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.

Oscillations and rhythms are measured in the brain through large-scale measures like EEG (electroencephalogram) and LFP (Local Field Potential). Particularly, cortical gamma rhythms (30-90 Hz) found in different brain regions are correlated with different cognitive states. Despite vast differences in the range frequencies in gamma rhythms, the regions communicate to complete high-level tasks. One way in which this takes place is entrainment, where the postsynaptic group phase-lock to the rhythmic input from the presynaptic group (constant phase-shift). Mathematical models of the neurons and the neural networks are proposed to uncover the mechanisms behind experimentally observed phenomena. Most works have used homogeneous models of spiking networks. These simplified models provide a valuable understanding of neural dynamics. However, neural heterogeneity (variation in the neural or network parameters) has been experimentally observed and shown to have a non-trivial effect on many neural processes. Few studies have dealt with the role of different types of neural heterogeneity in the entrainment of a large network, and how it affects the frequency range the neural network entrains to. In this project, we aimed to show how different types of network heterogeneity affect the ability of the networks to entrain to gamma frequencies. We used the Pyramidal-Interneuronal Network Gamma (PING) model, a model consisting of excitatory pyramidal cells (E-cells) and inhibitory interneurons (I-cells) that are synaptically connected and generate gamma oscillations. We show that heterogeneity in the synaptic conductance from excitatory neurons to inhibitory neurons greatly increases the frequency range over which the network can entrain. The mechanism that allows this to happen requires the heterogeneity to 1. Create an I-cell excitability gradient; 2. Introduce input synchrony difference among the I-cells. The entrained I-cell subsets formed under these two conditions are necessary for well-enhanced entrainment as they support the entrainment of the whole network through feedback inhibition. This improvement is shown to be robust in large parameter space.

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The present study was initiated with the purpose of determining and comparing the effect of gamma radiation on the capacity of yeast cells to grow and sporulate. Using a new technique by which irradiated and non-irradiated yeast cells could be scored directly, it was found that sporulating yeast cells were more sensitive to radiation than growing cells, and that the inactivation of the capacity of an irradiated yeast cell to sporulate did not affect its ability to grow. Observations on irradiated sporulating cells indicated that spore-formation and reduction division of the nucleus, are closely allied phenomena. A short discussion of a probable mechanism of action of gamma radiation on sporulating yeast is included, together with suggestions for future research. / Thesis / Master of Science (MS)

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)

It is the purpose of this paper to set forth the method and results of studying the gamma rays resulting from the bombardment of As75 with approximately 14 Mev neutrons. The source of these neutrons was the H3(d,n)He4 reaction. The deuterons of 325 Kev energy were obtained from a Van de Graff electrostatic accelerator. A NaI scintillation spectrometer was used to determine the gamma-ray energies.

This work aims to contribute to the study of the origins of cosmic rays, and broadly, to the advancement of both data analysis methods and instrumentation for very-high-energy γ-ray astronomy. First, reviewing the state of γ-ray astronomy, we show how gains in sensitivity can be achieved through sophisticated data analyses and improved instrumental designs. We then develop such an improved analysis method for the Very Energetic Radiation Imaging Telescope Array System (VERITAS) by combining Image Template Method (ITM) with Boosted Decision Trees (BDT), and study its performance, attaining a 30-50% improvement in integral sensitivity over the instrument’s standard analysis. Systematic issues in spectral reconstruction that the analysis displays are resolved satisfactorily by imposing a more stringent condition on the selection of its energy threshold. We employ the newly developed analysis to measure the γ-ray energy spectrum of the starburst galaxy M82, and combining our result with a measurement from the Fermi Large Area Telescope (Fermi-LAT), we find that a single power law fits the spectrum well between 100 GeV and 10 TeV, with no evidence for a spectral break or a cutoff. We conclude that this is in line with the current understanding that M82 is not a good proton calorimeter. Finally, we detail the design, implementation, and performance of the optical alignment system of the prototypeSchwarzschild-Couder Telescope (pSCT) for the Cherenkov Telescope Array (CTA), a novel two-mirror design that addresses many shortcomings of current instruments.

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.

This project aims to improve research in gamma-ray spectroscopy by using advanced detector systems. These systems are designed to reduce interference inherent in gamma-ray spectroscopy by rejecting Compton scattering events from high-energy gamma-rays, as well as look at cascading decays of gamma-rays through gamma-gamma coincidence counting. By combining these methods, one is able to lower detection limits for many elements than would otherwise be possible. This work also takes advantage of neutron activation analysis, which allows stable elements to be analyzed by activating them with neutrons, causing them to become unstable and decay with radioactive signatures. By analyzing these signatures, one is able to detect trace levels of elements with relatively small samples sizes (< 1g) and in a nondestructive manner. / text