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 DB: Mini-symposium on Pair and Cluster Correlations in Nuclei I |
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Sponsoring Units: DNP JPS Chair: Erich Ormand, Lawrence Livermore National Laboratory Room: Ritz-Carlton Hotel Salon 3 |
Tuesday, September 20, 2005 7:00PM - 7:30PM |
DB.00001: Di-neutron correlations in weakly bound and low density many-nucleon systems Invited Speaker: The correlations in nuclei often lead to spatial clusterizations. In the present talk we shall demonstrate this in connection with the pair correlation. A spatial correlation of the di-neutron type, which has been discussed originally for the two-neutron halo nuclei, may emerge rather generally in a wide class of neutron many-body systems, including the low density nuclear/neutron matters and the medium- mass neutron-rich nuclei containing several weakly bound neutrons. We shall also discuss that the systems with the di-neutron correlation exhibit characteristic modes of excitation. Our analysis is based on the coordinate-space HFB method for the description of the correlated ground state, and the continuum QRPA for the excitation modes. The HFB model predicts that neutron Cooper pairs e.g. in $^{84} $Ni, that contains six weakly bound neutrons above the N=50 gap, exhibit strong spatial correlation at short relative distances $< 2-3$fm. The continuum QRPA suggests that the soft dipole excitaition in this nuclei is characterized by a motion of the di-neutrons present in the skin region. The soft di-neutron mode appears also in the octupole multipolarity. In order to put these results in a wider context, we look into the superfluid low-density nuclear/neutron matters. It is shown that the size of the neutron Cooper pair (the coherence length) becomes comparable to or a little shorter than the average inter-neutron distance in a wide interval of densities corresponding to the neutron skin and the halo. We also discuss a possible connection between the strong di-neutron correlation and the BCS-BEC crossover phenomenon. [Preview Abstract] |
Tuesday, September 20, 2005 7:30PM - 7:45PM |
DB.00002: Pairing, Phase Transitions and Nuclear Level Densities Stefan Rombouts, Kris Van Houcke, Kris Heyde In a nuclear medium two protons or two neutrons can form a bound pair. Together with the average interactions with other nucleons this leads to the mean-field plus pairing Hamiltonian as a schematic model for correlations in nuclei. This Hamiltonian can be solved exactly even in large model spaces, using algebraic techniques or using quantum Monte Carlo methods (QMC). We will highlight two recent developments: the extension of the exactly solvable Richardson-Gaudin models to a more general separable pairing interaction, and a new QMC algorithm which is free of sign problems also for odd particle numbers. Combining algebraic solutions at low excitation energies with QMC results at high energies, we obtain accurate estimates of nuclear level densities, particularly in the region where a pairing phase transition might occur. Furthermore we can now evaluate symmetry-projected level densities, for specific parity and angular momentum. Results for Fe and Sn isotopes will be presented. [Preview Abstract] |
Tuesday, September 20, 2005 7:45PM - 8:00PM |
DB.00003: Invariant-Mass Spectroscopy of $^{14}\mathrm{Be}$ with a Carbon Target at 68.1~{\textit A}MeV T. Sugimoto, T. Nakamura, M. Miura, Y. Kondo, N. Fukuda We have studied the nuclear structure of $^{14}\mathrm{Be}$ using nuclear-breakup reaction with a carbon target at 68.1~\textit{A}MeV. In the neighboring beryllium isotope $^{12}\mathrm{Be}$, the disappearance of $N=8$ magic number was suggested, which was shown by observations of the low-lying first $2^+$ and the intruder $1^- $ state. On the other hand, no excited state has been observed for $^{14} \mathrm{Be}$. It is thus interesting to study such low excited state in $^{14} \mathrm{Be}$, in discussing the change of shell structure and the effect of neutron halo. The experiment was performed at the RIKEN Accelerator Research Facility. The secondary $^{14}\mathrm{Be}$ beam was produced and identified using RIPS beam line. The $^{14}\mathrm{Be}$ was broken up into $^{12}\mathrm{Be}$ and two neutrons by the carbon target. These decay particles were measured and identified using magnetic spectrometer and neutron detectors. The relative-energy spectrum of $^{12}\mathrm{Be}+2n$ system was extracted using invariant-mass method. In the spectrum we found a narrow peak in the unbound region of $^{14}\mathrm{Be}$. We also show the angular distribution of this transition in order to determine the spin/parity of the state. [Preview Abstract] |
Tuesday, September 20, 2005 8:00PM - 8:15PM |
DB.00004: Nuclear and Coulomb breakup of the Borromean nucleus $^{11}$Li Takashi Nakamura We have performed kinematically complete measurements of breakup reactions of $^{11}$Li with a $^{12}$C and with a Pb target at about 70~MeV/nucleon at the radioactive beam facility RIPS at RIKEN. Due to the loosly-bound nature of this nucleus, the breakup reactions will provide useful knowledge on the ground and low-lying states of $^{11}$Li, as well as that on the subsystems such as $^{10}$Li and di-neutron correlation. In the breakup with the C target, where nuclear interaction dominates over the Coulomb interaction, the knockout reaction becomes important. There, the $^{10}$Li nucleus is produced as an intermediate state. We show the relative energy spectrum of $^{10}$Li, which is important in understanding the structure of $^{11}$Li. In the breakup with the Pb target, where Coulomb breakup is dominant, we have observed strong E1 transition strengths at $E_{\rm rel}\sim$~0.3~MeV, which was missing in the previous experiments. In this presentation we also discuss the characteristics of reactions mechanisms of breakup reactions with halo nuclei. [Preview Abstract] |
Tuesday, September 20, 2005 8:15PM - 8:30PM |
DB.00005: A new look at the $\beta$-decay of $^{11}$Li Fred Sarazin, Caleb M. Mattoon Following the development of a more intense $^{11}$Li beam at ISAC/TRIUMF, the study of the $\gamma$ spectrum, following the $\beta$-decay of $^{11}$Li, was re-investigated with the 8pi spectrometer, an array of 20 Compton-suppressed HPGe detectors. The addition of an inner array of plastic scintillators allowed data-taking in $\beta$-$\gamma$ coincidences, which significantly improved the signal to background ratio. Since most of the decay strength is observed to proceed through $\beta$-delayed one-neutron emission, the $\gamma$-spectrum is dominated by the decay of bound excited states of $^{10}$Be. These transitions exhibit characteristic Doppler-broadened lineshapes, due to the recoiling effect induced by the neutron emission. Analysis of these lineshapes and results from the new experiment will be presented. [Preview Abstract] |
Tuesday, September 20, 2005 8:30PM - 8:45PM |
DB.00006: The structure of $^6$He studied by the proton inelastic scattering at 70 A MeV Yoshiko Hashimoto, Takashi Nakamura, Yoshitero Satou, Yosuke Kondo, Takashi Sugimoto, Nobuyuki Matsui, Toshifumi Okumura, Takumi Nakabayashi, Mayuko Shinohara, Toshio Kobayashi, Hideaki Otsu, Yohei Matsuda, Natsumi Endo, Mitsuhisa Kitayama, Takeo Onishi, Hooijin Ong, Susumu Shimoura, Mitsura Tamaki, Yasuhiro Togano, Syoko Kawai Inelastic proton scattering of the neutron-rich helium isotope $^6$He has been studied in inverse kinematics at 70 MeV/nucleon. $^6$He is a loosely bound Borromean nucleus. It is thus interesting to study its nuclear response, such as the three-body related discrete resonance and the continuum. The experiment was performed at the RIKEN Accelerator Research Facility. Secondary beam of $^6$He was produced and separated by the RIKEN Projectile Fragment Separator (RIPS). $^6$He bombarded a liquid hydrogen target. The momentum vectors of all the outgoing particles, $^4$He and two neutrons, were determined event by event. $^4$He was bent by a dipole magnet and was detected with a plastic scintillator hodoscope and drift chambers. The neutrons were detected by two walls of scintillator hodoscope. In this talk, we present the invariant mass spectrum of $^6$He and discuss its structure. [Preview Abstract] |
Tuesday, September 20, 2005 8:45PM - 9:00PM |
DB.00007: Tensor-correlated shell and cluster models and their applications to He isotopes Takayuki Myo, Kiyoshi Kato, Hiroshi Toki, Kiyomi Ikeda We investigate the effect of the tensor correlation (TC) in light nuclei. TC is mainly the correlation of proton-neutron pair and also related to the cluster correlation. In previous studies, we have extended the model space of $^4$He from $(0s)^4$ to $(0s)^4+(0s)^2(0p)^2$ in order to incorporate TC in model space description. We have also shown that TC produces about a half of the LS splitting in $^5$He (Prog.Theor.Phys.113(2005)783), and contributes to the breaking of the shell closure in $^{11}$Li(last JPS meeting). Here, these results depends on the amount of TC in nuclei. Then, in this talk, we examine how much our model represents TC. To do this, we extend the model space for $^4$He as follows; (1) we mix the $sd$ shell in addition to the $0s$+$0p$ shells, (2) we improve the single particle wave function from the harmonic oscillator basis to the one expanded with a finite number of the Gaussian bases. Again, we solve the ``tensor-correlated $^4$He cluster''+$n$ problem to see the coupling effect between TC and a valence neutron. [Preview Abstract] |
Tuesday, September 20, 2005 9:00PM - 9:15PM |
DB.00008: Pairing correlations in deformed nuclei close to the neutron drip line Masayuki Yamagami Study of pairing correlations in nuclei close to the neutron drip line is an extensively investigated subject currently. The presence of loosely-bound neutrons and the coupling to the nearby continuum enhance di-neutron correlations [1]. On the other hand, pairing correlations cause the change of the spatial structure of quasiparticle wave functions, and the correlations among the resultant two-quasiparticle states of low-$\ell$ neutrons lead to the large transition strength of the low-frequency shape oscillations in neutron drip line nuclei [2]. At present, however, these studies are restricted only for spherical nuclei. In my talk, by solving Hartree-Fock-Bogoliubov equation in coordinate space allowing axially symmetric deformation, the pairing effects in deformed neutron drip line nuclei is analyzed paying special attention to neutrons with small $\Omega$, that is the projection of the single-particle angular momentum along the symmetry axis, in connection with the possibility of neutron halos in medium mass and heavier neutron-rich nuclei.\\ \noindent [1] M. Matsuo, K. Mizuyama, and Y. Serizawa, preprint nucl-th/0408052.\\ \noindent [2] M. Yamagami, preprint nucl-th/0504059. [Preview Abstract] |
Tuesday, September 20, 2005 9:15PM - 9:30PM |
DB.00009: Structure of $^{17}\mathrm{B}$ studied by the inelastic scattering on proton M. Shinohara, T. Nakamura, Y. Satou, Y. Kondo, T. Sugimoto, N. Matsui, T. Okumura, Y. Hashimoto, T. Nakabayashi, T. Kobayashi, H. Otsu, Y. Matsuda, N. Endo, M. Kitayama, H. Sakurai, T. Onishi, H.J. Ong, S. Shimoura, M. Tamaki, Y. Togano, S. Kawai The structure of the neutron-rich isotope $^{17}\mathrm{B}$ has been investigated using the $^{17}\mathrm{B}$+p inelastic scattering at approximately 60 MeV/nucleon. We focus on extracting the deformation parameters independently for protons and neutrons for $^{17}\mathrm{B}$ by using the transition to the first excited state at 1.07(1) MeV. The phenomenon of different shapes in proton and neutron distributions was suggested for the neighboring nucleus $^{16}\mathrm{C}$. A comparison of the inelastic cross section of the current proton target with the one obtained in the previous $^{17}\mathrm{B}$+C experiment makes it possible to determine independently proton and neutron deformations. The experiment was performed using the RIPS beam line at RIKEN. A $^{17}\mathrm{B}$ beam delivered from the RIPS bombarded a liquid hydrogen target. The $\gamma$ rays emitted from the first excited state of $^{17}\mathrm{B}$ were detected by forty-eight NaI(Tl) scintillators. We have obtained the cross section and the angular distribution for the transition to the first excited state. We compare the present result with the one with the carbon target. [Preview Abstract] |
Tuesday, September 20, 2005 9:30PM - 9:45PM |
DB.00010: Pairing Energies of the High-Spin Isomers in N=83 Isotones A. Odahara, Y. Gono, T. Fukuchi, Y. Wakabayashi, H. Sagawa, W. Satula, W. Nazarewicz High-spin isomers in $N$=83 isotones have been systematically studied. Excitation energies of these isomers locate between 8.5 and 9.0 MeV. Their spins and parities are 49/2$^+$ and 27$^+$ for odd and odd-odd nuclei, respectively. Life times range between $\sim$10ns and $\sim$$\mu$s. High-spin isomers have stretch coupled configurations resulting from the breaking of a neutron magic 82 core, such as [$\nu$(f$_{7/2}$h$_{9/2}$i$_{13/2}$) $\pi$h$_{11/2}$$^2$]$_{49/2}$$^+$ for odd nuclei and [$\nu$(f$_{7/2}$h$_{9/2}$i$_{13/2}$) $\pi$(d$_{5/2}^{-1}$h$_{11/2}$$^2$)]$_{27}$$^+$ for odd-odd nuclei. These high-spin isomers have oblate shapes and are called to be high-spin shape isomers. The proton pairing correlations of high-spin isomers are empirically extracted for the first time by using the excitation energies of these isomers as well as binding energies, based on the odd-even mass difference. The experimental pairing correlations of high-spin isomers are found to be almost the same as those of the ground states in $N$=83 isotones. Theoretical studies of the excitation energies and the pairing correlations of high-spin isomers are also performed and compared with the experimental data. [Preview Abstract] |
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