Wuppertal, Universiẗat, Diss., 2001.

An apparatus for the study of positron annihilation containing some novel new features has been constructed and has been used to study disordered copper alloys in order to investigate the value of this technique in determining the electron momentum distribution. The concentrated alloys Cu₉₀Al₁₀ Cu₈₅Zn₁₅, Cu₇₀Zn₁₅Ni₁₅ and Cu₆₀Ni₄₀ have been studied and effects attributed to the details of the Fermi surface of these alloys observed. In particular, the [lll} necks in the Fermi surfaces of these alloys have been observed and the widths of these necks measured. The cutoffs corresponding to the Fermi radius kf of these alloys have been measured. An anisotropy in the annihilation of positrons with electrons in the filled Brillouin zones has been measured and an unusual effect attributed to the d-band electrons of these alloys has been observed. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Le sujet des travaux de recherche effectués au cours de mon doctorat consiste à contourner la difficulté liée au seuillage utilisé pour visualiser les images obtenues en tomographie par émission de positrons (TEP), de manière à obtenir des images tumorales plus précises qui par la suite pourraient être utilisées en radiothérapie. Pour cela, nous avons proposé de mettre à profit la simulation Monte Carlo que nous avions développée pour décrire le transport d’électrons et de particules lourdes chargées dans l’eau. Lors de cette étude, nous avons donc cherché à décrire dans les détails les plus fins (à l’échelle de la cellule) les dépôts d’énergie induits par le positron dans la matière biologique, et ce à partir de la simulation Monte Carlo de suivi de trace que nous avons déjà mise au point pour les électrons et les ions et qui a été modifiée pour décrire le suivi des positrons, notamment par la prise en compte du processus de capture qui vient d’être récemment calculé par notre équipe. L’intérêt de ce travail est multiple. D’une part, il permet de mesurer les risques encourus par le patient soumis à une irradiation +, ce qui est le cas lors d’examens médicaux comme la Tomographie par Emission de Positrons. D’autre part, la simulation numérique de transport de positrons dans la matière biologique, couplée à une simulation de suivi de photons dans l’eau, a permis d’appréhender avec précision la détection des photons après annihilation des positroniums (paires électron-positron formées lors du ralentissement du positron dans la matière). Une modélisation simple des paramètres géométriques et physiques de l’appareil de détection nous a ainsi permis de quantifier le nombre d’événements de chaque type détectés sur l’anneau à savoir le nombre de paires de photons dits diffusés, fortuits et vrais. A partir de ces données, il nous est alors possible de construire un sinogramme nous permettant la reconstruction d’images TEP. / The subject of my PhD work is to outline the problem linked to threshold used to visualize PET images, in order to obtain more accurate tumoral volumes which could be treated more efficiently in radiotherapy. To do that, we have proposed to use the Monte Carlo code we have already developed to study the following-up of electrons and ions in water. During this PhD study, we have also tried to describe with the best accuracy (at a cellular scale) the deposited energy induced by positrons in the biological matter thanks to the Monte Carlo simulation previously cited that we have adapted to describe the following-up of positrons, especially in adding the capture process which has recently been calculated by our team. In one hand, the goal of this work is to measure the risks taken by the patient during a medical exam (such as a PET acquisition). In another hand, the simulation of the following-up of positrons in the biological matter coupled to the simulation of the following-up of photons in water, has allowed the detection of photons after the Positronium (electron-positron pair formed during the positron slowing-down in the matter) annihilation. Then a simple modelling of the geometrical and physical parameters of the medical device has allowed a quantification of the number of events detected on the ring. More precisely, we have access to the number of true, scattered and random pairs. Thanks to these data, we are thus able to reconstruct a sinogram as well as the initial radiative volume. Then a comparison with some experimental results gives access to a semi-empirical relation between the volume and the threshlod.

Note:

No description available.

Thesis (Ph. D.)--University of Texas at Austin, 2000. / Vita. Includes bibliographical references (leaves 163-171). Available also in a digital version from Dissertation Abstracts.

No description available.

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

This thesis reports on experimental research on structural defects and magnetic species on the nanoscale. The latter project involved considerable development work on the production of a spin-polarised mono-energetic positron beam. The construction of the system is described through various trial steps with emphasis on the methods of maximum practical polarization of the positron beam and of electrons in the sample with the smallest possible loss of beam intensity. A new sodium-22 source capsule was purchased, having beryllium foil backing to minimise the depolarisation effects of backscattering, and the source-moderator spacing was increased. Different types of sample were tested, varying in atomic structure, purity, magnetic susceptibility, electronic structure, and electric conductivity - including iron of different purity and structure, mu metal and solid oxygen. After these tests measurements were taken on single crystal iron, and the results suggest that the positron response to magnetic structures is very small, and that prospects for depth profiling of dilute magnetic systems are not favourable at this time. A large number of other investigations have been performed on non-magnetic defect structures in various materials. Variable Energy Positron Annihilation Spectroscopy – here involving beam-based Doppler broadening – was applied to novel materials of relevance to photonic or electronic structures on the nanoscale. These included thin films of technological interest such as AlGaN and Ar plasma-treated TiO2: silicon and silicon-on-insulator samples implanted with He and Si ions to engineer vacancies: Si-rich SiO2 and SiN to form nanocrystals for photonic applications in which new findings on the evolution of the nanocrystals, and the role of the nanocrystal-oxide interface in optical emission, could be very useful in the technological development of such systems: and a study of the structural phase and nano-pore properties of water ice films grown from vapour on a cold copper surface. The variety of these experimental studies serves to underline the wide applicability of positron beam spectroscopy in research on defect and nanostructure structures. A list of papers published to date resulting from this work is given at the end of the thesis; a number of others are planned.

Positron studies of defects in thin films and semiconductors are reviewed. The results obtained from experimental studies of Doppler broadening of annihilation radiation (DBAR) from variable energies are presented. Normalisation methods for the DBAR parameters S and W have been developed, allowing for direct comparisons of the results for different samples taken over long periods of time. The evaluation of the P:V parameter, the peak-to-valley ratio in a full annihilation spectrum, has been improved via a correction method that produced a fourfold increase in sensitivity to o-Ps annihilation. The spectrum ratio curve technique was improved and developed to investigate the chemical composition of the environment surrounding a positron-trapping defect. By fitting to multiple-element and/or defect-type responses the percentage of that particular element or defect contained within the sample could be found. Ratio curves were found to rely on the positron affinity to different vacancy types. Beam-based Doppler broadening spectroscopy, variable-energy positron annihilation spectroscopy (VEPAS), was used as a probe of oxide film and film/substrate interface characteristics. Different film growth methods were found to play a significant role in defining the features of films and their interfaces. Vacancies have a profound effect on the properties of semiconductor-type devices. A range of different structures and the effects of implantation have been investigated. VEPAS has been found to be useful in studying the more exotic types of materials, silica aerogel and niobium.