Thesis (Ph. D.)--California Institute of Technology, 2004. / VLA and VLBA observations.

Thesis (M. S.)--Mechanical Engineering, Georgia Institute of Technology, 2006. / Ansari, Armin, Committee Member ; Wang, C.-K. Chris, Committee Member ; Hertel, Nolan, Committee Chair.

Thesis (M.S. in Physics)--United States Naval Postgraduate School, California. / "Unclassified Instrumentation." Includes bibliographical references (p. 37). 16

Thesis (Ph. D.)--University of Wisconsin--Madison, 1989. / Cover title. Includes bibliographical references (p. 253-256).

The interactions of gamma-rays with matter have been studied for many years and there are accurate mathematical representations of the physical processes involved. Tabulations of the total interaction cross-section and of the major constituent processes have been produced which have an uncertainty of a few percent. In recent years the development of the tomographic scanner, first at EMI and then worldwide, has lead to the measurement of material interaction cross-sections with a precision of less than one percent, which is much less than the available tabulations. The form of the tabulations has also meant that data points must be interpolated from standard energy values, and so a large data base must be maintained if the values are stored on a computer. This is a time consuming and fairly inefficient process, especially if a micro-computer is used for the data base. In 1981 a compact, portable computer program was developed for the calculation of total interaction cross-sections (Jackson and Hawkes (1981)), which reproduced the tabulated data with a high precision (typically better than 0.5%) over a wide range of atomic number (1 < Z < 54) and energy (15 < E < 1500 keV). Although the computer code reproduces the theoretical data with high precision, the uncertainty in the original data is relatively large, this has prompted this experimental study to determine the agreement of the theoretical data values with experimental measurements. The precision desired from the experiments at the outset was 1%, which has been achieved in some, although not all, of the results presented here. The range of atomic number and gamma-ray energies of the materials tested in this study are from carbon (Z=6) to lead (Z=82) and from 15 to 1500 keV. This range has been determined largely by the sources and samples that have been available within the department, but it also represents the range of materials and energies that have been used in the industrial tomography group within this department. Isotopic sources and high resolution solid state detectors have been used to measure total cross-sections for the twelve elements used and up to 25 energies per sample (depending on the range of thicknesses available for the material). Many repeat runs have been done on each sample to increase the precision of the measurements presented here. The use of total interaction cross-sections for materials analysis has also been investigated for several situations and these are discussed and it is shown which methodsare possible and which are not with the precision used in these experiments. This study has to be seen as part of an ongoing process of experimentally producing total interaction cross-sections of higher and higher precision for comparison with theoretical formulations so that a deeper understanding of the underlying processes can be obtained. Within that framework some directions for future work have been outlined which will benefit from further study and a more detailed exploration.

Two types of experiments involving the Mossbauer atom as a dilute impurity in a host lattice are discussed. For a zero-phonon experiment with Fe⁵⁷ in Pt¹⁹⁵ at room temperature, the expected shift of the central Mossbauer peak is [symbol omitted]/3000. The minimum time required to experimentally determine this shift to within 10% is found to be 16 weeks of counting with a 5 millicurie source. For a one-phonon experiment with Fe⁵⁷ in Pt¹⁹⁵, the count rate due to resonance scattering is found to be 3.14 x 10⁻⁴/sec., and the count rate due to Rayleigh scattering 2.2 x l0⁻²/sec. The case of Fe⁵⁷ in Be⁹ is also discussed. An air trough Mossbauer shift spectrometer constructed to perform the zero-phonon experiment is described. Vibrations present in this apparatus, making it inadequate for experimental work, are discussed. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The first project in this dissertation involves the search for and the study of excited superdeformed (SD) rotational bands in 154Dy. The data was obtained from an experiment at the 88-inch cyclotron of the Lawrence Berkeley National Laboratory via the fusion evaporation reaction 122Sn(36S, 4n) and using the Gammasphere spectrometer array to probe the high-spin states in 154Dy. With the help of large Compton-suppressed spectrometers such as GAMMASPHERE in the USA and EUROBALL in Europe, superdeformed nuclei have been identified in several regions of the chart of nuclides including 151Dy, 153Dy, 154Dy and 155Dy. The 154Dy nucleus, located in the center of A= 150 SD island, plays an important role in the study in this region. Only the lowest (yrast) SD band in 154Dy was known before this study. Three or four fold coincidence analysis was performed employing the conventional gating method as well as an automatic search routine. As the result, five excited SD bands were identified with intensities between 0.7 - 0.03 % of total reaction channel. Theoretical analysis based on Cranked Relativistic Mean Field calculations (CRMF) and the effective alignment method were carried out. The second project includes the spectroscopy of high-spin states in Hf nuclei, which were populated in another Gammasphere experiment using 128Te(48Ca, 5n) and 128Te(48Ca, 4n) reactions to obtain 171Hf and 172Hf, respectively. Three SD bands in 172Hf and one band with enhanced deformation (ED) in 171Hf have been extracted and published by our group. Subsequently, a complete highspin spectroscopy in 171Hf has been further carried out in which five new rotational bands have been identified. Their rotational properties, including spins, parities, excitation energies, aligned angular momentum and quasiparticle configurations were investigated within the framework of the cranked shell model, aided by a comparison with level structures in neighboring nuclei. The band crossings at very high rotational frequency (500 keV) are interpreted as the proton alignments. One band has been suggested to be associated with enhanced deformation based on the i13/2h9/2 proton orbitals and h9/2 neutron orbital. However, its decay pathways to known low-spin states could not be established.

One of the main challenges during the ultra-high count rate gamma-ray spectrometry is the large dead time. Using a LaBr3(Ce) detector (TRT 0.3 µs, TFT 0.5 µs), with an input count rate of 4.8×10E5 cps, the dead time could be as high as 87%. Such high dead time could significantly reduce the quality of the data collected as a considerable number of counts would be lost. Thus, this project aimed to reduce the dead time by modifying the detector system. Based on the setup used by previous research, the new system has its preamp, which is normally used for signal processing, removed. Experiments were made with calibration sources to optimize the new system. The calibration sources (Cs-137 and Co-60), Cs-137 resin sources, and Shephard Cs-137 sources were used to create different count rates, with the highest being 1.22×10E6 cps, for measurements. Side-by-side measurements were performed with the setup with preamp and the one without preamp at various count rates. The analysis, which focused on the dead time and resolution, shows the setup without preamp would have much lower dead time, especially during ultra-high count rate measurements. The method was proved to be successful, for, at 4.8×10E5 cps, the dead time decreased from 87% to 54%. / Thesis / Master of Science (MSc)

Excited states in the transitional nucleus 160Yb have been studied using gamma-ray and conversion electron spectroscopy following the beta+/EC decay of 160Lu. Excited states in the nuclei 160Tm and 160Er have been studied following the beta+/EC decay of 160Yb and 160Tm, respectively. The data for the present study were obtained at TRIUMF in Vancouver, Canada at the ISAC-1 facility through radioactive sources moved into the combination of the 8pi gamma-ray spectrometer array and the Pentagonal Array for Conversion Electron Spectroscopy (PACES). Analysis of gamma-ray gated gamma-ray, gamma-ray gated conversion electron and conversion electron gated gamma-ray spectra resulted in the discovery of a new first excited state and the establishment of a level scheme for 160Tm which differs from the one adopted; as well as a test of the rotational characteristics of 160Er with intensity comparisons to both the spin-5 beta-decaying isomer study of by Singh et al. and the spin-parity 1- beta decay study of by Strusny et al. and Bykov et al.

Beam profile monitors provide position and ux distribution information to facilitate the configuration of an experimental apparatus and are an important component of any accelerator facilities beam diagnostic system. Nuclear physics experiments typically involve the incidence of high energy particles or gamma-rays on some target material and the detection of the products of the ensuing interactions. Therefore, knowing the profile of the incident radiation beam is desirable. To address the need for a profile monitor for the High-Intensity Gamma-Ray Source, development of a CCD-based gamma-ray beam profiler was undertaken. The profiler consisted of plastic scintillator, a lens system and a Starlight Express MX5 CCD camera, all contained within a light tight box. The scintillation pattern, created by the interaction between the incident gamma-rays and the scintillator, could be focused onto the CCD. Simulations were used to determine the amount of power that would be absorbed for different beam energies and scintillator thicknesses. The use of a converter material, placed directly against the scintillator to improve power deposition, was also investigated. The system was tested in order to and the camera noise characteristics, the optical resolution and magnification and the systems responsivity to power absorption in the scintillator. Using a 137Cs source, preliminary beam proles were obtained. By combining the results of the testing and simulation, predictions of the required length of exposure were made. It was determined that a beam with a flux of 10^6/s and a diameter of 2.5 cm could be profiled, using 6.0 mm of plastic scintillator and 0.6 mm of iron converter, to within 5% error per 0.64 mm x 0.91 mm resolving unit, in less than 1 minute.