<p>Gamma ray spectroscopy has provided enormous amounts of information on the behaviour and structure of atomic nuclei [SHA88, BEA92, EBE08]. Most of the major discoveries in experimental nuclear physics over the last five decades are strongly associated with improvements in detector technologies. Inorganic scintilators led to the discovery in 1963 of the first excited states of a rotational band based on the ground state of 162Dy. Improvements in peak-to-background ratios and detector resolutions obtained with germanium led to the first evidence of backbending which is associated with a two quasi-particle excitation in 162Dy [SHA88]. More recently the development of composite and highly-segmented Ge detectors has significantly increased the performance and power of detection systems. The Clover detector is such a detector system and is in use at iThemba LABS. This study concerns the evaluation of the particle transport code MCNPX 2.5.0 as a tool to model complex composite detectors such as the Clover. Lanthanum silicate (LSO) and Lead tungstate (PbWO) are also evaluated as possible suppressor shield materials. It is shown that reasonable agreement between experiment and simulation is found when the experiment is accurately reproduced. However, when complex detection modes are implemented in the detector based on the number of elements that fire, MCNPX cannot be used to model the detector performance exactly. Differences between simulated and experimental results are found in suppressed add-back mode. It is proposed that the discrepancies are due to limitations in implementation of the pulse-height and special anti-coincidence tally in MCNPX. LSO and PbWO are compared to BGO as suppressor shield materials. It is found that LSO is not an ideal material for a suppression shield. PbWO is shown to give performance values similar to that of BGO. The back-plug is shown to have no effect on the Peak-to-Total ratio but is effective at reducing the background at lower energies.</p>

Im Rahmen eines vom Bundesministerium für Bildung und Forschung geförderten Projektes (Förderkennzeichen 02NUK010A) wurden an einem senkrechten, mit Flüssigkeit umströmten und beheizten Stabbündel gammadensitometrische Gasgehaltsmessungen durchgeführt. Es wurden zwei Messpositionen, zwei Volumenstromraten des umströmenden Fluides, zwei Unterkühlungswerte und elf Wärmestromdichten zur Messung gewählt. Der Bericht umfasst die Beschreibung des Versuchsstandes, die Messmethodik, Ergebnisse und deren Interpretation. Im Detail wird ebenfalls die Messunsicherheit bewertet.

Thesis (M.S.)--California Polytechnic State University, 2009. / Title from PDF title page; viewed on January 13, 2010. Major professor: William Ahlgren, Ph.D. "Presented to the Electrical Engineering faculty of California Polytechnic State University, San Luis Obispo." "In partial fulfillment of the requirements for the degree [of] Master of Science in Electrical Engineering." "December 2009." Includes bibliographical references (p. 81-87).

The large flashes of radiation that are observed in GRBs are generally believed to arise in a relativistic jetted outflow. This thesis addresses the question of how and where in the jet this radiation is produced. It further explores the jet properties that can be inferred from the observations made by the Fermi GST that regularly observes GRBs in the range 8 keV - 300 GeV.  In my analysis I focus on the observational effects of the emission from the jet photosphere. I show that the photosphere has an important role in shaping the observed radiation spectrum and that its manifestations can significantly vary between bursts. For bursts in which the photospheric  emission component can be identified, the dynamics of the flow can be explored by determining the  jet Lorentz factor and the position of the jet nozzle. I also develop the theory of how to derive the properties of the outflow for general cases. The spectral analysis of the strong burst GRB110721A reveals a two-peaked spectrum, with the peaks evolving differently. I conclude that three main flow quantities can describe the observed spectral behaviour in bursts:  the luminosity, the Lorentz factor, and the nozzle radius. While the photosphere can appear like a pure blackbody it can also be substantially broadened, due to dissipation of the jet energy below the photosphere. I show that Comptonisation of the blackbody can shape the observed spectra and describe its evolution. In particular this model can very well explain GRB110920A which has two prominent breaks in its spectra.  Alternative models including synchrotron emission leads to severe physical constraints, such as the need for very high electron Lorentz factors, which are not expected in internal shocks. Even though different manifestations of the photospheric emission can explain the data, and lead to ambiguous interpretations, I argue that dissipation below the photosphere is the most important process in shaping the observed spectral shapes and evolutions. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 4: In press. Paper 5: Submitted.</p>

This thesis focuses on the radiation mechanisms of non-thermal X-rays and soft gamma-rays of two types of thousands year old spin-down powered pulsars. The thousands year old pulsars have distinct radiation behaviors from the middle-aged gamma-ray pulsars. In the magnetosphere of the pulsar, the particles are accelerated by the electric field resulting from the rotation of the neutron star. These accelerated particles move along the magnetic field lines and emit GeV gamma-ray curvature photons. For the middle-aged pulsars, most of the curvature photons, whose observed spectra are described well by power law with exponential cut-off, can escape out of the light cylinder. In X-ray band, the middle-aged pulsars usually have black body radiation with a weak non-thermal component described by power law. On the other hand, for the thousands-year-old pulsars, the curvature spectra in GeV band, which obey power law with exponential cut-off, are smeared out by the pair creation or missed by the line of sight. The secondary pairs generated by pair creation processes spiral around the magnetic field lines and emit synchrotron photons, and the young pulsars have stronger non-thermal X-ray and soft gamma-ray radiation than the middle-aged ones. Seven young pulsars have been studied here, they are the Crab pulsar, PSRs B0540-69, B1509-58, J1846-0258, J1811-1925, J1617-5055 and J1930+1852. These seven fall into two categories: the Crab-like pulsars and soft gamma-ray pulsars. The Crab-like pulsars include the Crab pulsar and the Giant Crab PSR B0540-69, and the soft gamma-ray pulsars include the other five. The main difference between the two types of young pulsars is that the Crab-like pulsars’ spectra peak at E ≤ 1MeV while the soft gamma-ray pulsars’ spectra (in units of MeV/cm2/s) peak at E ∼ 10MeV. Their spectra also have different photon indices in X-ray band. The physics behind is two different pair creations, the photon-photon pair creation and the magnetic pair creation. The former happens when a high energy photon collides with a soft photon, and the latter happens when a high energy photon penetrates through strong perpendicular magnetic field. In the outer gap of the pulsar, a large mount of pairs are generated around the null charge surface via photon-photon pair creation, and the electric field separates the two charges to move in opposite directions. Therefore, there are outflow and inflow of particles in the magnetosphere, whose curvature photons are converted to pairs by photon-photon pair creation and magnetic pair creation respectively. For the Crab-like pulsar, the non-thermal X-rays and soft gamma-rays are emitted by the outgoing secondary pairs generated by photon-photon pair creation in the outer magnetosphere; for the soft gamma-ray pulsar, the radiating secondary pairs are generated below the null charge surface by the magnetic pair creation. / published_or_final_version / Physics / Doctoral / Doctor of Philosophy

Lithium ion batteries, due to their relatively high energy density, are now widely used as the power source for portable electronics. Commercial lithium ion cells currently employ layered LiCoO₂ as a cathode but only 50% of its theoretical capacity can be utilized. The factors that cause the limitation are not fully established in the literature. With this perspective, prompt gamma-ray activation analysis (PGAA) has been employed to determine the hydrogen content in various oxide cathodes that have undergone chemical extraction of lithium (delithiation). The PGAA data is complemented by data obtained from atomic absorption spectroscopy (AAS), redox titration, thermogravimetric analysis (TGA), and mass spectroscopy to better understand the capacity limitations and failure mechanisms of lithium ion battery cathodes. As part of this work, the PGAA facility has been redesigned and reconstructed. The neutron and gamma-ray backgrounds have been reduced by more than an order of magnitude. Detection limits for elements have also been improved. Special attention was given to the experimental setup including potential sources of error and system calibration for the detection of hydrogen. Spectral interference with hydrogen arising from cobalt was identified and corrected for. Limits of detection as a function of cobalt mass present in a given sample are also discussed. The data indicates that while delithiated layered Li[subscript 1-x]CoO₂, Li[subscript 1-x]Ni[subscript 1/3]Mn[subscript 1/3]Co[subscript 1/3]O₂, and Li[subscript 1-x]Ni[subscript 0.5]Mn[subscript 0.5]O₂ take significant amounts of hydrogen into the lattice during deep extraction, orthorhombic Li[subscript 1-x]MnO₂, spinel Li[subscript 1-x]Mn₂O₄, and olivine Li[subscript 1-x]FePO₄ do not. Layered LiCoO₂, LiNi[subscript 0.5]Mn[subscript 0.5]O₂, and LiNi[subscript 1/3]Mn[subscript 1/3]Co[subscript 1/3]O₂ have been further analyzed to assess their relative chemical instabilities while undergoing stepped chemical delithiation. Each system takes increasing amounts of protons at lower lithium contents. The differences are attributed to the relative chemical instabilities of the various cathodes that could be related to the position of the transition metal band and the top of the O²-:2p band. Chemically delithiated layered Li[Li[subscript 0.17]Mn[subscript 0.33]Co[subscript0.5-y]Ni[subscript y]]O₂ cathodes have also been characterized. The first charge and discharge capacities decrease with increasing nickel content. The decrease in the capacity with increasing nickel content is due to a decrease in the lithium content present in the transition metal layer and a consequent decrease in the amount of oxygen irreversibly lost during the first charge. / text

Gamma-ray bursts (GRBs) are relativistic explosions which originate at cosmological distances, and are among the most luminous transients in the universe. Following the prompt gamma-ray emission, a fading synchrotron "afterglow" is detectable at lower energies. While long-duration GRBs (duration > 2 sec) are linked to the deaths of massive stars, the progenitors of short-duration GRBs (duration < 2 sec) have remained elusive. Theoretical predictions formulated over the past two decades have suggested that they are the mergers of two compact objects, involving either two neutron stars (NS-NS) or a neutron star and a black hole (NS-BH). Such merging systems are also important to understand because they are premier candidates for gravitational wave detections with upcoming facilities and are considered likely sites of heavy element nucleosynthesis. The launch of the Swift satellite in 2004, with its rapid multi-wavelength monitoring and localization capabilities, led to the first discoveries of short GRB afterglows and therefore robust associations to host galaxies. At a Swift detection rate of ~8 events per year, the growing number of well-localized short GRBs enables comprehensive population studies of their afterglows and environments for the first time. In this thesis, I undertake a multi-wavelength observational campaign to address testable predictions for the progenitors of short GRBs. From their local environments, I show that short GRBs explode in diffuse regions of their host galaxies and are weakly correlated with the distribution of stellar mass and star formation in their host galaxies. I study the host galaxy demographics for the entire population and find that ~20-40% of short GRBs originate from elliptical galaxies, implying an older stellar progenitor. From their afterglows, I present evidence that some short GRBs are collimated in narrow jets of ~5-10 degrees, directly affecting the true energy scale and event rate. Finally, taking advantage of a decade of broad-band afterglow observations at radio through X-ray wavelengths, I find that short GRBs have median isotropic-equivalent energies of ~10^51 erg and that their local environments have low densities, ~10^-3-10^-2 cm^-3. Taken together, this thesis comprises several lines of independent evidence to demonstrate that short GRBs originate from the mergers of two compact objects, and also provides the first constraints on the explosion properties for a large sample of events. With the direct detection of gravitational waves from compact object mergers on the horizon, these studies provide necessary inputs to inform the next decade of joint electromagnetic-gravitational wave search strategies. / Astronomy

No description available.

No description available.

No description available.