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The NIFFTE fission Time Projection Chamber has been used to measure a number of cross section ratios and fission anisotropies over the past several years. Track information from fully reconstructed three-dimensional ionization profiles have allowed for sophisticated cross section analyses that were not previously possible. Initial measurements disregarded the shape of recorded ionization profiles, which should contain particle identification information. The utility of these charge profiles will be discussed, along with future measurements they might enable.

The normalized $^{238}$U(n,f)/$^{235}$U(n,f) cross section ratio has been measured using the NIFFTE fission Time Projection Chamber from the reaction threshold to $30$~MeV. The fissionTPC is a two-volume MICROMEGAS time projection chamber that allows for full three-dimensional reconstruction of fission-fragment ionization profiles from neutron-induced fission. The measurement was performed at the Los Alamos Neutron Science Center, where the neutron energy is determined from neutron time-of-flight. The $^{238}$U(n,f)/$^{235}$U(n,f) ratio reported here is the first cross section measurement made with the fissionTPC, and will provide new experimental data for evaluation of the $^{238}$U(n,f) cross section, an important standard used in neutron-flux measurements. Use of a development target in this work prevented the determination of an absolute normalization, to be addressed in future measurements. Instead, the measured cross section ratio has been normalized to ENDF/B-VIII.$\beta$5 at 14.5 MeV.

The weakly collective nucleus 94Mo has been considered an excellent example of quadrupole vibrational mixed-symmetry structure. Algebraic models such as the interacting boson model-2 have been largely unsuccessful in describing the structure of the 4 2 + state, due to the strong M1 transition between the 4 2 + and 4 1 + states, and the low energy of the 4 2 + state. In this Letter, we show that introducing g-bosons into the interacting boson model-2 allows for the first time a description of the electromagnetic decay properties of the low-lying 4 + states in 94Mo within an algebraic model. The hexadecapole degree of freedom is shown to be important for the description of the collective low-energy structure of nuclei in this region.

Excited states in 132,134Nd were populated in the β+/e decay of 132,134Pm and studied through off-beam γ-ray spectroscopy. Level spins and multipole mixing ratios of transitions were determined through an angular correlation analysis. In 132Nd, a new excited 0+ state is identified and in 134Nd the level scheme is significantly extended. Differences in the location of the 02+ state and the γ bandhead above and below N=82 suggest that the lighter isotopes are much more γ-soft than the heavier Nd and that a first-order phase transitional description is not applicable in the N

We measured the lifetimes of the ${2}_{1,2}^{+}$ and ${4}_{1}^{+}$ states in ${}^{98}$Ru in order to reduce their uncertainties and resolve the discrepancies in the literature for the lifetime of the ${4}_{1}^{+}$ state. Coulomb excitation in inverse kinematics was used to populate excited states in ${}^{98}$Ru, and the recoil distance doppler shift (RDDS) method was employed using the nNew Yale plunger device (NYPD). This technique combined with inverse Coulomb excitation requires several corrections due to relativistic and deorientation effects but yields high precision lifetimes. The determined ${B}_{4/2}=B({4}_{1}^{+}\ensuremath{\rightarrow}{2}_{1}^{+})/B({2}_{1}^{+}\ensuremath{\rightarrow}{0}_{1}^{+})=1.86(16)$ agrees well with the vibrational limit. In order to analyze the data, a new method for the deorientation correction of RDDS data was developed using the perturbation of experimental angular correlations. The simultaneous measurement of deorientation and lifetime of a given state and its application are discussed. The method is suitable for radioactive beam experiments.

The transient-field-perturbed angular correlation technique was used with Coulomb excitation in inverse kinematics to perform a systematic measurement of the $g$ factors of the first excited ${2}_{1}^{+}$ states in the stable even-$A$ isotopes $^{96\ensuremath{-}104}\mathrm{Ru}$. The measurements have been made relative to one another under matched kinematic conditions and include a measurement of $g({2}_{1}^{+})=+0.47(3)$ in $^{96}\mathrm{Ru}$.

The precise measurement of time has been a goal of physicists for centuries. With every new increase in our ability to measure time we have discovered new phenomena. The most advanced clocks available to us currently are atomic clocks that use electronic transitions to track the passage of time. In this proposal, I put forward the framework for the first nuclear clock estimated to be 1000 to 10000 times more precise than the current atomic clocks. This research will explore in detail the atomic nuclear interactions and help perfect and refine current atomic-nuclear interaction models. The realization of a {sup 229}Th nuclear clock will allow tests of cosmology by measuring the change of the fine structure constant as a function of time. The results of these experiments could dramatically alter our view of the universe, its past and future evolution. Precision clocks - with fundamental physics applications - require a long-lived quantum transition (two-level system) that is immune to external perturbations. Nuclear transitions would be better suited than atomic transitions for these applications except that nuclear transitions are typically much higher in energy and therefore cannot be accessed with table-top lasers. There is, however, one promising nuclear transition: the doublet between the ground and first excited states of the {sup 229}Th nucleus discovered by Helmer and Reich. This doublet has an energy splitting of 7.6 {+-} 0.5 eV, a spin difference of 1 h-bar, and an excited state half-life that could be as long as hours. A precision clock based on the {sup 229}Th nuclear doublet has been proposed by Peik et al. Their design is similar to the ion clock research being conducted at NIST in Boulder, CO. However, the NIST researchers use atomic transitions for their frequency standards. In the {sup 229}Th nuclear doublet transition is the frequency standard while atomic transitions are used to cool the ions and for probing the state of the {sup 229}Th nucleus. Recently, Campbell et al. have trapped and cooled {sup 232}Th{sup 3+} at Georgia Institute of Technology. This is a large step forward in the realization of a nuclear clock. The Georgia Tech group is already a collaborator on this project and we are in discussions with the NIST Boulder group about collaboration. In order to determine the suitability of the {sup 229}Th nuclear doublet for a precision clock, the half-life of the excited-state needs to be measured. Current estimates of the half-life vary from 10 {micro}s to 1000 hours. The longer the half-life, the narrower the natural linewidth of the state and the more desirable the transition is for potential applications. In this proposal, I outline the necessary research to be conducted to determine the half-life and exact wavelength of the nuclear doublet transition in {sup 229}Th. This research will lead to a deeper understanding of atomic-nuclear interactions important for our knowledge of high energy density science. It will provide a spectroscopy measurement of the lowest known nuclear transition ever and open the doorway for the development of a nuclear clock with unprecedented precision.

The energies of subsets of excited 0+ states in geometric collective models are investigated and found to exhibit intriguing regularities. In models with an infinite square well potential, it is found that a single formula, dependent on only the number of dimensions, describes a subset of 0+ states. The same behavior of a subset of 0+ states is seen in the large boson number limit of the Interacting Boson Approximation (IBA) model near the critical point of a first order phase transition, in contrast to the fact that these 0+ state energies exhibit a harmonic behavior in all three limiting symmetries of the IBA. Finally, the observed regularities in 0+ energies are analyzed in terms of the underlying group theoretical framework of the different models., 9 pages, 3 figures, added reference [38]

Excited, low-spin states in $^{130}\mathrm{Ce}$ are populated in the ${\ensuremath{\beta}}^{+}/\ensuremath{\epsilon}$ decay of $^{130}\mathrm{Pr}$ and studied through off-beam \ensuremath{\gamma}-ray spectroscopy at the Yale moving tape collector. New coincidence data lead to the construction of a substantially revised level scheme. The low-lying states of $^{130}\mathrm{Ce}$ are compared with the predictions of the X(5) critical-point model and the X(5)-${\ensuremath{\beta}}^{4}$ model, and the latter is found to give better agreement with the data in terms of energies. Discrepancies in the relative $B(E2)$ values in $^{130}\mathrm{Ce}$ and the geometrical models suggest that the \ensuremath{\gamma} degree of freedom may play an important role in this mass region.

A simple, empirical signature of a first order phase transition in atomic nuclei is presented, the ratio of the energy of the 6+ level of the ground state band to the energy of the first excited 0+ state. This ratio provides an effective order parameter which is not only easy to measure, but also distinguishes between first and second order phase transitions and takes on a special value in the critical region. Data in the Nd-Dy region show these characteristics. In addition, a repeating degeneracy between alternate yrast states and successive excited 0+ states is found to correspond closely to the line of a first order phase transition in the framework of the Interacting Boson Approximation (IBA) model in the large N limit, pointing to a possible underlying symmetry in the critical region., Comment: 4 pages, 4 postscript figures, uses revTeX

The $g$ factor of the ${2}_{1}^{+}$ state of $^{170}\mathrm{Hf}$ was measured by perturbed \ensuremath{\gamma}-\ensuremath{\gamma} angular correlation in a static external magnetic field. The result, $g({2}_{1}^{+})=0.28$(5), extends the systematics of $g$ factors of even-even Hf isotopes to $N=98$ and enables a better test of theoretical models. The $g({2}_{1}^{+})$ experimental values of these isotopes exhibit a remarkable constancy as a function of neutron number. This phenomenon, which was also observed for other isotopic chains in the Gd--W range, is explained in terms of a recently proposed empirical model.

Experiments were carried out at the Wright Nuclear Structure Laboratory at Yale University using the 21MV ESTU Tandem Van de Graaff accelerator with the purpose of studying 88Y. A beam of 18O impinged at laboratory energies of 60, 65 and 70 MeV on a 600 μg/cm2 74Ge target with a thick (10mg/cm2) 197Au backing. This experiment was performed with the specific aim of accessing medium spin states of the nucleus of interest. A second experiment was undertaken to populate the nucleus of interest in higher spin states by impinging the same 18O beam on a thin 62 μg/cm2 76Ge target with a 20 μg/cm2 carbon backing at a laboratory beam energy of 90 MeV. Gamma rays emitted following the decay of excited states in 88Y and other nuclei populated in the reactions were measured using the YRAST ball detector array, consisting of 10 Compton suppressed HPGe clover detectors. In conjunction with the experimental study presented here, nuclear shell model calculations using a truncated valence space have also been performed in an attempt to describe the single-particle make-up of the states observed. Preliminary results from these experiments and theoretical calculations are presented.

The g-Plunger technique has been introduced, which allows for the simultaneous measurement of lifetimes and absolute values of g-factors using a plunger device in inverse kinematics. Experiments on the 104,106,108Pd isotopes using Coulomb excitation resulted in g-factors in agreement with literature data. Lifetimes of 2 1 + states have in part significantly changed in this work.