Universiẗat, Diss., 2000--Dortmund. / Dateiformat: PDF.

Universiẗat, Diss., 1999--Dortmund. / Dateiformat: PDF.

Universiẗat, Diss., 2000--Dortmund. / Dateiformat: PDF.

Techn. Hochsch., Diss., 2002--Aachen.

Made available in DSpace on 2014-10-09T12:42:49Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:07:53Z (GMT). No. of bitstreams: 1 04994.pdf: 5757270 bytes, checksum: 17538e0c134d27fb78f39d761d00b604 (MD5) / Dissertacao(Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

Made available in DSpace on 2014-10-09T12:42:49Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:07:53Z (GMT). No. of bitstreams: 1 04994.pdf: 5757270 bytes, checksum: 17538e0c134d27fb78f39d761d00b604 (MD5) / Dissertacao(Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

Abstract The present thesis describes the development of a novel method, referred to as xenon porometry, for the determination of the structural properties of porous materials by means of xenon NMR spectroscopy. The method exploits the high sensitivity of the chemical shift of the ¹²⁹Xe isotope to its local environment. The purpose of the medium added to the sample is to slow down the diffusion of xenon so that the NMR signal of a xenon atom is characteristic of the properties of one pore, and the signals of all the atoms in the sample represent the distribution of the properties. Two types of porous materials (controlled pore glasses and silica gels) with well-known properties and three different media (acetonitrile, cyclohexane, and naphthalene) were used in the studies. The behavior of the medium and dissolved xenon at different temperatures around the melting point of the medium was explained. By varying the pore size of the material, three different correlations that make it possible to measure the pore sizes of unknown materials were experimentally determined. The chemical shift of xenon inside pockets built up in the pores during solidification of the medium turned out to be especially sensitive to pore size, and this correlation makes it possible to determine the pore size distribution. The curious behavior of the chemical shift as a function of pore size was explained by using a model based on the fast exchange between xenon adsorbed on the walls of the pockets and free xenon in the middle of the pockets. It was also proved that the porosity of the materials can be determined by comparing the intensities of two signals originating from xenon dissolved in a liquid medium. A comparison of the xenon porometry method with other methods used for pore size characterization leads to the following conclusions: The range of applications of the method is relatively wide, the measurements are fast and easy to do, the analysis of the spectra is simple on the basis of the information presented in this thesis, and the properties of the materials can be extracted from the spectral data with basic mathematical conversions. Because there are several different types of correlations available in the same spectra that represent the properties of the porous material, the complementary information of all the correlations make it possible to obtain a picture of the structures of very complex systems.

The prospects for enhanced Positron Emission Tomography imaging using liquid xenon (LXe) gamma ray detectors had been examined. Monte-Carlo simulations using GEANT4 were performed and the results were used to study the expected performance of a small animal PET scanner in comparison with a simulated conventional small animal scanner (LSO Focus 120). A NEMA-like cylinder phantom and an image contrast phantom were simulated with both scanners to compare performance characteristics. A Compton reconstruction algorithm was developed for the LXe scanner, and its performance and limitations studied. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

High Harmonic Generation (HHG) enables the production of bursts of coherent soft x-rays with attosecond pulse duration. This process arrises from the nonlinear interaction between intense infrared laser pulses and an ionizing gas medium. Soft x-ray photons are used for spectroscopy of inner-shell electron correlation and exchange processes, and the availability of attosecond pulse durations will enable these processes to be resolved on their natural time scales. The maximum or cutoff photon energy in HHG increases with both the intensity as well as the wavelength of the driving laser. It is highly desirable to increase the harmonic cutoff as this will allow for the generation of shorter attosecond pulses, as well as HHG spectroscopy of increasingly energetic electronic transitions. While the harmonic cutoff increases with laser wavelength, there is a corresponding decrease in harmonic yield. The first part of this thesis describes the experimental measurement of the wavelength scaling of HHG efficiency, which we report as lambda^(-6.3) in xenon, and lambda^(-6.5) in krypton. To increase the HHG cutoff, we have developed a 1.8 um source, with stable carrier envelope phase and a pulse duration of <2 optical cycles. The 1.8 um wavelength allowed for a significant increase in the harmonic cutoff compared to equivalent 800 nm sources, while still maintaing reasonable harmonic yield. By focusing this source into neon we have produced 400 eV harmonics that extend into the x-ray water window. In addition to providing a source of photons for a secondary target, the HHG spectrum caries the signature of the electronic structure of the generating medium. In krypton we observed a Cooper minimum at 85 eV, showing that photoionization cross sections can be measured with HHG. Measurements in xenon lead to the first clear observation of electron correlation effects during HHG, which manifest as a broad peak in the HHG spectrum centred at 100 eV. This thesis also describes several improvements to the HHG experiment including the development of an ionization detector for measuring laser intensity, as well as an investigation into the role of laser mode quality on HHG phase matching and efficiency.

An abundance of evidence from a wide range of astrophysical and cosmological observations suggests the existence of nonluminous cold dark matter, which makes up about 83% of the matter and 27% of the mass-energy of the Universe. Weakly Interacting Massive Particles (WIMPs) have been one of the most promising dark matter candidates. Various detection techniques have been used to directly search for the interaction in terrestrial detectors where WIMP particles are expected to scatter off target nuclei. Over the last fifteen years, dual-phase time projection chambers (TPCs) with liquid xenon (LXe) as target and detection medium have led the WIMP dark matter search. The XENON dark matter search project is a phased program focused on the direct detection of WIMPs through a series of experiments employing dual-phase xenon TPCs with increasing target mass operated at the Gran Sasso underground laboratory (LNGS) in Italy. The XENON1T experiment is the most recent generation, completed at the end of 2018. The XENON1T dark matter search results from the one ton-year exposure have set the most stringent limit on the WIMP-nucleon spin-independent elastic scatter cross-section over a wide range of masses, with a minimum upper limit of 4.1 x 10⁻⁴⁷ cm² at 30 GeV · c⁻² and a 90% confidence level. XENON1T is the first WIMP dark matter experiment which has deployed a dual-phase xenon TPC at the multi-ton scale, with 3.2 t of LXe used. The large xenon mass posed new challenges in reliable and stable xenon cooling, in achieving and maintaining ultra-high purity as well as in efficient and safe xenon storage, transfer and recovery. The Cryogenic Infrastructure was designed and constructed to solve these challenges. It consists of four highly interconnected systems --- the Cryogenic System, the Purification System, the Cryostat and Cryogenic Pipe, and the ReStoX System. The XENON1T Cryogenic Infrastructure has performed successfully and will continue to serve the next generation experiment, called XENONnT, with a new Cryostat containing a total of 8.4 tons of xenon. I first give an instrument overview of the systems in XENON1T. I then review the cooling methods in LXe detectors which led to the design of the cooling system implemented in the XENON1T experiment, and suggest a design of the cooling system for future LXe dark matter experiments at the 50 tons scale. I describe and discuss in detail the design and the performance of the XENON1T Cryogenic Infrastructure. Finally, I describe the detector stability and the corresponding data selection in all three XENON1T science runs, and describe the dark matter search results from the one ton-year exposure.