Bulletin of the American Physical Society
3rd Joint Meeting of the APS Division of Nuclear Physics and the Physical Society of Japan
Volume 54, Number 10
Tuesday–Saturday, October 13–17, 2009; Waikoloa, Hawaii
Session LH: Mini-Symposium on Direct Reactions Involving Unstable Nuclei II |
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Chair: Bob Tribble, Texas A&M University Room: Kings 3 |
Saturday, October 17, 2009 2:00PM - 2:30PM |
LH.00001: Direct Reactions studies at RIBF new facility Invited Speaker: The RIKEN RI Beam Factory (RIBF) is the firs realization of new-generation facilities with beams of unstable nuclei. It is based on heavy-ion primary beams accelerated to 345 MeV/nucleon for all elements up to uranium. When the goal intensity, 1 p$\mu $A, is reached, RIBF allows production of about thousand new isotopes with the yield higher than 1 particle par day. Since the RI beam energy after production by in-flight fission and/or projectile fragmentation is be around 200-300 MeV/nucleon, the direct reaction is one of the useful processes for spectroscopy of uncle very far from the stability. Several experiments were proposed and a few of them have been performed with intense (currently) $^{48}$Ca primary beams. The ZeroDegree Spectrometer, which is already operational, can be used to identify the product of a direct reaction in inverse kinematics coupled with, for example, measurement of $\gamma $-rays from excited states in the product. Construction of SAMURAI, a large-acceptance spectrometer, has been started. Decays of unbound states or breakup products from various types of direct reaction will be measured in coincidence. Experimental and theoretical issues to be considered for the specific conditions in this new opportunity will be discussed together with brief overview of near-term research. [Preview Abstract] |
Saturday, October 17, 2009 2:30PM - 2:45PM |
LH.00002: Low-lying states in $^{32}$Mg studied by proton inelastic scattering Satoshi Takeuchi Low-lying excited states in the neutron-rich nucleus $^{32}$Mg were studied by proton inelastic scattering in inverse kinematics via an in-beam $\gamma $-ray spectroscopy technique. Populated states were identified by measuring de-excitation $\gamma $ rays, in which five new states were found by $\gamma -\gamma $ coincidence analyses. The differential cross sections were analyzed by using coupled-channel calculations to determine the transferred angular momenta and the amplitudes of individual transitions. The spin and parity of the 2321-keV state was assigned as 4$^{+}$. The ratio between the energies of the 2$^{+}$ and 4$^{+}$ states indicates that $^{32}$Mg is a transitional nucleus rather than an axially deformed rigid rotor. A candidate for the 3$^{-}$ state was found at an excitation energy of 3115 keV, which is lower than the 3$^{-}$ energies in other N = 20 isotones. A small B(E3) value of 0.6 W.u. suggests a single-particle nature. The collectivities in the nucleus $^{32}$Mg with N = 20 are discussed based on the results obtained in the present experiment. [Preview Abstract] |
Saturday, October 17, 2009 2:45PM - 3:00PM |
LH.00003: Mechanisms in knockout reactions D. Bazin, R.J. Charity, R.T. de Souza, M.A. Famiano, A. Gade, V. Henzl, D. Henzlova, S. Hudan, J. Lee, S. Lukyanov, W.G. Lynch, S. McDaniel, M. Mocko, A. Obertelli, A.M. Rogers, L.G. Sobotka, J.R. Terry, J.A. Tostevin, M.B. Tsang, M.S. Wallace We report on the first detailed study of the mechanisms involved in knockout reactions, via a coincidence measurement of the residue and fast proton in one-proton knockout reactions, using the S800 spectrograph in combination with the HiRA detector array at the NSCL. Results on the reactions $^{9}$Be($^{9}$C,$^{8}$B+X)Y and $^{9}$Be($^{8}$B,$^{7}$Be+X)Y are presented. They are compared with theoretical predictions for both the diffraction (elastic breakup) and stripping (inelastic breakup) reaction mechanisms, as calculated in the eikonal model. The data shows a clear distinction between the two reaction mechanisms, and the observed respective proportions are very well reproduced by the reaction theory. This agreement supports the results of knockout reaction analyses and their applications to the spectroscopy of rare isotopes. In particular, this add considerable support to the use of the eikonal model as a quantitative tool, able, for example, to determine single-particle spectroscopic strengths in rare isotopes. [Preview Abstract] |
Saturday, October 17, 2009 3:00PM - 3:15PM |
LH.00004: Studying the neutron-unbound $^{18}$B A. Spyrou, T. Baumann, D. Bazin, G. Christian, S. Mosby, M. Strogman, M. Thoennessen, J. Brown, P.A. DeYoung, A. DeLine, J.E. Finck, A. Russel, N. Frank, E. Breitbach, R. Howes, W.A. Peters, A. Schiller The decay of the neutron-unbound ground state of $^{18}$B was studied for the first time through a single-proton knockout reaction from a 62 MeV/u $^{19}$C beam. The decay energy spectrum was reconstructed from coincidence measurements between the emitted neutron and $^{17}$B fragment using the MoNA/Sweeper setup. An $s$-wave line shape was used to describe the experimental spectrum resulting in an upper limit for the scattering length of $-50$ fm which corresponds to a decay energy $< 10$ keV. Observing an $s$-wave decay of $^{18}$B provides an experimental verification that the ground state of $^{19}$C includes a large $s$ component. In addition, our results show that the $s-d$ mixing proposed for $^{19}$C is also present in $^{18}$B, therefore no clear signs of an inversion between the $s_{1/2}$ and $d_{5/2}$ orbitals can be suggested. [Preview Abstract] |
Saturday, October 17, 2009 3:15PM - 3:30PM |
LH.00005: Two-particle correlations in continuum dipole transitions in Borromean nuclei K. Hagino, H. Sagawa, T. Nakamura, S. Shimoura We discuss the energy and angular distributions of two emitted neutrons from the dipole excitation of typical weakly-bound Borromean nuclei, $^{11}$Li and $^6$He. To this end, we use a three-body model with a density dependent contact interaction between the valence neutrons. Our calculation indicates that the energy distributions for the valence neutrons are considerably different between the two nuclei, although they show similar strong dineutron correlations in the ground state to each other. This different behaviour of the energy distribution primarily reflects the interaction between the neutron and the core nucleus, rather than the interaction between the valence neutrons. That is, the difference can be attributed to the presence of $s$-wave virtual state in the neutron-core system in $^{11}$Li, which is absent in $^6$He. It is pointed out that the angular distribution for $^{11}$Li in the low energy region shows a clear manifestation of the strong dineutron correlation, whereas the angular distribution for $^{6}$He exhibits a strong anticorrelation effect. [Preview Abstract] |
Saturday, October 17, 2009 3:30PM - 3:45PM |
LH.00006: Subsystem correlations in soft E1 excitation of $^{11}$Li Yuma Kikuchi, Takayuki Myo, Masaaki Takashina, Kiyoshi Kat\={o}, Kiyomi Ikeda The $^{11}$Li nucleus has characteristc features of neutron-rich nuclei such as two-neutron halo structure and large $s$-wave mixing in the ground state, and has been studied with keen interest from both theoretical and experimental sides. Experimentally, the Coulomb breakup reactions have been performed to investigate the exotic features of $^{11}$Li, and significant $E1$ strength was measured at low excitation energy. However, the nature of this soft $E1$ excitation for $^{11}$Li is not clearly understood. To understand the nature of the soft $E1$ excitation, it is necessary to understand the complicated structure of $^{11}$Li, which contains both $^9$Li-$n$ and $n$- $n$ correlations. In the present study, we investigate soft $E1$ excitation for $^{11}$Li based on the core+$n$+$n$ three-body model. We analyze the $E1$ strength as a function of relative energies in binary subsystems in $^{11}$Li, and discuss the correlations of $^9$Li-$n$ and $n$-$n$ subsystems through the soft $E1$ excitation. [Preview Abstract] |
Saturday, October 17, 2009 3:45PM - 4:00PM |
LH.00007: Inclusive Coulomb breakup of $^{22}$C and $^{31}$Ne Nobuyuki Kobayashi, Takashi Nakamura, Yosuke Kondo, Yoshiteru Sato One-neutron removal cross section of $^{31}$Ne and two neutron- removal cross section of $^{22}$C on Pb and C targets were measured at about 230 MeV/nucleon at RIBF(RI-Beam Factory) at RIKEN. This experiment aims at extracting the inclusive Coulomb breakup cross sections of these nuclei to see if these nuclei can involve halo structures. The enhancement of low- energy E1 strength, called ``soft E1 excitation'' is a unique property of halo nuclei. Hence, the enhanement of Coulomb breakup cross section can be used as a direct signal for halo structuresi. This measurement was made as one of experiments of ``Day-One campaign'' using $^{48}$Ca primary beam at 345MeV/nucleon. The typical secondary-beam intensities of 5-10 cps were obtained oth for $^{22}$C and $^{31}$Ne. The result indeed showed the significant enhancement of 1n(2n) removal cross sections for $^{31}$Ne($^{22}$C). This result thus shows the occurence of soft E1 excitations, which suggests the 1n and 2n halo structures for $^{31}$Ne and $^{22}$C, respectively. We discuss also the possible shell melting in 31Ne using both nuclear and Coulomb breakup data. [Preview Abstract] |
Saturday, October 17, 2009 4:00PM - 4:15PM |
LH.00008: Inelastic Proton Scattering on 21Na in Inverse Kinematics Roby Austin R.A.E. Austin, R. Kanungo, S. Reeve, Saint Mary's University; D.G. Jenkins, C.Aa.Diget, A. Robinson, A.G. Tuff, O. Roberts, University of York, UK; P.J. Woods, T. Davinson, G. J. Lotay, University of Edinburgh; C.-Y. Wu, Lawrence Livermore National Laboratory; H. Al Falou, G.C. Ball, M. Djongolov, A. Garnsworthy, G. Hackman, J.N. Orce, C.J. Pearson, S. Triambak, S.J. Williams, TRIUMF; C. Andreiou, D.S. Cross, N. Galinski, R. Kshetri, Simon Fraser University; C. Sumithrarachchi, M.A. Schumaker, University of Guelph; M.P. Jones, S.V. Rigby, University of Liverpool; D. Cline, A. Hayes, University of Rochester; T.E. Drake, University of Toronto; We describe an experiment and associated technique [1] to measure resonances of interest in astrophysical reactions. At the TRIUMF ISAC-II radioactive beam accelerator facility in Canada, particles inelastically scattered in inverse kinematics are detected with Bambino, a $\Delta$E-E silicon telescope spanning 15-40 degrees in the lab. We use the TIGRESS to detect gamma rays in coincidence with the charged particles to cleanly select inelastic scattering events. We measured resonances above the alpha threshold in {$^{22}$Mg} of relevance to the rate of break-out from the hot-CNO cycle via the reaction {$^ {18}$Ne}($\alpha$,p){$^{21}$Na}. [1] PJ Woods et al. Rex-ISOLDE proposal 424 Cern (2003). [Preview Abstract] |
Saturday, October 17, 2009 4:15PM - 4:30PM |
LH.00009: Elastic scattering of polarized protons from neutron-rich helium isotopes at 71 MeV/A Satoshi Sakaguchi In recent studies of unstable nuclei, increasing interest has been atracted by the manifestations of spin-dependent interactions in nuclei. Spin-dependent interactions such as tensor, spin-orbit, and three-nucleon forces have been investigated via direct reactions of polarized light ions. Application of this method to the physics of unstable nuclei should provide us with new information on the effects of spin-dependent interactions on the structure and reaction mechanisms involving unstable nuclei. For this purpose, we have successfully developed a solid polarized proton target which can be operated under a low magnetic field of 0.1~T and at high temperature of 100~K. Making use of this target, we measured vector analyzing powers for the elastic scattering of polarized protons from neutron-rich helium isotopes, $^6$He and $^8$He, at 71~MeV/A. Features of spin-orbit potential between protons and neutron- rich helium isotopes have been extracted by the phenomenological optical model analysis. [Preview Abstract] |
Saturday, October 17, 2009 4:30PM - 4:45PM |
LH.00010: Relativisitic impulse approximation analysis of unstable nickel isotopes: 48-82Ni Kaori Kaki Recent relativistic mean-field calculations have provided nuclear distributions of Ni isotopes whose mass numbers are 48 through 82. We calculate observables of proton elastic scattering from these unstable isotopes and discuss relations between observables and nuclear distributions of such unstable nuclei. The calculations are based on relativistic impulse approximation (RIA) at incident proton energies: 300 and 400 MeV where predictions of RIA have been shown to provide good agreement with experimental data. [Preview Abstract] |
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