Theses and Dissertations from UMD
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Item Elliptic Flow Measured with the PHOBOS Spectrometer at RHIC(2008-07-28) Bindel, Richard Thomas; Mignerey, Alice C; Mignerey, Alice; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory provides experiments with the most energetic nucleus-nucleus collisions ever achieved in a laboratory. These have been used to investigate the phase diagram of nuclear matter at very high temperature and low baryon chemical potential. Under such conditions, quantum chromodynamics predicts a deconfinement of quarks from their hadronic boundaries, and this is believed to result in a phase transition to a quark gluon plasma (QGP). The characterization of the substance in a microscopic collision system is difficult because the matter undergoes significant changes as it rapidly inflates and cools. The collective expansion of the medium perpendicular to the collision axis is a revealing feature that can be related to the early stages of the system evolution. Arising as a consequence of the natural spatial asymmetry in non-head-on collisions, the back-to-back "elliptic flow" is a particularly informative mode of the expansion. The collective movement is characterized in terms of the relaxation of a compressed liquid. The magnitude of the elliptic flow constrains the parameters of various hydrodynamics-based models, and these suggest that the matter behaves as an ultra low-viscosity liquid, achieving local thermal equilibrium very early in the collision evolution. This thesis presents measurements of the elliptic flow anisotropy parameter, v2, for Au+Au and Cu+Cu collisions at center-of-mass energies of 200 GeV and 62 GeV per nucleon pair. The data was taken at the PHOBOS experiment at RHIC using the spectrometer in conjunction with the ring and octagon multiplicity detectors. A Monte Carlo Glauber model is used to establish the eccentricity of the overlap region in non-head-on collisions. When this geometry is taken into account, the elliptic flow is shown to evolve smoothly between collision systems. This behavior is evident, not only in the elliptic flow as a function of reaction centrality, but also as a function of the transverse momentum. The agreement lends support to the prevailing theory of a smooth progression with increasing system size and collision energy towards a hydrodynamic limit.Item Structure of Fe isotopes at the limits of the pf shell(2008-07-11) Hoteling, Nathan James; Walters, William B; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)An investigation of the structure of Fe isotopes up to neutron number 40 has been carried out at Argonne National Laboratory with the application of a thick-target deep-inelastic reaction experiment. A beam of 450 MeV 64Ni was incident on a 55 mg/cm2 target of enriched 238U located at the center of the Gammasphere spectrometer. Triple coincidence data obtained from the experiment were sorted into four time-correlated cubes and used to identify new levels in 61,62,64Fe. Angular correlations were used to both confirm previously-assigned spin and parity assignments and to assign values to excited states established in this work for the first time. The effect of g9/2 neutron excitations on the structure of low-lying yrast states in these isotopes was studied with a comparison of experimental levels with the results from shell model calculations within both pf and truncated pfg configuration spaces. The effective interactions used in this work were derived from the N3LO nucleon-nucleon potential. These calculations indicate a strong influence from the g9/2 orbital, beginning at moderate energy and spin in 62Fe and extending to the low-lying states of 64Fe. New levels identified above a 239-ns, 9/2+ isomer in 61Fe appear to be consistent within a rotation-aligned coupling scheme, with prolate deformation beta ~ 0.24, a value supported with both the shell model and Particle-triaxial rotor model. The data from this work mark a significant achievement in terms of understanding the evolution of nuclear structure in this region and the possible onset of deformation near N = 40. Still, more theoretical work is needed in order to better characterize experimentally observed features of this region. In addition to the Fe experiments described in the body of this thesis, another measurement was carried out in which the structure of 128Cd was investigated. This nucleus, like 64Fe, can be viewed as two proton and two neutron holes in a double-magic system. The identification of isomeric decay, and a confirmation of 2+ and 4+ level energies is described.Item Investigation of Shell Model States in Exotic Odd-A Sb Isotopes(2005-03-23) Shergur, Jason; Walters, William B; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The level structures of 134Sb and 135Sb were studied in the b- decays of the very-neutron-rich 134,135Sn isotopes. Tin isotopes were produced by proton- or neutron-induced fission at CERN/ISOLDE and ionized using a resonance ionization laser ion source. Following mass separation of the Sn nuclei, it was possible to collect g-ray singles and g-g coincidence data as a function of time for the decays of 134Sn and 135Sn. From these data, the position of the 7- b-decaying isomer in 134Sb has been established at 279 keV, and the 5- and 6- members of the f7/2g7/2 multiplet in 134Sb have also been identified, along with three new 1- levels at higher energy. New structure information was also obtained for 135Sb. New important levels in 135Sb were established at 282, 440, and 798 keV, which are given tentative spin and parity assignments of 5/2+, 3/2+ and 9/2+, respectively. The level structures of both 134Sb and 135Sb were compared with shell model calculations. Following b-delayed neutron decay of the parents, half-lives (T1/2) for 135,136,137Sn were measured to be 530(10) ms, 300(15) ms, and 250(30) ms, respectively. The corresponding b-delayed neutron probabilities (Pn) for these nuclides were measured to be 20(2)%, 27(4)%, and 33(12)%, for 135,136,137Sn decay. Limited data indicated an upper limit of 150 ms for the T1/2 of 138Sn. Low-spin levels in 111Sb and 113Sb were identified at Argonne National Laboratory following the b+/EC decay of 111Te and 113Te, respectively. 111Te and 113Te were produced and separated by the Fragment Mass Analyzer via the 56Fe(58,60Ni,2pn)111,113Te reactions. Analysis of the g-singles, g-g coincidence, and g-time data resulted in the identification of new levels in 111Sb, including levels at 487 and 881 keV that have been tentatively assigned spins and parities of 1/2+ and 3/2+, respectively. In addition, a more precise T1/2 for 111Te decay was measured from the decay of the two most intense g transitions to be 26.2(6) s. The positions of these new levels were compared with shell model calculations for the odd-A Sb isotopes from A = 105 to A = 111. Several new g rays and levels in 113Sb were identified, with data supporting a 3/2+ and 7/2+ assignment for the previously ambiguous 1019- and 1181-keV levels, respectively. Identification of the spins and parities of these states suggest a 5/2+ assignment for the ground state of 113Te.Item A GAS-INJECTION NEGATIVE ION SOURCE FOR ACCELERATOR MASS SPECTROMETRY(2004-11-04) Tumey, Scott; Mignerey, Alice C; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Accelerator mass spectrometry is an ultra-sensitive isotopic analysis technique that allows for the determination of rare long-lived radionuclides such as radiocarbon. Historically, the technique has required that samples be processed into graphite prior to analysis. The processing is time and labor intensive and limits the technique due to contamination. There has been recent interest to develop an ion source that would allow direct injection of gaseous samples, thus eliminating the processing. Previous attempts to build such an ion source have been limited by low mass usage efficiency and severe memory effects. This dissertation describes the development of a plasma negative ion source that allows direct injection of gaseous material. This ion source was able to produce a negative carbon ion beam of approximately 4.5 uA from carbon dioxide. This beam intensity is lower than both of the competing designs. However, this ion source was able to operate with a mass efficiency of nearly 0.15% which exceeds one of the competing designs. It was also free from significant memory effects which plagued the other competing design. There are many improvements to the design of this ion source that would further enhance its performance. These results are promising and show that this type of ion source could be used effectively in an accelerator mass spectrometry system.Item Charged Antiparticle to Particle Ratios Near Midrapidity in d+Au and p+p Collisions at sqrt(s_NN) = 200 GeV(2004-10-28) Bickley, Abigail Anne; Mignerey, Alice C; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Experiments at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory are designed to investigate the behavior of strongly interacting matter at high temperatures and densities. The conditions created during a heavy ion collision at RHIC energies are predicted to be sufficient to form a quark-gluon plasma. As part of this investigation, smaller collision systems need to be studied to aid in the understanding and interpretation of results from the more complicated heavy ion collisions. This thesis reports the ratios of the yields of antiparticles to particles for primary charged pions, kaons, and protons emitted in p+p and d+Au collisions at sqrt{s_NN}=200GeV. In the d+Au collision system the results are measured as a function of collision centrality. The data analyzed were collected by the PHOBOS detector during the 2003 run of the Relativistic Heavy Ion Collider. Comparison of the results obtained in this thesis with the antiparticle to particle ratios measured in 200GeV Au+Au collisions allows the effects of final state interactions on the produced particle yields to be inferred. Furthermore, measurement of the antiproton to proton ratio allows the relative influence of the baryon number transport and the antibaryon-baryon pair production mechanisms on the collision process to be investigated. The measured antiparticle to particle ratios represent the ratio of the yields averaged over the rapidity range of 0.1<y_pi<1.3 and 0<y_{K,p}<0.8, and for transverse momenta of 0.1<p_{T}^{pi,K}<1.0GeV/c and 0.3<p_{T}^p<1.0GeV/c. In the d+Au collision system it is found that the relative yields of antiparticles to particles are independent of centrality for all three particle species, pions, kaons and protons. The <pi-/pi+> ratio at all centralities is consistent with one. In the top 10% most central events <pi-/pi+>=1.016+/-0.007(stat.)+/-0.019(syst.) and in the 60-100% most peripheral events <pi-/pi+>=0.996+/-0.008(stat.)+/-0.013(syst.). The <K-/K+> ratio ranges from 0.97+/-0.03(stat.)+/-0.04(syst.) in the top 10% most central events to 1.00+/-0.04(stat.)+/-0.03(syst.) in the 60-100% most peripheral events. The <pbar/p> ratio ranges from 0.86+/-0.02(stat.)+/-0.04(syst.) in the top 10% most central events to 0.85+/-0.02(stat.)+/-0.03(syst.) in the 60-100% most peripheral events. The results obtained for the p+p collision system are consistent with the values measured in d+Au collisions. Ratios of <pi-/pi+>=1.000+/-0.012(stat.)+/-0.019(syst.), <K-/K+>=0.93+/-0.05(stat.)+/-0.03(syst.), and <pbar/p>=0.85+/-0.04(stat.)+/-0.03(syst.) have been measured. The data are compared to results from model calculations, other collision systems and other collision energies.