Bulletin of the American Physical Society
2005 2nd Joint Meeting of the Nuclear Physics Divisions of the APS and The Physical Society of Japan
Sunday–Thursday, September 18–22, 2005; Maui, Hawaii
Session DC: Mini-symposium on Structure Changes of Asymmetric Nuclear Systems I |
Hide Abstracts |
Sponsoring Units: DNP JPS Chair: Hiroyoshi Sakurai, RIKEN Room: Ritz-Carlton Hotel Salon 2 |
Tuesday, September 20, 2005 7:00PM - 7:30PM |
DC.00001: Properties of Excited States in Exotic Nuclei Probed by Direct Reactions Invited Speaker: Direct reactions have been commonly used to populate excited states having characteristic quantum numbers depending on the probes such as proton, deuteron, alpha, Coulomb field, and so on. In RI-beam experiments, these probes are prepared as experimental target and excited states of beam or beam-like nucleus are identified by measuring all the decay products including $\gamma$-rays where the invariant masses are reconstructed to determine the excitation energies. It is noted that the Doppler-shift correction of the $\gamma$-ray from the moving nucleus is equivalent to reconstruct the invariant mass of the $\gamma$ and the residual nucleus. The combination of the inverse kinematics and the invariant mass spectroscopy has several advantages to overcome poor energy resolutions, weak intensities and a poor purity of RI beams. One of the advantages in an experimental point of view is that physical probe can be changed by replacing the experimental target without changing the configuration of the detector system. Another advantage is that spectroscopy of an exotic nucleus via different reaction processes such as inelastic scatterings, stripping and knockout reactions from different nuclei in an RI beam at the same time. Those facts provide us characteristics of excited states by directly comparing excitation spectra from different reactions and/or probes with the same acceptance. I present some of the recent experimental results on properties of exotic nuclei by using the above-mentioned techniques. [Preview Abstract] |
Tuesday, September 20, 2005 7:30PM - 7:45PM |
DC.00002: Lifetime measurement of $^{12}$Be(2$^+_1$) Nobuaki Imai We have measured the lifetime of the first $2^+$ state of $^{12}$Be in flight via the Doppler-shift attenuation method (DSAM). This is the first application of DSAM to the intermediate-energy unstable nucleus beam. The lifetime is inversely proportional to the $B$($E2$) value, which relates to the quadrupole deformation of charge distribution. It was suggested that the magic number of $N$=8 disappeared in $^{12}$Be. Besides, the large matter deformation was inferred from the proton inelastic scattering. The measurement of $B$($E2$) value would provide further understanding of the exotic structure. The detail of the experimental method and the result will be presented. [Preview Abstract] |
Tuesday, September 20, 2005 7:45PM - 8:00PM |
DC.00003: Ground State Wave Function of $^{12}$Be W.A. Peters, T. Baumann, N. Frank, J.-L. Lecouey, A. Schiller, M. Thoennessen, K. Yoneda, P. DeYoung, G. Peaslee, J. Brown, K. Jones, B. Luther, W. Rogers Spectroscopic factors are important quantities in establishing the shell structure in nuclei. We measured the spectroscopic factor for the neutron knockout reaction of a $^{12}$Be beam into the neutron unbound d$_{5/2}$ state of $^{11}$Be using neutron-fragment coincidence measurements. The secondary $^{12}$Be beam was produced from a 120 MeV/nucleon $^{18}$O beam from the Coupled Cyclotron Facility at the NSCL. The $^{10}$Be fragments were detected and identified using the MSU/FSU sweeper magnet while the neutrons were detected by the Modular Neutron Array (MoNA). From the reconstructed invariant mass spectra the relative contributions from different states in $^{11}$Be can be extracted. [Preview Abstract] |
Tuesday, September 20, 2005 8:00PM - 8:15PM |
DC.00004: Gamma Spectroscopy of $^{13}$B via Proton Transfer Reaction at 50~MeV/nucleon Shinsuke Ota We have investigated proton single particle states in $^{13}$B via a proton transfer reaction on $^{12}$Be at 50~MeV/nucleon. A candidate was found at the excitation energy of 4.8~MeV in $^{13}$B whose spin and parity was deduced to be 1/2$^{+}$ from a Distorted Wave Born Approximation (DWBA) analysis of the observed angular distribution of the differential cross sections. The experiment was performed in RIKEN Accelerator facility with RIKEN Projectile-fragment Separator (RIPS). A $^{12}$Be beam bombarded a liquid helium target of 120 mg/cm$^2$-thickness. Gamma rays from excited states on $^{13}$B were detected with six Germanium detectors, a part of Gamma-Ray detector Array with Position and Energy sensitivity (GRAPE) located at upstream of the target. The incident and outgoing particles were identified event-by-event by measuring their velocities and energies. In this talk, we report the detail of experiment and data analysis, and discuss about the single-particle nature of the observed state. [Preview Abstract] |
Tuesday, September 20, 2005 8:15PM - 8:30PM |
DC.00005: Measurement of Proton Transfer Reaction for Single-Particle States in $^{23}$F S. Michimasa, N. Aoi, H. Baba, M. Kurokawa, T. Minemura, T. Motobayashi, S. Takeuchi, Y. Yanagisawa, A. Yoshida, S. Shimoura, M. Tamaki, S. Kubono, A. Saito, H. Iwasaki, H.J. Ong, H. Sakurai, S. Ota, N. Iwasa, S. Kanno, K. Kurita, E. Takeshita, M. Notani We have studied excited states in neutron-rich $^{23}$F using a one-proton transfer reaction onto $^{22}$O at 35 MeV/nucleon. The experiment was performed at RIKEN Accelerator Facility, where an intense RI beam bombarded a liquid helium target in inverse kinematics. Incident and outgoing particles were identified for tagging the secondary reaction channel, and de-excitation $\gamma$ rays from reaction products were detected by an array of 150 NaI(Tl) scintillators (DALI2) surrounding a secondary target in order to identify excitated states of reaction products. In addition to the transfer reaction, we could simultaneously measure the inelastic scattering of $^{23}$F at 41.5 MeV/nucleon and the neutron-knockout reaction of $^{24}$F at 36 MeV/nucleon, because the RI beam contained several nuclei around $^{23}$F and induced several reactions which populated excited states in $^{23}$F. We identifiled proton single-particle states in $^{23}$F by comparing population strengths in the different reactions. In the present talk, we will report details of the experiment and discuss the proton shell structure in neutron-rich $^{23}$F. [Preview Abstract] |
Tuesday, September 20, 2005 8:30PM - 8:45PM |
DC.00006: New Coulomb Excitation Measurement of $^{18}$Ne Kazunari Yamada Reduced transition probability $B({\rm E2};0^{+}_{\rm g.s.} \rightarrow 2^{+}_{1})$ of $^{18}$Ne has been measured by the intermediate-energy Coulomb excitation. The previous work on Coulomb excitation measurement$^{1)}$ reported a $B({\rm E2})$ value that remarkably disagreed with two lifetime measurements$^{2,3)}$. The present study is aimed at providing a new Coulomb excitation data with a precise angular distribution to clarify if the discrepancy is accidental or due to the reaction mechanism. A secondary $^{18}$Ne beam at 54~$A$MeV produced by RIPS in RIKEN irradiated a lead target. De-excitation $\gamma$-rays were measured by an array of 68 NaI (Tl) detectors surrounding the lead target in coincidence with the incident beam and scattered particles. The $B({\rm E2})$ value was extracted with the help of DWBA analysis, where the nuclear excitation component was constrained from $B({\rm E2})$ of $^{18}$O, the mirror of $^{18}$Ne, assuming the isospin symmetry. \\ {\bf References}\\ 1) L.A.~Riley et al., Phys. Rev. C {\bf 62}, 034306 (2000).\\ 2) A.B.~McDonald et al., Nucl. Phys. {\bf A258}, 152 (1976).\\ 3) L.A.~Riley et al., Phys. Rev. C {\bf 68}, 044309 (2003).\\ [Preview Abstract] |
Tuesday, September 20, 2005 8:45PM - 9:00PM |
DC.00007: Search for the First Excited State of $^{24}$O N. Frank, P.G. Hansen, J.-L. LeCouey, W.A. Peters, A. Schiller, C. Simenel, J.R. Terry, M. Thoennessen, K. Yoneda, P. DeYoung, J. Brown, J. Hinnefeld, R. Howes, R.A. Kryger, B. Luther The location of the first 2$^+$ excited state of $^{24}$O is important evidence for the doubly magic nature of this nucleus predicted by shell model calculations. Previous searches for the first excited state of $^{24}$O utilizing in-beam $\gamma$-ray spectroscopy have been unsuccessful indicating that the state is located above the neutron separation energy. We populated states in $^{24}$O by two-proton knockout from a beam of $^{26}$Ne. Neutrons were detected by the Modular Neutron Array MoNA in coincidence with $^{23}$O fragments deflected by the MSU/FSU sweeper magnet and detected in a set of charged-particle detectors. The decay energy and thus the location of unbound excited states can be reconstructed from the measured energies and angles of the $^{23}$O fragments and the neutrons. [Preview Abstract] |
Tuesday, September 20, 2005 9:00PM - 9:15PM |
DC.00008: Two-neutron knockout reaction to final levels in the $T_{\rm z}$~=~-2 nuclei $^{24}$Si, $^{28}$S, and $^{32}$Ar Kenichiro Yoneda, P.G. Hansen, D. Bazin, B.A. Brown, C.M. Campbell, J.M. Cook, D.C. Dinca, A. Gade, T. Glasmacher, T.E. Hoagland, J.L. Lecouey, W.F. Mueller, H. Olliver, B.M. Sherrill, J.R. Terry, P.D. Cottle, K.W. Kemper, R. Reynolds, B.T. Roeder, J.A. Tostevin Two-proton knockout from a neutron-rich nucleus has recently been shown$^1$ to proceed as a direct reaction. While the inclusive and partial cross sections could be understood in a simple theory, a more precise description can be based on a scheme that combines the full shell-model two-nucleon spectroscopic amplitudes with eikonal reaction theory$^2$. We report here a first attempt to investigate the analogous two-neutron knockout from a proton-rich nucleus at energies around 100~MeV/nucleon. The projectiles $^{26}$Si, $^{30}$S, and $^{34}$Ar lead to products that have a 2$^+$ level as the only bound excited state, and are thus well suited for an accurate test of the theory. The experiment carried out at the NSCL observed this gamma peak in coincidence with the projectile residues in all three cases. The partial cross sections to the 0$^+$ and 2$^+$ levels will be discussed and compared with theory. This work was supported by NSF grants PHY-0110253, PHY-9875122, PHY-0244453, and PHY-0342281.\\ 1. D.~Bazin et al., Phys. Rev. Lett. 41 (2003) 012501.\\ 2. J.~A.~Tostevin et al., Phys. Rev. C 70, 064602 (2004). [Preview Abstract] |
Tuesday, September 20, 2005 9:15PM - 9:30PM |
DC.00009: Study of neutron-rich A$\sim $30 nuclei in multi-nucleon transfer reactions Mathis Wiedeking, P. Fallon, E. Rodriguez-Vieitez, R.M. Clark, M. Cromaz, M. Descovich, I-Y. Lee, M-A. Deleplanque, A.O. Macchiavelli, F.S. Stephens, D. Ward, M.P. Carpenter, D. Cline, R. Teng, C.Y. Wu The structure of neutron-rich s-d-f shell nuclei in the mass A$\sim $30 region has been investigated through multi-nucleon transfer in the $^{208}$Pb($^{36}$S,X$\gamma )$ reaction at 230 MeV. Gamma-radiation from the reactions was detected using GAMMASPHERE [1]. Mass identification of the target-like and projectile-like products as well as an event-by-event Doppler-shift correction was possible by utilizing the excellent spatial and timing resolution of the heavy-ion counter CHICO [2]. The level schemes of several nuclei are significantly expanded. The measurements are compared to shell-model calculations to test the current understanding and to provide new information on the neutron-proton interaction in neutron-rich s-d-f shell nuclei. [1] I-Y Lee, Nucl. Phys. A520, 641c (1990). [2] M.W. Simon \textit{et al}., Nucl. Inst. Meth. Phys. A 452, 205 (2000). [Preview Abstract] |
Tuesday, September 20, 2005 9:30PM - 9:45PM |
DC.00010: Neutron Knockout from $^{30,32}$Mg and $^{26,28}$Ne at Intermediate Energies J.R. Terry, B.A. Brown, C.M. Campbell, J.A. Church, J.M. Cook, A.D. Davies, D.C. Dinca, T. Glasmacher, P.G. Hansen, H. Olliver, B.M. Sherrill, D. Bazin, J. Enders, A. Gade, J.L. Lecouey, W.F. Mueller, K. Yoneda, J.A. Tostevin The nuclides $^{30,32}$Mg and $^{26,28}$Ne have been studied by single-neutron knockout at the Coupled Cyclotron Facility at Michigan State University. These nuclides span a transitional region between the suspected pronounced N=16 sub-shell closure and the island of inversion around $^{31}$Na. The aim of this study is to directly observe and quantify the extent of \textit{pf} intruder configurations in this region. For $^{26}$Ne with 16 neutrons, present results are in good agreement with USD shell model calculations. An upper limit of 0.2 is placed on the 3/2$^{+}_{1}$ spectroscopic factor in the residue $^{25}$Ne, in good agreement with the predicted enhanced sub-shell closure at N=16. However, for $^{28}$Ne with 18 neutrons, USD calculations predict a strong population of the 3/2$^{+}$ ground state and a single 1/2$^{+}$ excited state is the residue $^{27}$Ne while observation reveals that the 1/2$^{+}$ strength is shared by two states at 765 and 885 keV. For the magnesium isotopes, preliminary results are available for $^{30}$Mg. Beta-decay work for $^{29}$Mg suggests a negative parity assignment for the two levels at 1094.5 and 1430.6 keV, both of which have been populated in the present work. Details of the analysis and further results will be discussed. [Preview Abstract] |
Tuesday, September 20, 2005 9:45PM - 10:00PM |
DC.00011: In beam $\gamma$-ray spectroscopy via $^{32}$Mg($\alpha ,\alpha$')reaction Toshiaki Fukui It is well known that $^{32}$Mg nucleus exhibits a disappearance of magic number at $N$=20. While the spin and parity of the first excited state are known, those of the higher excites states have not been determined. In order to investigate level structure of $^{32}$Mg, an alpha inelastic scattering on $^{32}$Mg was performed at RIKEN. The alpha inelastic scattering is a good probe in determining the spin and parity of an excited state because of the selectivity in exciting natural parity states. A secondary beam of $^{32}$Mg at $\sim $42MeV/nucleon bombarded a liquid He target. The ejectile was identified event-by-event with TOF-dE-E method. The scattering angle of ejectile was measured by sets of parallel plate avalanche counters. Gamma rays from the ejectile were detected by the 17 position sensitive Ge detector array (GRAPE). Gamma rays from the first and the higher excited states in $^{32} $Mg were clearly observed. Angular distributions of differential cross section are compared with DWBA calculation. We will present the result of the analysis and discuss the collectivity of $^{32}$Mg. [Preview Abstract] |
Tuesday, September 20, 2005 10:00PM - 10:15PM |
DC.00012: In-beam gamma-ray studies of neutron-rich N$\sim $20 nuclei in fragmentation reactions E. Rodriguez-Vieitez, R.M. Clark, M. Cromaz, M.-A. Deleplanque, M. Descovich, P. Fallon, I.-Y. Lee, A.O. Macchiavelli, F.S. Stephens, D. Ward, M. Wiedeking, S.G. Prussin, D. Bazin, C.M. Campbell, J.M. Cook, D.-C. Dinca, A. Gade, T. Glasmacher, W.F. Mueller, K. Yoneda Neutron-rich sodium, neon, and fluorine nuclei were produced in a two-step fragmentation/knockout experiment conducted at Michigan State University. A 140 MeV/A $^{48}$Ca primary beam bombarded a $^{9}$Be target to produce a ``cocktail'' of secondary beams ($^{29}$Na/$^{30}$Mg/$^{32}$Al and $^{32}$Mg/$^{33}$Al/$^{35}$Si). The secondary beams underwent fragmentation/knockout reactions on a second $^{9}$Be target located at the center of the Segmented Germanium Array (SeGA), which was used to measure the prompt gamma-ray decays tagged to specific fragments detected at the S800 focal plane. New high quality data on a range of nuclei were obtained, e.g. $^{30,31}$Na, $^{28,29,30}$Ne, and $^{25,26}$F. In $^{30}$Na, for example, we observed seven gamma-ray transitions and the statistics allowed the study of gamma-gamma coincidences. These data will provide new information on the structure of these exotic nuclei and specifically the role of collective degrees of freedom and the strong n-p spin-isospin interaction. [Preview Abstract] |
Tuesday, September 20, 2005 10:15PM - 10:30PM |
DC.00013: Proton inelastic scattering on $^{34}$Si and $^{32}$Mg. Satoshi Takeuchi The inelastic scattering of $^{34}$Si and $^{32}$Mg on a liquid hydrogen target was measured in inverse kinematics at energy of about 64 MeV/nucleon. In combination with B($E2$) values obtained from Coulomb excitation experiments$^{1,2)}$, information on the collective motions of the protons and neutrons can be studied separately. Experiment was performed at the RIPS beam line in RIKEN. A recently developed time-of-flight spectrometer and a NaI(Tl) array (DALI2) were used for particle identification of scattered particles and for $\gamma$-ray detection, respectively. De-excitation $\gamma$ rays from known 2$^+$ states and other higher states are clearly observed in $\gamma$-ray spectra for both nuclei. Deduced (p,p') cross sections from ground state to the 2$^+$ states are compared with those of the Coulomb excitation to study the neutron and proton quadrupole matrix elements. \\ {\bf References}\\ 1) R.~W.~Ibbotson et al., Phys. Rev. Lett. {\bf80}, 2081 (1998).\\ 2) T.~Motobayashi et al., Phys. Lett. B {\bf 346}, 9(1995).\\ [Preview Abstract] |
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