Bulletin of the American Physical Society
4th Joint Meeting of the APS Division of Nuclear Physics and the Physical Society of Japan
Volume 59, Number 10
Tuesday–Saturday, October 7–11, 2014; Waikoloa, Hawaii
Session DE: Mini-Symposium on Unbound Nuclear States and Correlations Beyond the Drip Lines |
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Chair: Koichi Hagino, Tohoku University Room: King's 1 |
Thursday, October 9, 2014 9:00AM - 9:30AM |
DE.00001: One- and two neutron decay of light neutron-rich nuclei Invited Speaker: Michael Thoennessen Direct reactions with radioactive beams have been used very successfully to populate and measure nuclei beyond the neutron dripline and neutron unbound excited states of nuclei close to the neutron dripline. The use of different reactions (for example neutron removal and proton removal) to populate the same final nucleus can be used to selectively populate different states. Recent results from the MoNA-LISA setup at the NSCL, including $^{10}$He, $^{10,11}$Li, and $^{12,13}$Be will be presented. [Preview Abstract] |
Thursday, October 9, 2014 9:30AM - 9:45AM |
DE.00002: Determining the Energy Gap Between the sd-pf Neutron Shells in $^{25}$O Michael Jones, Nathan Frank, Paul DeYoung, Thomas Baumann, Zach Kohley, Jenna Smith, Artemis Spyrou, Krystin Stiefel, Anthony Kuchera, Michael Thoennessen The excited states of $^{25}$O, particularly those of negative parity, are of great interest for determining the evolution of the \emph{sd-pf} shell gap in and around the ``island of inversion.'' Shell Model (WBBS) calculations tuned to nearby $^{27}$Ne predict the 3/2$^{-}$ state in $^{25}$O to be only 500 keV above the ground state, implying the \emph{sd-pf} shell gap to be small. Hence it is likely for nuclei beyond N=16 to have mixing between the 0d$_{3/2}$ and 1p$_{3/2}$ orbitals. A recent experiment, performed at the NSCL, populated $^{25}$O through use of a $(d,p)$ reaction. Using the Ursinus College Liquid Hydrogen Target, an $^{24}$O beam impinged on a deuterium target cell with a thickness of 400 mg/cm$^{2}$ at a rate of approximately 30 pps to produce $^{25}$O, which decayed immediately by neutron emission. The resulting charged fragments were bent by the Sweeper magnet into a suite of charged particle detectors, while the neutrons traveled unimpeded towards MoNA (Modular Neutron Array) and LISA (Large multi-Institutional Scintillator Array). Together, MoNA-LISA and the Sweeper provide a full kinematic measurement from which the decay energy of the 2-body system can be determined. Preliminary results will be discussed. [Preview Abstract] |
Thursday, October 9, 2014 9:45AM - 10:00AM |
DE.00003: Experimental study of $^{25}$O and $^{26}$O by using SAMURAI at RIBF Yosuke Kondo The unbound nuclei $^{25}$O and $^{26}$O are studied by means of invariant mass method by using the SAMURAI spectrometer at RIBF. These nuclei are located in the region of the neutron drip line anomaly, where limit of nuclear stability suddenly changes from oxygen to fluorine. According to recent theoretical studies, three nucleon forces are essential to explain the location of the neutron drip line of oxygen isotopes. Available experimental information on oxygen isotopes beyond the drip line is not sufficient to test these theories. In addition, $^{26}$O has attracted much attention as a candidate of two neutron radioactivity. Currently, experimental studies provide only upper limit of the ground state energy, which is important to understand possible long lifetime of $^{26}$O. In the present study, $^{25}$O and $^{26}$O are produced by one-proton removal reactions from $^{26}$F and $^{27}$F, respectively. Decay products, $^{24}$O and neutrons are detected by heavy ion detectors and the large acceptance neutron detector array NEBULA. Experimental results will be discussed in the presentation. [Preview Abstract] |
Thursday, October 9, 2014 10:00AM - 10:15AM |
DE.00004: Simulation of two neutron detection for invariant mass spectroscopy of unstable nuclei Jyunichi Tsubota Two neutron detection in invariant mass spectroscopy is essential to study neutron rich nuclei near and beyond neutron drip line. Recently, Coulomb breakup measurements of ${}^{19}$B and ${}^{22}$C, and study of the unbound nucleus ${}^{26}$O were performed at RIBF. Goal of the Coulomb breakup measurements is to study di-neutron like correlation, while ${}^{26}$O is interesting as a candidate of two neutron radioactivity. In these measurements, decay products, ${}^{24}$O and two neutrons from ${}^{26}$O, for example, are detected in coincidence by SAMURAI spectrometer. The neutrons are detected by large acceptance plastic scintillator array NEBULA. If a neutron scatters twice or more, this may cause a fake signal (crosstalk), and become a background. The crosstalk background can be eliminated by causality cut using time, position, pulse height information. The cut condition is investigated by a Monte-Carlo simulation based on the Geant4 tool kit to obtain high detection efficiency with small crosstalk background. The simulation is compared with experimental data of quasi-monoenergetic neutrons at 200 MeV and 250 MeV produced in the ${}^7$Li(p,n)${}^7$Be(g.s.+0.43 MeV) reaction. A new algorithm of crosstalk cut will also be discussed. [Preview Abstract] |
Thursday, October 9, 2014 10:15AM - 10:30AM |
DE.00005: One-neutron knockout reaction from 20C Jongwon Hwang Recent researches in neutron-rich nuclei have demonstrated that the depth of each single-particle level varies from that in stable nuclei : some of the well-known magic numbers disappear and new shell closures develop. Cross-shell excitation, transition of a nucleon across a shell gap, can be exploit to probe changes in shell structure. The present work aims at exploration of neutron-unbound states of $^{19}$C, especially a hole- state populated by cross-shell excitation, via a one-neutron knockout reaction. The experiment was performed at the RIBF facility in RIKEN. A $^{20}$C secondary beam produced by BigRIPS with an energy of 280 MeV/nucleon impinged on a carbon target placed before the SAMURAI spectrometer. By taking full advantage of the analyzer system comprised of a large-acceptance super-conducting dipole magnet, associated tracking detectors, and a large volume neutron detector system, an invariant mass spectrum for the system of $^{18}$C$ + n$ was reconstructed. Three unbound excited states in $^{19}$C were identified including the unknown $1/2^{+}_{1}$ state at 2.90 MeV in excitation energy. Details of the measurement and analysis along with results will be presented. [Preview Abstract] |
Thursday, October 9, 2014 10:30AM - 10:45AM |
DE.00006: Search for unbound $^{15}$Be states in the 3-neutron decay channel A.N. Kuchera, A. Spyrou, J.K. Smith, T. Baumann, J. Brown, P.A. DeYoung, N. Frank, M. Jones, Z. Kohley, M. Thoennessen The first observation of $^{15}$Be has been made recently using a (d,p) reaction from a $^{14}$Be beam. The observed resonance at 1.8(1) MeV, which decays to the ground state in $^{14}$Be, was tentatively assigned to have a spin and parity of 5/2$^{+}$. Prior to this result, a two-proton knockout reaction from $^{17}$C, which was expected to populate the 3/2$^{+}$ state, did not produce a significant number of one-neutron decay events from $^{15}$Be. It was suggested that any states populated in this way should then decay through the first excited unbound state of $^{14}$Be [2]. Both experiments used the Modular Neutron Array (MoNA) together with the Sweeper dipole magnet and charged particle detectors to make measurements of the neutron-fragment coincidences from the decay of the neutron-unbound systems. The two-proton knockout data were recently analyzed to search for the predicted, yet unobserved, 3/2$^{+}$ state in the 3-neutron decay channel to $^{12}$Be. Through simultaneous fitting of the reconstructed decay energy spectra and other experimental observables, limits on the potential 3/2$^+$ state will be presented.\\[4pt] [1] J. Snyder et al., Phys. Rev. C 88, 031303(R) (2013).\\[0pt] [2] A. Spyrou et al., Phys. Rev. C 84, 044309 (2011). [Preview Abstract] |
Thursday, October 9, 2014 10:45AM - 11:00AM |
DE.00007: Dinucleon correlation enhancement in p-shell nuclei Fumiharu Kobayashi, Yoshiko Kanada-En'yo Dinucleon (dineutron or diproton) correlation is one of the most attractive phenomena in the physics of unstable nuclei. A dinucleon is a pair of two nucleons coupled to spin-singlet having a strong spatial correlation, considered to be a bosonic cluster. Dinucleon correlation would be significant for the description of the valence nucleon motion at the nuclear surface of unstable nuclei. To clarify the formation mechanism and the universal properties of dinucleon correlation, we have constructed a framework of dinucleon condensate (DC) wave function, which can describe the detailed dinucleon motion around a core which can be deformed and excited. In this work, we use the antisymmetrized molecular dynamics (AMD) wave functions and the DC wave functions to analyze the effect of the core structure, especially the occupied orbits by the core nucleons, on dinucleon formation in p-shell neutron-rich and proton-rich nuclei (e.g. neutron-rich Li isotopes and proton-rich O isotones). We will show that the LS-favored $0p_{3/2}$ orbits play an important role to dissociate dinucleon components to $j$-$j$ coupling shell-model components without spatial correlation and that the occupation probability of $0p_{3/2}$ would be a key to the enhancement of dinucleon correlation in p-shell nuclei. [Preview Abstract] |
Thursday, October 9, 2014 11:00AM - 11:15AM |
DE.00008: tetra neutron system studied by exothermic double-charge exchange reaction $^4$He($^8$He, $^8$Be)4n Keiichi Kisamori A possible existence of the tetra-neutron system as a resonance state is still an open and fascinating question, while theoretical papers using ab-initio calculation suggests that the bound tetra-neutron does not exist. We have performed a missing-mass spectroscopy of the 4n system via the exothermic double-charge exchange reaction $^4$He($^8$He,$^8$Be)4n. The experiment was carried out at the RIBF at RIKEN using the SHARAQ spectrometer and the liquid He target system. Since the secondary beam, $^8$He at 190A MeV, has a large internal energy, it is possible to produce the 4n system in small momentum transfers of less than 20 MeV/c. In the present analysis, a new analytical framework to treat multi-particles under high beam rate condition (2 MHz) was developed for good statistics. At the SHARAQ spectrometer, $^8$Be can be identified by measuring the invariant mass of the coincident two-alpha particle with a good signal-to-noise ratio. About several tens of candidate events are obtained above the 4n threshold. We will show the preliminary result of missing-msss spectrum and discuss the shape of spectrum. [Preview Abstract] |
Thursday, October 9, 2014 11:15AM - 11:30AM |
DE.00009: Microscopic study of soft and giant dipole resonances in $^{6}$He Wataru Horiuchi, Daichi Mikami, Yasuyuki Suzuki We present our recent work on the electric dipole response (E1) of $^{6}$He with a fully microscopic six-body calculation [Ref: Phys. Rev. C 89, 064303 (2014)]. The wave functions of the ground and excited states are expressed as a combination of explicitly correlated Gaussians. Final state interactions of three-body decay channels are explicitly taken into account. The ground state properties and the low-lying E1 strength are obtained consistently with observations. Two-peak structure appears in the strength function. The peak at the high-energy region indicates a typical macroscopic picture of the giant dipole resonance, the out-of-phase proton-neutron motion, whereas the lower-lying peaks exhibit in-phase proton-neutron motion in the internal region, out-of-phase motion near the surface region, and spatially extended neutron oscillation, indicating a soft-dipole mode and its vibrationally excited mode. The compressional dipole strength is also examined and discussed in relation to the soft-dipole mode. [Preview Abstract] |
Thursday, October 9, 2014 11:30AM - 11:45AM |
DE.00010: Two-neutron decay from the $2^+_1$ state of $^6$He Yuma Kikuchi, Takuma Matsumoto, Kosho Minomo, Kazuyuki Ogata Two-neutron correlations in neutron-rich nuclei have attracted much attention. In paricular, in two-neutron halo nuclei, such two-neutron correlations are characterized as a sptially-correlated $n$-$n$ pair, the so-called dineutron, and have extensively studied from both theoretical and experimental sides. In this contribution, we investigate the dineutron correlation in the excited resonant state of $^6$He. To do it, we calculate the breakup cross section of $^6$He+$^{12}$C reaction at 240 MeV/nucleon, in which the $2^+_1$ resonant state is populated. From the results, we pin down the decay mode of the $2^+_1$ state, and discuss the exsistence of a dineutron in the excited state of $^6$He. [Preview Abstract] |
Thursday, October 9, 2014 11:45AM - 12:00PM |
DE.00011: Coupled-cluster computations of unbound states in neutron rich calcium isotopes Gaute Hagen In this talk I will present microscopic coupled-cluster computations of weakly bound and unbound states in the neutron rich calcium region. Starting from state-of-the-art nucleon-nucleon and schematic three-nucleon forces, the role of continuum on ordering of states close to and above the threshold is discussed. In particular several new level orderings are predicted that contrast the na\"ive shell-model ordering of states. We also discuss the possibility for Efimov physics around the very neutron rich calcium-62 by merging input from coupled-cluster calculations with halo effective-field-theory. [Preview Abstract] |
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