We have reexamined our previously published data to search for evidence of correlations between the rates for the alpha, beta-minus, beta-plus, and electron capture decays of 22Na, 44Ti, 108Agm, 121Snm, 133Ba, and 241Am and the Earth?Sun distance. We find no evidence for such correlations and set limits on the possible amplitudes of such correlations substantially smaller than those observed in previous experiments.

In addition to \textsuperscript{235,238}U and \textsuperscript{239}Pu, \textsuperscript{241}Pu is considered a major actinide in regards to nuclear fuel and systems. Despite contributing up to $\sim$10$\%$ of energy generated in nuclear reactors towards the end of a cycle, nuclear data for this isotope is limited. Due to its 14 year half-life, target manufacturing for this isotope is difficult, and as a result the (n,xn) cross section and fission neutron multiplicity as a function of incident neutron energy for $^{241}$Pu does not exist in the published literature. Using the surrogate ratio method, experimental difficulties associated with target manufacturing can be circumvented in measuring n + $^{241}$Pu reactions by using inelastic scattering of $\alpha$ particles incident on $^{242}$Pu to create the same compound nucleus. The NeutronSTARS detection array in Cave 4 of the Texas A$

The existence of a low energy excited state in ^{229<\super>Th was first postulated in 1975. The energy of the state was revised over the years from less than 100 eV in 1975, to 3.5 ± 1.0 eV in 1994, to 7.8 ± 0.5 eV in 2009, among others. Despite this progress, the decay of the state has never been observed, and the half life is unknown. While no measurement described in this dissertation was successful in these experiments either, many areas were searched and exclusion plots were produced. Measurements included looking for the internal conversion electron from the isomer, as well as any possible γ emission. 229m<\super>Th was produced via the α decay of 233<\super>U, and the recoiling 229<\super>Th was collected on a catcher. Short time scales were probed using an α coincidence measurement that was sensitive from tens of nanoseconds to almost a millisecond. Intermediate time scales used a mechanical shutter that was sensitive from 2 milliseconds to a few seconds. Longer time scale measurements moved the catcher from the source to the detector, and were sensitive from a few seconds to a few days. The proper function of the system was confirmed by measuring 235m<\super>U, which generated new results for the variation in its half life based on catcher material.}

The carbon burning nuclear reactions 12C(12C, alpha)20Ne, 12C(12C,p)23Na, and 12C(12C,n)23Mg occur during the carbon burning phase of sufficiently large stars and during explosive events such as supernovae. The Gamow window for these reactions is typically around 1.5 MeV, however direct measurements at energies in this range are very difficult due to the large Coulomb barrier between the carbon atoms. The compound nucleus formed during these reactions is 24Mg. For energies around the Gamow window this compound nucleus has an excitation energy of 15 to 16 MeV. A surrogate experiment which produces the compound nucleus can measure the branching ratios for the excited compound nucleus and thus the ratios between the carbon burning reaction cross sections. An experiment was performed using inelastic scattering of 40 MeV alpha particles to produce 24Mg excited up to 27 MeV and the applicability of these results as a surrogate for the carbon burning reactions is examined. A successful surrogate measurement, which this experiment partially achieves, will both provide the branching ratios within the Gamow window and aid future direct measurements of carbon burning within the Gamow window.

Nuclear excitation by electronic transition (NEET) is a rare nuclear excitation that is theorized to occur in numerous isotopes. One isotope in particular, U-235, has been studied several times over the past 40 years and NEET of U-235 has never been conclusively observed. These past experiments generated conflicting results with some experiments claiming to observe NEET of U-235 and others setting limits for the NEET rate.

This dissertation discusses the latest attempt to measure NEET of U-235. If NEET of U-235 were to occur, U-235m would be created. U-235m decays by internal conversion with a decay energy of 76 eV and a half-life of 26 minutes. A pulsed Nd:YAG laser operating at 1064 nm with a pulse energy of 789 mJ and a pulse width of 9 ns was used to generate a uranium plasma. The plasma was captured on a catcher plate and electrons emitted from the catcher plate were accelerated and focused onto a microchannel plate detector. A decay of 26 minutes would suggest the creation of U-235m and the possibility that NEET occurred. However, measurements performed using a variety of uranium targets spanning depleted uranium up to 99.4% enriched uranium did not observe a 26 minute decay. Numerous other decays were observed with half-lives ranging from minutes up to hundreds of minutes. While NEET of U-235 was not observed during this experiment, an upper limit for the NEET rate of U-235 was determined. In addition, explanations for the conflicting results from previous experiments are given. Based on the results of this experiment and the previous experiments looking for NEET of U-235, it is likely that NEET of U-235 has never been observed.

The Cryogenic Underground Observatory for Rare Events (CUORE) is an experiment that will search for neutrinoless double-beta (0&nu&beta&beta) decay of ¹³⁰Te and other rare processes. Observing 0&nu&beta&beta decay would establish that neutrinos are massive Majorana fermions, demonstrate that lepton number is not conserved, and constrain the neutrino mass scale and hierarchy. The CUORE detector, currently being constructed underground at the Gran Sasso National Laboratory in Italy, is an array of 988 high-resolution TeO₂ bolometers. Each bolometer is comprised of a thermistor and a TeO₂ crystal that serves as both the source and the detector of 0&nu&beta&beta decay. The 0&nu&beta&beta decay signature for ¹³⁰Te is a peak at the Q-value 2528 keV. Observation of 0&nu&beta&beta decay requires that the background at the peak be ultra-low. Background-source identification and characterization are therefore extremely important.

One source of background that is poorly characterized is activation of the TeO₂ crystals by sea-level cosmic-ray neutrons. This process, known as cosmogenic activation, produces long-lived radioisotopes that can obscure the 0&nu&beta&beta peak. Existing cross-section data is insufficient to estimate this background; therefore, an additional cross-section measurement has been performed in which a TeO₂ target was irradiated with a neutron spectrum similar to that of cosmic-ray neutrons at sea-level. Analysis of the radionuclides produced reveals that ^110mAg will dominate the cosmogenic activation background in CUORE. Estimations using the measured cross section for ^110mAg indicate this source will be negligible compared with other contributions to the CUORE background.

The work presented in this dissertation used a variety of facilities and methods to make precise measurements of neutron reaction cross sections and γ-ray branching ratios important for the study of high energy density environments. While measurements related to thulium used commercially available detectors, others required the development of a new detection

system. A 4π gas proportional beta counter was designed and characterized specifically for the measurement of fission product branching ratios. Measurements of increasing precision were performed for 147 Nd and 95 Zr. The instrumentation and techniques developed in this dissertation have set the path for an ongoing series of fission product branching ratio measurements. With work towards refinements mentioned here, precision will continue to improve in the measurements and results of future studies with these devices.