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 1WE: Workshop 5A: Nuclear States Under Extreme Conditions of Binding and Isospin Asymmetry |
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Sponsoring Units: DNP JPS Chair: Don Geesaman, Argonne National Laboratory Room: Ritz-Carlton Hotel Amphitheatre |
Sunday, September 18, 2005 9:00AM - 9:30AM |
1WE.00001: Proton radioactivity Invited Speaker: Proton radioactivity was discovered 35 years ago [1]. However, most of the data on over 40 proton-emitting states known to date were obtained recently using electromagnetic separators selecting recoiling fusion-evaporation reaction products. Observation of fine structure in proton emission [2-5] is particularly important since the deformation and structure of wave function of involved states can be determined from observed properties. Recent results obtained at Oak Ridge on rare-earth nuclei as well as the perspectives of further studies at the next generation radioactive ion beam facilities will be presented.\\ 1. K.P Jackson et al., Phys.Lett. B33,1970,281.\\ 2. A.A. Sonzogni et al., Phys. Rev. Lett. 83,1999,1116.\\ 3. M.Karny et al., Phys. Rev. Lett. 90,2003,012502.\\ 4. T.N.Ginter et al., Phys. Rev. C68, 2003, 034330.\\ 5. R.K. Grzywacz et al., ENAM04 conf.,EPJDirect 2005, in press. [Preview Abstract] |
Sunday, September 18, 2005 9:30AM - 10:00AM |
1WE.00002: Gamow Shell Model description of weakly bound and resonant nuclei Invited Speaker: Recently, the shell model in the complex k-plane (the so-called Gamow Shell Model) has been formulated using a complex Berggren ensemble representing bound single-particle states, single-particle resonances, and non-resonant continuum states. The single-particle basis used is that of the Hartree-Fock potential generated self-consistently by the finite-range residual interaction. In this framework, we shall discuss binding energies and energy spectra of neutron-rich helium and lithium isotopes, as well as spectroscopic factors and spin-orbit splitting in the helium region. It is shown that correlations due to scattering components cannot be neglected. In collaboration with Witek Nazarewicz, HRIBF Scientific Director and Marek Ploszajczak, CEA researcher. [Preview Abstract] |
Sunday, September 18, 2005 10:00AM - 10:30AM |
1WE.00003: Correlation of halo neutrons studied via Coulomb breakup of $^{11}$Li Invited Speaker: Strong E1 excitation to the low-lying continuum is one of the manifestations of nuclear halo phenomena. This excitation, which has been observed in the Coulomb breakup, is now well understood for one-neutron halo nuclei as in the case of $^{11}$Be~[1]. On the other hand, the case of two neutron halo nucleus such as $^{11}$Li is still in the controversial situation. For instance, there have been discrepancies among three experimental results obtained on the Coulomb breakup of $^ {11}$Li at MSU~[2], RIKEN~[3], and GSI~[4]. The understanding of Coulomb breakup of halo nuclei is of great importance since it should have information on the three-body properties such as two-neutron correlations in the Borromean system. We have thus studied the Coulomb dissociation of $^{11}$Li on a Pb target at 70~MeV/nucleon at the ratioactive beam facility RIPS at RIKEN, with much higher statistics and much less ambiguities caused by cross talk events in detecting two neutrons. The momentum vectors of incident $^{11}$Li as well as outoging $^{9}$Li, and two neutrons, and $\gamma$ rays were all measured in coincdence. Hence, we could obtain the excitation energy spectrum in combination with the angular distribution of $^{11}$Li. In addition, relative energies between two neutrons as well as those of $^{9}$Li-n $^{10}$Li were extracted. In the excitation energy spectrum of $^{11}$Li, we have observed a strong peak at the relative energy $E_{\rm rel}\sim$ 0.3 MeV. The corresponding $B$(E1) strength has been extracted to be 1.6~$\pm$~0.1 e$^2$fm$^2$ using the conventional equivalent photon method. The non-energy weighted E1 cluster sum rule is applied to extract the neutron-neutron correlation in the ground state. Spectra of $^9$Li-n, and n-n relative energies are also shown to discuss the two-body correlations in $^{11}$Li.\\ \noindent [1] N.Fukuda, T.Nakamura et al., Phys. Rev. C. \textbf{70}, 054606 (2004). \\ \noindent [2] K. Ieki et al., Phys. Rev. Lett. \textbf{70}, 730 (1993). \\ \noindent [3] S. Shimoura et al., Phys. Lett. B \textbf{348}, 29 (1995). \\ \noindent [4] M. Zinser et al., Nucl. Phys. A \textbf{619}, 151 (1997). [Preview Abstract] |
Sunday, September 18, 2005 10:30AM - 11:00AM |
1WE.00004: Spatially extended coherence induced by pairing correlation in low-frequency vibrational excitations of neutron drip line nuclei Invited Speaker: Study of low-frequency vibrational excitations in neutron drip line nuclei is one of the most interesting issues in nuclear structure physics. In contrast to vibrations in stable nuclei, the contributing single-particle states are loosely-bound states, resonant and non-resonant continuum states in neutron drip line region. Therefore the two-quasiparticle states (or one-particle - one-hole states in closed shell nuclei) have rich variety of the spatial structure, and the correlations among them may bring about qualitatively new aspects of low-frequency vibrational excitations. In my talk, novel effects of pairing correlations for emergency of low-frequency vibrational excitations in neutron drip line nuclei is discussed paying special attentions to neutrons with small orbital angular momentum $\ell$. By solving the Hartree-Fock-Bogoliubov (HFB) equation in coordinate space, we discuss the change of the spatial structure of quasiparticle wave functions induced by pairing correlations; the pairing anti-halo effect in the lower component [1] and the broadening effect in the upper component [2]. The resultant broad localization of the two-quasiparticle states of low-$\ell$ neutrons produces the coherence in the spatially extended region, and the correlations can cause the large transition strength. By performing HFB plus quasiparticle random phase approximation calculation for the first $2^+$ states in neutron rich Ni isotopes, the unique role of pairing correlations is examined [2].\\ \noindent [1] K. Bennaceur, J. Dobaczewski, and M. Ploszajczak, Phys. Lett. B {\bf 496}, 154 (2000).\\ \noindent [2] M. Yamagami, preprint nucl-th/0504059. [Preview Abstract] |
Sunday, September 18, 2005 11:00AM - 11:30AM |
1WE.00005: Absolute spectroscopic factors from nucleon knockout with radioactive projectiles Invited Speaker: Many experiments now demonstrate that experiments with rare-isotope beams reveal effects that were not observable in the classical nuclear-physics experiments, limited as they were to experiments with stable targets. Exploration of the drip lines is therefore an essential element in reaching a full understanding of nuclear structure. This presentation will discuss the use of one- and two-nucleon knockout reactions in inverse kinematics to measure partial cross sections and to deduce spectroscopic factors for approximately 50 nuclear states. By far the most interesting result to emerge is that, owing to the firm theoretical basis and essentially parameter-free description offered by eikonal reaction theory, it is possible to obtain absolute spectroscopic factors, accurate to 5-15\%, and hence to obtain information on the physical occupancy of shell model states. While knockout reactions with electrons on stable targets had suggested a constant reduction factor $R_s$ relative to the shell model of 0.6-0.7, we find a strong dependence on isospin. Weakly-bound halo states have $R_s$ close to unity. In contrast, some deeply-bound states near the $N=Z$ line have $R_s$ as low as 0.25, but in spite of this the nuclear level assignments and energies are still well described by the effective-interaction model. In other words, the quasiparticles of the model are, for these cases, only to 25\% physical particles. Other recent results concern single-particle structure for neutron-rich nuclei near $N=20$, where the shell gap vanishes, and near N=28, where a new doubly magic nucleus $^{42}$Si with a large shell gap above (14,28) appears. We have demonstrated that two-proton knockout on a neutron-rich projectile proceeds as a direct reaction. In this case, the cross sections to individual final states carry detailed information about particle-particle correlations in the many-body wave function. [Preview Abstract] |
Sunday, September 18, 2005 11:30AM - 12:00PM |
1WE.00006: Cluster aspect in C isotopes Invited Speaker: We discuss structure of ground and excited states of C isotopes, while paying attention to cluster aspect. In the neutron-rich nuclei near the drip line, there exist loosely bound three-body systems called as Borromean systems. Also in stable nuclei, we can regard $^{12}$C as a Borromean system when we consider the 3$\alpha$ system, where no bound system consists of two of the three clusters. Recently, the properties of the second $0^+$ state of $^{12}$C has been successfully described by a gas-like 3$\alpha$ structure. It leads us to expect that loosely bound three-body states may appear in other C isotopes. We propose that the third $3/2^-$ state of $^ {11}$C is a candidate of such a gas-like state. It is supported by the recent observation of small Gamov-Teller transition strength, $B(GT;^{11}B\rightarrow ^ {11}C^*)$, measured by charge exchange reactions. We give a discussion of cluster aspect in $^{11}$C. In contrast to the development of three-body clustering in the highly excited states, the clusters are considered to be tightly bound in the low- lying states of C isotopes. In the neutron-rich C isotopes, it is suggested that spatial development of 3$\alpha$ may not appear, instead, compact proton structure is favored. This feature may lead to decoupling of core and valence neutrons in neutron- rich C and has a good contrast to low-lying states of neutron-rich Be which has the strong coupling nature of 2$\alpha$ core and valence neutrons. We study structure of neutron-rich C isotopes and discuss the decoupling of proton and neutron deformations. [Preview Abstract] |
Sunday, September 18, 2005 12:00PM - 12:30PM |
1WE.00007: Single particle gaps and the structure of neutron-rich nuclei above $^{48}$Ca Invited Speaker: Recent investigations have shown that single-particle level spacing and residual interactions in very neutron rich nuclei may significantly differ from those inferred from many years of studies of nuclei near the valley of stability. This presentation will focus on the region of neutron-rich nuclei above $^{48}$Ca and explore issues such as the appearance of new shell gaps and the onset of deformation in the Ca-Ti-Cr region. The nuclei of interest are extremely difficult to produce and our understanding is only emerging by combing data from experiments using very different techniques ranging from beta decay of fragmentation products to gamma-ray studies following deep-inelastic reactions, and from fusion-evaporation with radioactive targets to Coulomb excitation at relativistic energies. This symbiotic relationship is a template for future exploration far from stability. [Preview Abstract] |
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