Bulletin of the American Physical Society
2012 Fall Meeting of the APS Division of Nuclear Physics
Volume 57, Number 9
Wednesday–Saturday, October 24–27, 2012; Newport Beach, California
Session 1WA: Workshop on And Here Be Dragons: Understanding the Nature of Unstable Isotopes |
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Chair: Jutta Escher, Lawrence Livermore National Laboratory, and Calvin Johnson, San Diego State University Room: Plaza I |
Wednesday, October 24, 2012 8:30AM - 9:00AM |
1WA.00001: Nuclear structure beyond the neutron drip line Invited Speaker: Artemis Spyrou Nuclei far from stability were shown to exhibit new structure characteristics as well as exotic decay modes. Modern facilities can populate and study light nuclei all the way to the neutron drip line and even beyond. At the National Superconducting Cyclotron Laboratory at Michigan State University the MoNA/Sweeper setup is used to measure the decay of neutron unbound nuclei and study their ground and excited state properties. In addition, for two-neutron unbound nuclei the correlations between the emitted neutrons can reveal the decay mode, i.e. whether the two-neutron decay corresponds to two sequential decays or to a simultaneous emission of the two neutrons. In this talk the most recent results of the MoNA collaboration will be presented, including the two-neutron decay of $^{26}$O, $^{16}$Be and $^{13}$Li. The results will be compared to theoretical predictions in order to provide constraints to these calculations. [Preview Abstract] |
Wednesday, October 24, 2012 9:00AM - 9:30AM |
1WA.00002: Living at the edge of stability: The role of continuum and three-nucleon forces Invited Speaker: Gaute Hagen Nuclear structure at and reactions at the dripline is a challenging undertaking. In order to give reliable predictions for future experiments and describing the exotic phenomena that occurs as we move beyond the valley of stability we need a theory that properly accounts for; (i) the effects of three nucleon forces, (ii) the presence of open decay channels and particle continuum, and (iii) many-nucleon correlations. We aim to fill this gap by using interactions from chiral effective field theory together with a schematic potential that accounts for the effects of three-nucleon forces, a Berggren basis that treats bound and continuum states on equal footing, and coupled-cluster theory to properly account for many-nucleon correlations. We apply this approach to the computation of binding energies, radii and excited states in the neutron rich oxygen [1] and calcium isotopes [2]. We show that effects of three-nucleon forces are essential in placing the dripline at $^{24}$O and for explaining the shell closure in $^{48}$Ca. We find a weak shell closure in $^{54}$Ca, assign spin and parities to several unknown levels in oxygen and calcium isotopes, and in particular we find due to continuum coupling the level ordering of the states in the \textit{gds} shell are reversed compared to the na\"ive shell model picture as we move towards $^{60}$Ca. We also note that a saturation of total binding sets in around $^{60}$Ca, indicating that the $^{60}$Ca is either a very weakly bound or unbound nucleus. By computing overlap functions we have also extended our approach to compute reaction observables, and we present results for low-energy elastic proton scattering on calcium isotopes. The results are promising and open up new possibilities to perform predictive and microscopic structure and reaction calculations towards the dripline. \\[4pt] [1] G. Hagen, M. Hjorth-Jensen, G. R. Jansen, R. Machleidt, and T. Papenbrock, Phys. Rev. Lett. (2012), arXiv:1204.3612v1 \\[0pt] [2] G. Hagen, M. Hjorth-Jensen, G. R. Jansen, R. Machleidt, and T. Papenbrock, arXiv:1202.2839v1 (2012) [Preview Abstract] |
Wednesday, October 24, 2012 9:30AM - 10:00AM |
1WA.00003: Linking Nuclear Reactions and Nuclear Structure on the Way to the Drip Line Invited Speaker: Willem Dickhoff The present understanding of the role of short- and long-range physics in determining proton properties near the Fermi energy for stable closed-shell nuclei has relied on data from the (e,e'p) reaction. Hadronic tools to extract such spectroscopic information have been hampered by the lack of a consistent reaction description that provides unambiguous and undisputed results. The dispersive optical model (DOM), originally conceived by Claude Mahaux, provides a unified description of both elastic nucleon scattering and structure information related to single-particle properties below the Fermi energy. The DOM provides the starting point to provide a framework in which nuclear reactions and structure data can be analyzed consistently to provide unambiguous spectroscopic information including its asymmetry dependence. Recent extensions of this approach include the treatment of non-locality to describe experimental data like the nuclear charge density based on information of the spectral density below the Fermi energy, the application of the DOM ingredients to the description of transfer reactions, a comparison of the microscopic content of the nucleon self-energy based on Faddeev-RPA calculations emphasizing long-range correlations with DOM potentials, and a study of the relation between a self-energy which includes the effect of short-range correlations with DOM potentials. The most recent Dom implementation currently in progress abandons the constraint of local potentials completely to allow an accurate description of various properties of the nuclear ground state. [Preview Abstract] |
Wednesday, October 24, 2012 10:00AM - 10:30AM |
1WA.00004: Resonant states in proton-rich nuclei for nuclear astrophysics Invited Speaker: J.C. Blackmon Our understanding of stellar explosions like novae and X-ray bursts is limited in large part by the dearth of experimental information on resonant states in proton-rich nuclei that govern nuclear reaction rates affecting energy production and nucleosynthesis. The Array for Nuclear Astrophysics Studies with Exotic Nuclei is a charged-particle detector array that is being used to measure (alpha,p) and proton scattering reactions to improve our understanding of the nuclear reaction rates important in stellar explosions. Results from the first experiments using ANASEN with beams of radioactive nuclei from the RESOLUT facility at Florida State University will be presented. Plans for experiments with ANASEN at the National Superconducting Cyclotron Laboratory will also be presented, along with the development of the Separator for Capture Reactions (SECAR) that will enable measurements of proton capture reactions using low energy beams of radioactive nuclei. [Preview Abstract] |
Wednesday, October 24, 2012 10:30AM - 10:45AM |
1WA.00005: COFFEE BREAK
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Wednesday, October 24, 2012 10:45AM - 11:15AM |
1WA.00006: Reaction Theory Advances For FRIB Invited Speaker: Ian Thompson Much interest in rare isotopes centers on how they interact with neutrons, in particular concerning their neutron capture rates in many slow and fast processes. Since neutrons do not make a target, we must use indirect methods, and use reaction theory to bridge the gap. Many of these indirect methods are transfer reactions, in which the rare isotope has a neutron or two added or subtracted. Most common is the (d,p) reaction to add one neutron, just as in a capture reaction. The deuteron is {\em itself} weakly bound, however, so I will review recent work on few-body methods to describe deuterons incident on heavy nuclei. When bound neutron states are measured, we can predict direct capture rates for neutrons. Continuum states for the neutron may also be produced in (d,p) reactions. If individual resonances can be distinguished, new theory can describe the transfer cross section in terms of R-matrix parameters for the neutron state. If, however, we have a high level density, then we must use energy averaging, giving a complex optical potential for the neutron. I review the theory of partial fusion needed in this case to describe the competition between breakup and compound-nucleus production. Neutron pickup reactions such as ($^3$He,$\alpha$) and (p,t) are also useful to probe hole states, and do not have competition with direct breakup. [Preview Abstract] |
Wednesday, October 24, 2012 11:15AM - 11:45AM |
1WA.00007: Nucleus as an Open System: New Effects and Theoretical Challenges Invited Speaker: Vladimir Zelevinsky As nuclear science moves in the direction of nuclei far from stability, the studies of nuclear structure and nuclear reactions become more and more interrelated. The main theoretical challenge is to find a consistent description of the nucleus as an open mesoscopic system coupled with continuum through real decay channels and through virtual excitations. The method using the effective non-Hermitian Hamiltonian [see review article: N. Auerbach and V. Zelevinsky, Rep. Prog. Phys. 74, 106301 (2011)] is one of the most promising theoretical approaches; it can be strictly derived from quantum many-body theory, it allows for calculating bound states, resonances and reaction cross sections in the unified framework, and it is quite flexible in practical applications. After explaining the main features of this theory, I will show the method at work (continuum shell model with predictions recently confirmed by the experiments with exotic oxygen isotopes, phenomenon of super-radiance, relation to the idea of doorway states, quantum signal transmission through mesoscopic systems) and discuss new theoretical challenges. [Preview Abstract] |
Wednesday, October 24, 2012 11:45AM - 12:15PM |
1WA.00008: Surrogate reactions for neutron capture with radioactive ion beams Invited Speaker: Jolie A. Cizewski Neutron capture reactions are responsible for most of the elements heavier than iron, through either the slow or rapid processes of nucleosynthesis. The r process in particular proceeds through very short-lived nuclei on which neutron capture reaction measurements will never be possible. Knowledge of neutron capture cross sections on short-lived nuclei is also important for applications such as nuclear energy, nuclear forensics, and stockpile stewardship science. When the level density at the neutron separation energy is relatively low, for example near closed neutron shells, direct neutron capture often dominates and direct neutron transfer reactions can provide the spectroscopic information needed to calculate the direct capture. However, when the level density is higher, a compound nucleus is formed and statistical mechanisms dominate the decay. While the formation of the compound nucleus can be calculated with optical models, modeling of the decay is less robust. Because of the importance of neutron capture on nuclei away from stability, there have been efforts to validate surrogate reactions for neutron capture that exploit the availability of beams of radioactive nuclei that interact with light targets where reaction products are measured in coincidence with gamma radiation. This talk would summarize efforts to validate a surrogate for neutron capture and the techniques being developed to measure these reactions with beams of radioactive ions. [Preview Abstract] |
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