs d’amas globulaires, sans succès à présent. L’analyse des courbes de lumière et des propriétés spectrales des huit MSPs détectés révèle que leur rayonnement ? est relativement similaire à celui des pulsars ordinaires, et est vraisemblablement produit dans la magnétosphère externe. Cette découverte suggère que certaines sources non-identi?ées sont des MSPs, pour l’instant inconnus. / The Fermi observatory was launched on June 11, 2008. It hosts the Large Area Telescope (LAT), sensitive to ? -ray photons from 20 MeV to over 300 GeV. Before the LAT began its activity, six young and energetic pulsars were known in ? rays. At least several tens of pulsar detections by the LAT were predicted before launch. The LAT also allowed the study of millisecond pulsars (MSPs), never ?rmly detected in ? rays before Fermi. This thesis ?rst presents the pulsar timing campaign for the LAT, in collaboration with large radiotelescopes and X-ray telescopes, allowing for high sensitivity pulsed searches. Furthermore, it lead to quasi-homogeneous coverage of the galactic MSPs, so that the search for pulsations in LAT data for this population of stars was not affected by an a priori bias. We present a search for pulsations from these objects in LAT data. For the ?rst time, eight galactic MSPs have been detected as sources of pulsed ? -ray emission over 100 MeV. In addition, a couple of good candidates for future detection are seen. A similar search for globular cluster MSPs has not succeeded so far. Comparison of the phase-aligned ? -ray and radio light curves, as well as the spectral shapes, leads to the conclusion that their ? -ray emission is similar to that of normal pulsars, and is probably produced in the outer-magnetosphere. This discovery suggests that many unresolved ?-ray sources are unknown MSPs.

Le système EURITRACK, basé sur la technique de la particule associée, vise à détecter des explosifs et des drogues dans les conteneurs maritimes avec des neutrons de 14 MeV produits par la réaction D(T,)n. La particule alpha et le neutron sont émis environ à 180° l'un de l'autre. Les réactions induites par le neutron produisent des rayonnements gamma qui sont détectés en coïncidence avec la particule alpha pour déterminer la direction et le temps de vol neutronique, et ainsi remonter à l'origine des rayonnements gamma dans le conteneur. La composition chimique est obtenue par déconvolution du spectre gamma en signatures élémentaires (C, O, N, Fe,…). Les rapports des nombres de coups du carbone, de l'oxygène et de l'azote sont convertis en proportions chimiques, afin de distinguer les matières organiques bénignes et illicites, via des facteurs calculés par simulation Monte Carlo et validés expérimentalement. Ils prennent en compte la modération neutronique et l'atténuation photonique dans les marchandises transportées. L'application à la caractérisation élémentaire des déchets radioactifs est aussi étudiée par simulation, avec des écrans et collimateurs pour limiter le bruit dû à l'émission radiologique des colis. / The EURITRACK inspection system, based on the associated particle technique, aims at detecting explosives and narcotics in cargo containers with 14 MeV neutrons produced by the D(T,)n reaction. Alpha particle and neutron are emitted almost back to back. Reactions induced by fast neutrons produce gamma rays which are detected in coincidence with the alpha particle to determine the neutron direction. Neutron time-of-flight allows to determine gamma-ray origin inside the container. Information concerning material composition is obtained by unfolding the gamma spectrum into elemental signatures using a database of elemental spectra (C, O, N, Fe…). Carbon, oxygen, and nitrogen count ratios are converted into chemical proportions to distinguish illicit and benign organic materials. Conversion factors based on Monte Carlo simulations have been calculated and validated experimentally, taking into account neutron slowing down and photon attenuation in cargo materials. Application to the elemental characterisation of radioactive wastes is also studied by numerical simulation, with shields and collimators to limit the background due to waste radiations.