Bulletin of the American Physical Society
10th Annual Meeting of the Northwest Section of APS
Volume 53, Number 6
Thursday–Saturday, May 15–17, 2008; Portland, Oregon
Session B4: Nuclear Physics |
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Chair: Kara Keeter, Idaho State University Room: Olin 102 |
Friday, May 16, 2008 2:00PM - 2:36PM |
B4.00001: The fusion of neutron-rich nuclei with $^{208}$Pb Invited Speaker: I report the results of two experiments dealing with the fusion of n-rich nuclei with $^{208}$Pb. In the first experiment, the fusion excitation function was measured for the interaction of $^{32,36}$S with $^{208}$Pb to determine the isospin dependence of fusion enhancement. The deduced fusion barriers for the $^{32,36}$S + $^{208}$Pb reactions were 152.0 and 142.5 MeV, respectively, a 9.5 MeV downward shift for the n-rich projectile. The implications of this shift for the synthesis of n-rich heavy nuclei will be presented. In the second study, the sub-barrier fusion of $^{9}$Li with $^{70}$Zn and $^{208}$Pb was measured, leading to a finding of enhanced sub-barrier fusion beyond that predicted by coupled channel calculations. The importance of this finding for nuclear astrophysics will be discussed. [Preview Abstract] |
Friday, May 16, 2008 2:36PM - 3:12PM |
B4.00002: Taking a Broad Look at the Nuclear Equation of State Invited Speaker: The relation between pressure and density in hadronic matter, known as the nuclear equation of state (EoS), plays a major role in nuclear and astrophysical systems. In particular, microscopic studies of nuclear matter under extreme conditions of isospin asymmetry (that is, different neutron and proton concentrations), are of great contemporary interest. Together with parallel experimental effort, they can help constrain models of the nuclear force through the unusual topologies of rare isotopes. Recent efforts in my group have been aimed at exploring the EoS through a broad spectrum of theoretical studies involving isospin asymmetries as well as other exotic states of nuclear matter, such as the presence of strange baryons. The approach we take is microscopic and relativistic. The predicted EoS properties are derived self-consistently from realistic nucleon- nucleon forces. I will review recent results and point out the need for more stringent constraints. [Preview Abstract] |
Friday, May 16, 2008 3:12PM - 3:24PM |
B4.00003: The Strong and Weak Lithium Anomaly in BBN Patrick Bruskiewich The measured abundance for $^{7}$Li is within a factor of two agreement with the standard Big Bang Nucleosynthesis (BBN) models, however for the more fragile $^{6}$Li, its abundance has been observed at a level three orders of magnitude above those predicted by standard BBN model. These discrepancies are known as the \textit{Lithium Anomaly}. The standard BBN model predicts an abundance ratio for [$^{7}$Li / $^{6}$Li] on the order of a 1000 or greater. Precise measurements of isotopic ratio indicate that [$^{7}$Li / $^{6}$Li] $\approx $ 12.3. This discrepancy is the \textit{Strong Lithium Anomaly}. The measured abundance [$^{7}$Li / $^{1}$H] $\approx $ (1.5 \underline {+} 0.3) x 10$^{-10 }$is a factor of two lower than the abundance [$^{7}$Li/$^{1}$H] $\approx $ (3.82 \underline {+} 0.70) x 10$^{-10 }$predicted by the standard BBN calculations, and this discrepancy is the \textit{Weak Lithium Anomaly}. A quick review will be done of the reactions that have been included in the BBN calculations of Lithium-6 abundance. I will discuss an experiment to address the \textit{Lithium Anomaly}, via the $^{7}$Li($^{3}$He, $^{4}$He)$^{6}$Li reaction using the TACTIC detector at TRIUMF. [Preview Abstract] |
Friday, May 16, 2008 3:24PM - 3:36PM |
B4.00004: Lifetimes of states in 19Ne above the 15O + alpha threshold Mythili Subramanian, Barry Davids, Tom Alexander, Gordon Ball, M. Chicoine, Ravuri Chakrawarthy, Randy Churchman, Jim Forster, S. Gujrathi, Greg Hackman, Derek Howell, Ritu Kanungo, J. Leslie, Elizabeth Padilla, Chris Pearson, Chris Ruiz, Gotz Ruprecht, Mike Schumaker, Isao Tanihata, Chris Vockenhuber, Pat Walden, Stan Yen The 15O(alpha,gamma)19Ne reaction plays a role in the ignition of Type I x-ray bursts on accreting neutron stars. The lifetimes of states in 19Ne above the 15O + alpha threshold of 3.53 MeV are important inputs to calculations of the astrophysical reaction rate. These levels in 19Ne were populated in the 3He(20Ne,alpha)19Ne reaction at a 20Ne beam energy of 34 MeV. The lifetimes of six states above the threshold were measured with the Doppler shift attenuation method (DSAM). The measurement, methods of analysis and implications of the results will be discussed. [Preview Abstract] |
Friday, May 16, 2008 3:36PM - 3:50PM |
B4.00005: BREAK
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Friday, May 16, 2008 3:50PM - 4:26PM |
B4.00006: Solving a Five Decade-Old Mystery: Why is there Carbon Dating? Invited Speaker: Carbon dating is due to the fact that the half-life of 14C is unusually long, namely, 5730 years, after which it decays in to 14N. \textit{A priori} one would not expect the beta decay of 14C to extend over archaeological times, because the quantum numbers of the initial and final states satisfy the selection rules for an allowed Gamow-Teller transition. The expected half-live would therefore be in the order of minutes or hours. The corresponding nuclear transition matrix element is very small, but it has been a mystery for half a century why it is so small. In a recently published paper [1], we have shown that by incorporating hadronic medium modifications into the one-boson-exchange model of the nuclear force the decay of 14C is strongly suppressed, explaining the long life-time. The medium modifications are based upon Brown-Rho scaling [2], which predicts that hadron masses decrease at finite nuclear density due to the partial restoration of chiral symmetry. \newline [1] J.W. Holt, G.E. Brown, T.T.S. Kuo, J.D. Holt, and R. Machleidt, Phys. Rev. Lett. \textbf{100}, 062501 (2008). \newline [2] G.E. Brown and M. Rho, Phys. Rev. Lett. \textbf{66}, 2720 (1991). [Preview Abstract] |
Friday, May 16, 2008 4:26PM - 4:38PM |
B4.00007: Ultra-Low Background Measurements using AMS Daniel J. Robertson, John D. Baker, Philippe Collon, Jaret Heise, Kara J. Keeter, Christopher J. Schmitt, Ed Tatar, Charles Taylor Current-generation experiments studying rare processes such as neu- trino and dark matter interactions require ultra-low levels of radioactive background. Accelerator Mass Spectrometry (AMS) shows promise in achieving the ultra-low sensitivity required for detector material selec- tion. One project interested in such techniques is SNO+, which proposes to modify the existing SNO detector to study low-energy solar neutrinos as well as other neutrino properties via double-beta decay using a liquid scintillator called linear alkylbenzene (LAB). Due to the lower energy threshold of the detector, the present materials need to be reevaluated for concentrations of $^{40}$K. Ultra-pure copper cathodes as well as sam- ples of materials to be used in the detector have been prepared at Idaho State University and Idaho National Laboratory. These materials are being tested for levels of $^{40}$K at the Notre Dame AMS facility. Results from the first set of measurements will be discussed. [Preview Abstract] |
Friday, May 16, 2008 4:38PM - 4:50PM |
B4.00008: Fast Analysis of Potential Scintillators Using Ion Time Of Flight Brian Milbrath, Yanwen Zhang The development of scintillators for radiation applications such as national security, medical imaging, and experimental nuclear/particle physics has historically been rather slow, principally due to the developmental time necessary for large crystal growth. Scintillator crystals must achieve dimensions of a few mm before important characterizations, such as gamma ray energy resolution, can be performed. In order to facilitate accelerated discovery, we developed a time of flight (TOF) telescope for use on an ion beam. This allows individual determination of the ion energies prior to impinging the crystal, which may be a very thin prototype material. With such a technique, the scintillator performance in terms of energy resolution, light yield, decay time, and spectrum, can be determined quickly over a broad energy range. Though the analysis is performed using ions rather than the gamma-rays whose detection is the ultimate aim of the materials investigated, we have found useful correlations between the ion and gamma responses of the materials we have investigated (CaF2:Eu, YAP:Ce, BGO, CsI:Tl, and plastic scintillator). The technique appears to be able to rapidly determine whether a scintillator material has promise for further development. [Preview Abstract] |
Friday, May 16, 2008 4:50PM - 5:02PM |
B4.00009: Nuclear Resonance Fluorescence Response of U-235 Glen Warren Nuclear resonance fluorescence (NRF) is a physical process that provides an isotopic-specific signature that could be used for the identification and characterization of materials. The technique involves the detection of prompt discrete-energy photons emitted from a sample, which is exposed to photons in the MeV energy range. Potential applications of the technique range from detection of high explosives to characterization of special nuclear materials. Pacific Northwest National Laboratory and Passport Systems have collaboratively conducted a set of measurements to search for an NRF response of U-235 in the 1.5 to 9 MeV energy range. Results from these measurements will be presented. [Preview Abstract] |
Friday, May 16, 2008 5:02PM - 5:14PM |
B4.00010: Defect Measurements and Performance Testing of CZT Detectors Using I-DLTS, TCT, I-V, C-V and $\gamma $-Ray Spectroscopy Rubi Gul, Zheng Li, Kara Keeter, Rene Rodriguez, Ralph James Performance testing and the measurement of the defect levels of CZT detectors from different manufacturers are investigated by means of I-DLTS (Current Deep Level Transient Spectroscopy), TCT (Transient Charge Technique), I-V, C-V and $\gamma $-Ray Spectroscopy. Studies include measuring defects parameters such as energy levels in the band gap, carrier capture cross-section and defect density have been determined by using I-DLTS. The space charge density, induced current and collected charge were obtained by using TCT. Detector's electric characteristics and detection performance tests are carried out by using I-V, C-V and $\gamma $-Ray Spectroscopy. I-DLTS data is collected in the temperature range of 10-330 K. Different mid-band energy traps, ranging from E$_{c}$-0.01 eV to E$_{c}$-0.31 eV, mostly native acceptor defects, are identified. The TCT parameters are obtained by measuring laser induced current transients with 50-1100 V bias voltages across the detector. The maximum value of collected electrons and the typical measured space charge density, at 1100V is found of the order of 10$^{10}$. Resistivity, performance and depletion of the detectors are determined by I-V and $\gamma $-Ray Spectroscopy. [Preview Abstract] |
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