Bulletin of the American Physical Society
2015 Fall Meeting of the APS Division of Nuclear Physics
Volume 60, Number 13
Wednesday–Saturday, October 28–31, 2015; Santa Fe, New Mexico
Session KC: Nuclear Astrophysics III |
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Chair: Andrew Rogers, University of Massachusetts Lowell Room: Sweeney Ballroom B |
Friday, October 30, 2015 2:00PM - 2:12PM |
KC.00001: Total absorption spectroscopy of neutron-rich nuclei around the A=100 mass region Alexander Dombos, Alejandro Algora, Thomas Baumann, Jaclyn Brett, Benjamin Crider, Tom Ginter, Ulrike Hager, Elaine Kwan, Sean Liddick, Braden Marks, Farheen Naqvi, Wei Jia Ong, Jorge Pereira, Christopher Prokop, Stephen Quinn, Anna Simon, Dustin Scriven, Artemis Spyrou, Chandana Sumithrarachchi, Paul DeYoung Accurate modeling of the r-process requires knowledge of properties related to the $\beta$-decay of neutron-rich nuclei, such as $\beta$-decay half-lives and $\beta$-delayed neutron emission probabilities. These properties are related to the $\beta$-decay strength distribution, which can provide a sensitive constraint on theoretical models. Total absorption spectroscopy is a powerful technique to accurately measure quantities needed to calculate the $\beta$-decay strength distribution. In an effort to improve models of the r-process, the total absorption spectra of neutron-rich nuclei in the mass region around A=100 were recently measured using the Summing NaI(Tl) (SuN) detector at the NSCL in the first ever total absorption spectroscopy measurement performed in a fragmentation facility. Total absorption spectra will be presented and the extracted $\beta$-decay feeding intensities will be compared to theoretical calculations. [Preview Abstract] |
Friday, October 30, 2015 2:12PM - 2:24PM |
KC.00002: The First GRIFFIN Experiment: An investigation of the $s$-process yields in the Cd-In-Sn region from $^{115}$Cd Ryan Dunlop In the $s$-process, it is assumed that He-shell flashes give rise to neutron bursts at two different thermal energies $(kT\sim10$ keV and $kT\sim25$ keV). The contribution to the isotopic abundance of $^{116}$Cd from the higher temperature neutron bursts are calculated assuming thermal equilibrium between the ground state and the long-lived isomeric state of $^{115}$Cd. However, it is unknown if the equilibrium between these states is present at the low temperature of the first burst, which would significantly decrease the calculated $s$-process yields of $^{116}$Cd. To answer this question, we are searching for gateway levels at slightly higher excitation energy than the isomer in $^{115}$Cd that could be populated from the isomeric state via $(\gamma,\gamma^\prime)$ reactions within stars. In November 2014, the GRIFFIN spectrometer was commissioned at TRIUMF's Isotope Separator and Accelerator. GRIFFIN is a state-of-the-art array consisting of 16 HPGe clovers, with a large $\gamma$-ray efficiency of roughly 17\% at 1 MeV. In this first experiment, a beam of $^{115}$Ag was delivered to GRIFFIN in order to search for transitions between gateway levels following the $\beta$ decay of $^{115}$Ag. In this talk, results from this first GRIFFIN experiment will be presented. [Preview Abstract] |
Friday, October 30, 2015 2:24PM - 2:36PM |
KC.00003: Neutron Capture Cross Sections for Radioactive Nuclei Anton Tonchev, Peter Bedrossian, Jutta Escher, Nicholas Scielzo Accurate neutron-capture cross sections for radioactive nuclei near or far away from the line of beta stability are crucial for understanding the nucleosynthesis of heavy elements. However, neutron-capture cross sections for short-lived radionuclides are difficult to measure due to the fact that the measurements require both highly radioactive samples and intense neutron sources. Essential ingredients for describing the $\gamma$ decays following neutron capture are the $\gamma$-ray strength function and level densities. We will compare different indirect approaches for obtaining observables that can constrain Hauser-Feshbach statistical model calculations of capture cross sections. Specifically, we will consider photon scattering, transfer reactions, and beta-delayed neutron emission. Challenges that exist on the path to obtaining neutron-capture cross sections for reactions on isotopes far from stability will be discussed. [Preview Abstract] |
Friday, October 30, 2015 2:36PM - 2:48PM |
KC.00004: Global Monte Carlo Calculations for r-process Nucleosynthesis Matthew Mumpower, Rebecca Surman, Ani Aprahamian The rapid neutron capture process is believed to be responsible for the production of approximately half of the heavy elements above iron on the periodic table. Nuclear physics properties (e.g. nuclear masses, neutron capture rates, $\beta$-decay rates, and $\beta$-delayed neutron emission branching ratios) are critical inputs that go into theoretical calculations of this nucleosynthesis process. We highlight the current capabilities of nuclear models to reproduce the pattern of solar $r$-process residuals by performing global Monte Carlo variations of the uncertain nuclear physics inputs. We also explore the reduction in uncertainties that may arise from new measurements or improved modeling and discuss the implications for using abundance pattern details to constrain the site of the $r$ process. [Preview Abstract] |
Friday, October 30, 2015 2:48PM - 3:00PM |
KC.00005: Time-Dependent Collective Neutrino Oscillations in Supernovae Sajad Abbar, Huaiyu Duan Neutrinos can experience self-induced flavor conversion in core-collapse supernovae due to neutrino-neutrino forward scattering. Previously a stationary supernova model, the so called ``neutrino bulb model,'' was used exclusively to study collective neutrino oscillations in the core-collapse supernova. We show that even a small time-dependent perturbation in neutrino fluxes on the surface of the proto-neutron star can lead to fast varying collective oscillations at large radii. This result calls for time-dependent supernova models for the study of collective neutrino oscillations. [Preview Abstract] |
Friday, October 30, 2015 3:00PM - 3:12PM |
KC.00006: Characteristics of hybrid compact stars with a sharp hadron-quark interface Sophia Han, Mark Alford We describe two aspects of the physics of hybrid stars that have a sharp interface between a core of quark matter and a mantle of nuclear matter. Firstly, we analyze the mass-radius relation. We describe a generic ``CSS'' parameterization of the quark matter equation of state (EoS), in which the speed of sound is independent of density. In terms of the three parameters of the CSS EoS we obtain the phase diagram of possible forms of the hybrid star mass-radius relation, and we show how observational constraints on the maximum mass and typical radius of neutron stars can be expressed as constraints on the CSS parameters. Secondly, we propose a mechanism for the damping of density oscillations, including r-modes, in hybrid stars with a sharp interface. The dissipation arises from the periodic conversion between quark matter and nuclear matter induced by the pressure oscillations in the star. We find the damping grows nonlinearly with the amplitude of the oscillation and is powerful enough to saturate an r-mode at very low saturation amplitude, of order $10^{-10}$, which is compatible with currntly-available observations of neutron star spin frequencies and temperatures. [Preview Abstract] |
Friday, October 30, 2015 3:12PM - 3:24PM |
KC.00007: Heavy Ion Collisions and Tests of the Supernova Equation of State K. Hagel, M. Hempel, J.B. Natowitz, G. R\"opke, S. Typel, S. Wuenschel, R. Wada, M. Barbui, K. Schmidt Understanding the evolution of core-collapse supernovae and the properties of the neutrinosphere requires systematic information on the properties of nuclear matter at a wide range of densities and temperatures. Central collisions in heavy ion reactions at intermediate energies produce nuclear matter on a microscopic scale that has a wide range of density and temperature and thus provide the possibility of probing conditions similar to those of core-collapse supernovae. Hot early reaction stage sources in violent collisions of heavy ion reactions, denoted as femtonovae, are identified and analyzed in the context of a coalescence model. The analysis yields various quantities indicate that temperature and density similar to those near the neutrinosphere are achieved. These results from these analyses are compared to the results of various supernovae simulations and thus provide insight into the supernova equation of state and thus indicate which ingredients in the simulations are important. [Preview Abstract] |
Friday, October 30, 2015 3:24PM - 3:36PM |
KC.00008: Self Assembly and Elasticity of Nuclear Pasta Matthew Caplan, Chuck Horowitz, Don Berry, Andre da Silva Schneider While the outer crust of a neutron star is likely a solid ion lattice, the core consists of uniform nuclear matter at or above saturation density. In between, nuclei adopt exotic non-spherical geometries called ``nuclear pasta'' in order to minimize the nuclear attraction and Coulomb repulsion between protons. These structures have been well studied with both classical and quantum molecular dynamics, and their geometry can be predicted from the density, temperature, and proton fraction. Recent classical molecular dynamics simulations find evidence for a phase transition at $T \approx 0.5$ MeV, where simulations with low proton fractions undergo a solid-liquid phase transition, while simulations with high proton fractions under a glass-rubber phase transition. This is expected to have nontrivial consequences for the elastic properties of the pasta. Additionally, recent observations indicate that the structure of nuclear pasta may be related to structures observed in biophysics, specifically self assembling lipid membranes. [Preview Abstract] |
Friday, October 30, 2015 3:36PM - 3:48PM |
KC.00009: Photo-Induced depopulation of the $^{180m}$Ta isomer Megha Bhike, Fnu Krishichayan, W. Tornow The $^{180m}$Ta nucleus is the rarest isotope in the universe, existing only in an isomeric state at 77.2 keV (J$^\pi$ = 9$^{-}$) with half-life of greater than 7.1$\times $10$^{15}$ years. The stellar production of this high-spin isomer has been a challenging astrophysical problem. Cross-section measurements for the depopulation of the $^{180m}$Ta isomer with monoenergetic photon beams of energies 2.5 and 3.1 MeV have been carried out at the HI$\gamma $S facility. The activated Ta foils of natural abundance and containing 14.4 mg of $^{180m}$Ta were $\gamma $-ray counted at TUNL's low background facility using a 13\% planar HPGe detector. A 8$''$$\times $12$''$ NaI detector in combination with the standard HI$\gamma $S scintillator paddle system was employed for absolute photon-flux determination. Preliminary results will be discussed, and measurements at lower energies are planned. [Preview Abstract] |
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