Bulletin of the American Physical Society
APS April Meeting 2017
Volume 62, Number 1
Saturday–Tuesday, January 28–31, 2017; Washington, DC
Session J12: Nuclear Reactions and Astrophysics II |
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Sponsoring Units: DNP Chair: David Dean, ORNL Room: Roosevelt 4 |
Sunday, January 29, 2017 10:45AM - 10:57AM |
J12.00001: $\beta^-$-Decay Study of the Deformed, Neutron-Rich Nucleus $^{160}$Eu D.J. Hartley, F.G. Kondev, G. Savard, A.D. Ayangeakaa, S. Bottoni, M.P. Carpenter, J.A. Clark, C.R. Hoffman, R.V.F. Janssens, T. Lauritsen, S. Zhu, P. Copp, C. Hu, H. Zhang, Y. Zhu, A. Nystrom, R. Orford, J. Sethi A new experimental program at Argonne National Laboratory has been initiated in the pursuit of understanding the structure of deformed nuclei in the $A\approx 160$ region. Of particular interest is the evolution of collectivity towards mid shell ($N=104$). In addition, these results will provide valuable information for understanding the rare-earth ``pygmy peak" in the r-process abundance distribution. A high-purity beam of $^{160}$Eu from the CARIBU facility was directed onto the SATURN moving tape system. The X-Array spectrometer (with multiple Ge clover detectors and plastic scintillators) was placed around the tape system for $\gamma$ and $\beta$ detection. Two $\beta$-decaying states in $^{160}$Eu were observed, and the decay scheme of the $^{160}$Gd daughter nucleus was greatly extended. Multi-quasiparticle blocking calculations were performed to help interpret the structure of the parent and daughter states. [Preview Abstract] |
Sunday, January 29, 2017 10:57AM - 11:09AM |
J12.00002: Determining fragmentation dynamics through a study of neutron multiplicity at the NSCL Sharon Stephenson, Peter Christ, Maria Mazza In nuclear fragmentation reactions the number of neutrons and the excitation energy of the final fragment are related to the excitation energies of prefragments, which are produced in the reaction target but not directly observed. The MoNA Collaboration designed and performed an experiment to measure the number of neutrons in coincidence with charged projectile fragments to determine the excitation mechanisms of specific prefragments. All prior MoNA experimental campaigns concentrated on neutrons emitted from discrete levels in near dripline nuclei and treated any evaporation neutrons as an underlying background. This experiment capitalizes on those evaporation neutrons, focusing on sodium, neon and fluorine reaction products. For the experiment a $^{32}$Mg secondary beam with energy 86 MeV/u was incident on a Be reaction target. This target is upstream from the Sweeper, a superconducting dipole steering magnet with a bending angle of 43$^{\circ}$ and a vertical gap that permits forward-focused neutrons to get to the MoNA, the Modular Neutron Array. The rigidity of the Sweeper was varied during this experiment to increase the detection range. Analysis of the neutron-neutron hit distribution in coincidence with each sodium, neon, or fluorine charged fragment will be presented. [Preview Abstract] |
Sunday, January 29, 2017 11:09AM - 11:21AM |
J12.00003: Supernovae neutrino pasta interaction Zidu Lin, Charles Horowitz, Matthew Caplan, Donald Berry, Luke Roberts In core-collapse supernovae, the neutron rich matter is believed to have complex structures, such as spherical, slablike, and rodlike shapes. They are collectively called "nuclear pasta". Supernovae neutrinos may scatter coherently on the "nuclear pasta" since the wavelength of the supernovae neutrinos are comparable to the nuclear pasta scale. Consequently, the neutrino pasta scattering is important to understand the neutrino opacity in the supernovae. In this work we simulated the "nuclear pasta" at different temperatures and densities using our semi-classical molecular dynamics and calculated the corresponding static structure factor that describes $\nu$-pasta scattering. We found the neutrino opacities are greatly modified when the "pasta" exist and may have influence on the supernovae neutrino flux and average energy. Our neutrino-pasta scattering effect can finally be involved in the current supernovae simulations and we present preliminary proto neutron star cooling simulations including our pasta opacities. [Preview Abstract] |
Sunday, January 29, 2017 11:21AM - 11:33AM |
J12.00004: Low energy excitations of the neutron star core Sanjay Reddy I will summarize recent work on low energy excitations in cold dense matter and its implications for thermal and transport properties, and seismology of neutron stars. I argue that a low energy Lagrangian with a handful of low energy constants (LECs) provides an adequate framework for calculations. The LECs can be related to the equation of state of dense matter at zero temperature. [Preview Abstract] |
Sunday, January 29, 2017 11:33AM - 11:45AM |
J12.00005: Astrophysical reaction rates from a symmetry-informed first-principles perspective Alison Dreyfuss, Kristina Launey, Robert Baker, Jerry Draayer, Tomas Dytrych With a view toward a new unified formalism for studying bound and continuum states in nuclei, to understand stellar nucleosynthesis from a fully \emph{ab initio} perspective, we studied the nature of surface $\alpha$-clustering in $^{20}$Ne by considering the overlap of symplectic states with cluster-like states. We compute the spectroscopic amplitudes and factors, $\alpha$-decay width, and absolute resonance strength -- characterizing major contributions to the astrophysical reaction rate through a low-lying $1^-$ resonant state in $^{20}$Ne. As a next step, we consider a fully microscopic treatment for the $\mathrm{n}+^4$He system, based on the successful first-principles No-Core Shell Model/Resonating Group Method (NCSM/RGM) for light nuclei, but with the capability to reach intermediate-mass nuclei. The new model takes advantage of the symmetry-based concept central to the Symmetry-Adapted No-Core Shell Model (SA-NCSM) to reduce computational complexity in physically-informed and methodical way, with sights toward first-principles calculations of rates for important astrophysical reactions, such as the $^{23}\mathrm{Al}(\mathrm{p},\,\gamma)^{24}\mathrm{Si}$ reaction, believed to have a strong influence on X-ray burst light curves. [Preview Abstract] |
Sunday, January 29, 2017 11:45AM - 11:57AM |
J12.00006: A measurement of the parity violating asymmetry in the neutron capture on $^3$He at the SNS. Latiful Kabir Studies of parity violating (PV) observables in hadronic systems offer a unique probe of nucleon structure, complementary to other probes of low-energy non-perturbative QCD. The n-$^3$He experiment at the Spallation Neutron Source at the ORNL measures the PV asymmetry of the recoil proton momentum $\vec{k}_p$ with respect to the neutron spin $\vec{\sigma}_n$ in the reaction $\rm n + {}^3He \to p + T + 764\,keV$. This asymmetry is sensitive to the isospin-conserving and isospin-changing ($\Delta$I = 0, 1) channels of the Hadronic Weak Interaction, and is expected to be extremely small ($\sim 10^{-7}$). The experiment will determine this PV asymmetry with the statistical sensitivity of the order of $10^{-8}$. Challenges like beam fluctuation, pedestal and background subtraction, instrumental interference, detector correlations and many others must be considered very carefully in the analysis to achieve this precision. I will discuss the data analysis and a method to extract the value for the PV asymmetry. [Preview Abstract] |
Sunday, January 29, 2017 11:57AM - 12:09PM |
J12.00007: PUSHing Core-Collapse Supernovae to Explosions in Spherical Symmetry: Explodability and Nucleosynthesis Yields Sanjana Sinha, Kevin Ebinger, Carla Frohlich, Albino Perego, Matthias Hempel, Matthias Liebendoerfer, F.-K. Thielemann Core-collapse supernovae (CCSNe) are the highly energetic deaths of massive stars. They play a vital role in the synthesis and dissemination of many chemical elements. CCSN nucleosynthesis calculations have previously relied on artificial explosion methods that do not adequately capture the physics of the innermost stellar layers. Multidimensional simulations currently being performed to fully unravel the explosion mechanism of CCSNe are very computationally expensive. The PUSH method, calibrated against SN1987A, provides parametrized spherically symmetric models that follow the consistent evolution of the proto-neutron star as well as the electron fraction of the ejecta. This method is computationally affordable and captures the physics relevant for nucleosynthesis calculations. Here, we present the results of a broad study that investigates the explodability and nucleosynthesis yields of progenitors covering a wide range of ZAMS masses. Comparisons of the predicted explosion properties and yields with observational CCSNe and metal-poor star data will also be presented. The complete set of nucleosynthesis yields will be a valuable input to models of galactic chemical evolution. [Preview Abstract] |
Sunday, January 29, 2017 12:09PM - 12:21PM |
J12.00008: Ab initio predictions of the symmetry energy and recent constraints. Francesca Sammarruca The symmetry energy plays a crucial role in the structure and the dynamics of neutron-rich systems, including the formation of neutron skins, the location of neutron drip lines, as well as intriguing correlations with the structure of compact stars. With experimental efforts in progress or being planned to shed light on the less known aspects of the nuclear chart, microscopic predictions based on ab initio approaches are very important. In recent years, chiral effective field theory has become popular because of its firm connection with quantum chromodynamics and its systematic approach to the development of nuclear forces. Predictions of the symmetry energy obtained from modern chiral interactions will be discussed in the light of recent empirical constraints extracted from heavy ion collisions at 400 MeV per nucleon at GSI [1]. Applications of our equations of state [2] to neutron-rich systems will also be discussed, with particular emphasis on neutron skins, which are sensitive to the density dependence of the symmetry energy. [1] P. Russotto {\it et al.}, arXiv:1608.04332. [2] F. Sammarruca {\it et al.}, Phys. Rev. C {\bf 91}, 054311 (2015). [Preview Abstract] |
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