Doctor of Philosophy / Department of Physics / Daniel Rolles / Imaging the structures of molecules, understanding the molecular dynamics in onization and dissociation processes and, most importantly, observing chemical reactions, i.e. the making and breaking of chemical bonds in real time, have become some of the most exciting topics in the atomic and molecular physics. The rapid advances of experimental tools such as synchrotron radiation light sources, free-electron lasers and continuing advances of tabletop femtosecond ultrashort lasers that provide laser pulses at a variety of wavelengths have opened new avenues for understanding the structure of matter and the dynamics of the chemical interactions. In addition, significant improvements in computational techniques and molecular dynamic simulations have provided complementary theoretical predictions on structures and chemical dynamics. The Coulomb explosion imaging method, which has been developed and applied in many studies in the last three decades, is a powerful way to study molecular structures. The method has mostly been applied to small diatomic molecules and to simple polyatomic molecules. In this thesis, Coulomb explosion imaging is applied to study the structure of isomers, molecules that have the same chemical formula but different chemical structures. Specifically, by taking inner-shell photoionization as well as strong-field ionization approaches to ionize and fragment the molecules and by using coincidence electron-ion-ion momentum imaging techniques to obtain the three-dimensional momentum of fragment ions, structures of isomers are distinguished by using the correlations among product ion momentum vectors. At first, the study aims to understand if the Coulomb explosion imaging of geometrical isomers can identify and separate cis and trans structures. Secondly, in order to extend the application of the Coulomb explosion imaging method to larger organic molecules to test the feasibility of the method for identifying structural isomers, photoionization studiesof 2,6- and 3,5-difluoroiodobenzene have been conducted. In addition, using the full three-dimensional kinematic information of multi-fold coincidence channels, breakup dynamics of both cis/trans geometric isomers and structural isomers, and in particular, sequential fragmentation dynamics of the difluoroiodobenzene isomers are studied. Furthermore, for each study, Coulomb explosion model simulations are conducted to complement the experimental results. The results of the Coulomb explosion imaging reseach in this thesis paves the way for future time-resolved Coulomb explosion imaging experiments aiming to understand the transient molecular dynamics such as photoinduced ring opening reactions and cis/trans isomerization processes in gas-phase molecules.

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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.

When a nuclear explosion occurs certain radionuclides are emitted, notably xenon. Due to the fact that xenon is a noble gas, it is hard to contain and can therefore be detected far from the explosion site. There are four isotopes of xenon that are of interest in the detection of a nuclear explosion: 131mXe, 133mXe, 133Xe and 135Xe. By constantly measuring the amount of these isotopes in the air, changes in the concentration in an indication that a nuclear explosion has occurred. In this thesis a detector was modelled in GEANT4 and focuses on one kind of noble gas detector: SAUNA - the Swedish Automatic Unit for Noble gas Acquisition. SAUNA uses the coincidence technique in order to determine the concentration of xenon there is in the air. By using the coincidence technique, it is possible to reduce the impact of the background radiation and therefore increase the efficiency of the detector. 133Xe has a coincidence when it first undergoes beta decay, with an endpoint energy of 346 keV, and then emits a 80 keV gamma particle. 135Xe has also a dual coincidence, a beta decay with an endpoint energy of 910 keV together with a 250 keV gamma-ray. However both these isotopes have a triple coincidence decay that also can be exploited: for 133Xe, a beta particle with endpoint energy of 346 keV, a 30 keV X-ray and a 45 keV conversion electron, while for 135Xe there is instead of the gamma particle a 30 keV X-ray and a 214keV conversion electron that can be emitted together with the beta particle. The 30 keV X-ray together with the beta particle for 133Xe can also be used as a dual coincidence, in that case the conversion electron is ignored. For 133Xe, when a beta particle, a 45 keV conversion electron, and a 30 keV X-ray are emitted, the model was able to detect all three particles in 69.2% ± 0.1 of the cases. However, when only the particles with a detected energy within a 5 keV interval of their generated energies are considered to be in coincidence, then for 133Xe triple coincidence occurs in 22.9% ± 0.2 of the cases. For 135Xe the model was able to detect the triple coincidence (between a beta, 214 keV CE and 30 keV X-ray) in 63.5% ± 0.1 of the cases. This work shows that adding another particle in a coincidence reduces the chance to detect the coincidence. The positive effect of adding another particle in a coincidence is that the minimum detectable concentration of xenon should be smaller. The goal for future detectors should be to make it possible for the detector to take advantage of the triple coincidences but at the same time be also able to use the dual coincidences.

Barbora Vřeťonko PHOTOGRAPHY AS A REPRESENTATION Photography, although it is technology, is also an idea that implies a specific interpretation of a 'representation'. If there is photography, the world becomes photogenic and therefore it is possible to possess the world in the way of photography. That also requires a network of photographic terms. Reality is ineliminable in photography, we are moving between an image as representation and an image as the "reality." According to Maurice Merleau-Ponty the image is reversing invisible into visible. Photography, in contrary, is reversing visible into visible and so it is a special kind of zero degree image. This thesis will discuss the problems of the existence of photography as a created visual medium. Later it will focus on the relationship between photography and reality, photography and the possibility or impossibility of representation and then, in particular, the relationship between photography and a viewer. The research will take place in a discourse of the contemporary philosophical thinking as it is presented by Miroslav Petricek's book Myšlení obrazem (Thinking by the Images). The main thesis we will develop in this context is a definition of photography as the futile desire to see something as itself and now.

In dem Bericht werden Strahlungsquelle aufgeführt, die den Untergrund bei der Messung geringer Radioaktivitäten verursachen und dargestellt wie zur Unterdrückung der kosmischen Strahlung Laboratorien untertägig eingerichtet werden und wie durch Koinzidenz und Antikoinzidenzverfahren der verbleibende Untergrund weiter reduziert werden kann. / The radiation sources are described, which causes the background at the measurement of low radioactivities. To reduce the influence of cosmic rays counting devises are installed in underground laboratories. The remaining background may be further reduced by coincidence and anti-coincidence methods.

Dans le cadre du Traité d'Interdiction Complète des Essais Nucléaires (TICE), le Système de Surveillance International (SSI) a été développé. Ce réseau consiste en plusieurs centaines de stations de mesures dont le rôle est de détecter la signature d'un essai nucléaire clandestin. Quatre types de mesures sont effectués: sismique, hydro-acoustique, infrason et radionucléide. Dans le cas particulier d'un essai nucléaire souterrain, seule la mesure des radionucléides est adéquate pour mettre en évidence le caractère nucléaire de l'explosion. Quatre radionucléides du xénon sont produits en masse lors d'un essai nucléaire: le 131m-Xe, le 133m-Xe, le 133-Xe, et le 135-Xe. Une faible proportion de ces gaz parvient à diffuser à travers les fissures de la cavité et se retrouver dans l'atmosphère. A cause de la dilution atmosphérique, les concentrations de ces quatre radionucléides sont extrêmement faibles et par conséquent difficiles à détecter. De plus, le bruit de fond atmosphérique ne cesse d'augmenter car ces gaz radioactifs sont également produits et relâchés par les usines de production de radio-isotopes médicaux et les centrales nucléaires. Afin de renforcer le réseau de surveillance, le CEA-DAM a proposé de développer un système de détection des radionucléides du xénon ultra-compact et mobile, afin de pouvoir effectuer des mesures de vérification directement sur site suspect, et ainsi contourner la contrainte de dilution atmosphérique. Ces travaux présentent le développement et l'optimisation de ce système. Le système a d'abord été modélisé sur ordinateur et ses performances en terme d'efficacité de détection ont été simulées par méthode de Monte-Carlo avec le logiciel Geant4. Les mesures se font en utilisant la technique de mesure en coïncidences électron/photon. En parallèle, une chaîne d'acquisition ultra-compacte a été développée, ainsi qu'un réseau de communication permettant de synchroniser en temps les différents spectromètres numériques utilisés pour le traitement du signal (protocole IEEE 1588 PTP). Les concentrations minimales détectables de ce système sont inférieures à 5 mBq/m³ pour une acquisition de 12 h, et ce pour les quatre radionucléides du xénon d'intérêt. / As part of the Comprehensive nuclear Test Ban Treaty (CTBT), an International Monitoring System (IMS) has been developed. This network consists of several hundred measuring stations whose role is to detect the signatures of a clandestine nuclear test. Four types of measurements are carried out: seismic, hydroacoustic, infrasound and radionuclide. In the particular case of an underground nuclear test, only the radionuclide measurement is adequate to detect the explosion and quantify its yield. Four xenon radionuclides with long half-lives are produced during a nuclear test: the 131m-Xe, the 133m-Xe, the 133-Xe and the 135-Xe. A small proportion of these gases may diffuse through the cracks in the cavity and end up in the atmosphere. Due to atmospheric dilution, the concentrations of these four radionuclides are extremely low and therefore difficult to detect. In addition, atmospheric background continues to increase as these radioactive gases are also produced by medical radioisotope facilities and nuclear power plants. In order to strengthen the IMS, the CEA-DAM has proposed to develop an ultra-compact and mobile system for detecting xenon radionuclides, in order to be able to carry out verification measurements directly on site, and thus to avoid the problem of atmospheric dilution. This work presents the development and optimization of this system. The system was first modelled on a computer and its performances in terms of detection efficiency were simulated by the Monte-Carlo method using Geant4 software. Measurements are made in electron-photon coincidence mode. In parallel, an ultra-compact acquisition chain has been developed, as well as a communication network to synchronize in time the various digital spectrometers used for signal processing (IEEE 1588 PTP protocol). The minimum detectable concentrations of this system are less than 5 mBq/m³ for a 12 h acquisition, for the four xenon radionuclides of interest.

This work documents the author's interest in drawing and is based on the mental background of his artistic creation. The work is guided by the interest in the physical conditions of the drawing itself (materiality of the drawing), but also in the study of drawing as a fluid material of artistic creation, entering into the relationship with the image (drawing as a material). At the same time it deals with the process of creation itself and the phenomenon of repetition. The primary goal of the whole work is not the deliberate laying of pre-formulated questions and the subsequent search for their answers, the presentation of some finished truths, or the mere illustration of a certain issue, but rather the process of finding questions that emerge during the process of creation itself.

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