Bulletin of the American Physical Society
2008 Annual Meeting of the Division of Nuclear Physics
Volume 53, Number 12
Thursday–Sunday, October 23–26, 2008; Oakland, California
Session LA: Frontiers in Rare Isotope Science |
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Chair: Brad Sherrill, National Superconducting Cyclotron Laboratory/Michigan State University Room: Simmons Ballroom 2-3 |
Sunday, October 26, 2008 8:30AM - 9:06AM |
LA.00001: $\beta$ decay of N=Z isotopes $^{96}$Cd, $^{98}$In and $^{100}$Sn Invited Speaker: The $\beta$-decay properties of the N=Z isotopes $^{96}$Cd, $^{98}$In and $^{100}$Sn have been studied. The isotopes were produced at the National Superconducting Cyclotron Laboratory (NSCL) by fragmenting a 120 MeV/u $^{112}$Sn primary beam in a Be target. The resulting radioactive beam was filtered in the A1900 and the newly commissioned Radio Frequency Fragment Separator to achieve a purity level suitable for decay studies. The observed production cross sections of these isotopes are lower than expected by factors of 10 to 30. The $^{100}$Sn cross section is 0.25(15) pb, in sharp contrast with the 120 pb lower limit established at 63 MeV/u incident energy of the same primary beam. The half-life of $^{96}$Cd, which was the last experimentally unknown waiting point half-life of the astrophysical rp-process, is 1.03$^{+0.24}_{-0.21}$ s. The implications of the experimental T$_{1/2}$ value of $^{96}$Cd on the abundances predicted by the rp-process and the origin of A=96 isotopes such as $^{96}$Ru are explored. The measured half-lives of $^{98}$In are 47(13) ms and 0.66(40) s, and 0.55$^{+0.70}_{-0.31}$ s for $^{100}$Sn. They are in agreement with previous determinations and lead to an improved precision. [Preview Abstract] |
Sunday, October 26, 2008 9:06AM - 9:42AM |
LA.00002: New decay studies near the doubly-magic $^{78}$Ni Invited Speaker: The nucleus $^{78}$Ni, with a closed proton shell at Z=28 and a closed neutron shell at N=50, is the most neutron-rich doubly-magic nucleus identified to date [1,2]. Spectroscopic studies of nuclei around $^{78}$Ni are important for understading both the evolution of nuclear structure in neutron rich matter and the rapid neutron capture nucleosynthesis process. Additionaly, the beta-delayed neutron emission from neutron-rich fission products contributes to the total number of neutrons inducing fission in nuclear fuel and should be accounted for when running power reactors. The neutrons filling the large 1g$_{9/2}$ shell between N=40 and N=50 impact the spin-orbit splitting of the respective proton orbital pairs, 2p$_{3/2}$-2p$_{1/2}$ and 1f$_{7/2}$-1f$_{5/2}$. This can trigger a change in the ground-state proton configuration of very neutron rich nuclei above Z=28 [3,4]. Further, the energy difference beetwen the 2d$_{5/2}$ and 3s$_{1/2}$ neutron orbitals above N=50 is decreasing when approaching the $^{78}$Ni region possibly resulting in the appearance of a new subshell closure at N=58. Nuclei in the $^{78}$Ni region are produced at the Holifield Radioactive Ion Beam Facility (HRIBF, Oak Ridge National Laboratory) by means of an on-line isotope separation technique using the fission of a $^{238}$U target induced by a 50 MeV, 10 microAmp proton beam. The decay studies performed at the HRIBF profitted from the post-acceleration of mass-separated radioactive beams to about 200 MeV. A novel method, the so-called {\it ranging- out} technique, allowed us to separate the most neutron-rich component of the isobaric cocktail beam [5,6]. New results on the decay of A=76 to A=79 Cu isotopes and of A=83 to A=85 Ga isotopes will be presented. In particular, the measured beta-delayed neutron branching ratios for the Cu isotopes are two to four times larger than previously reported [7]. An energy of 247 keV was established for the 3s$_{1/2}$ neutron state above the 2d$_{5/2}$ ground- state in the N=51 isotone $^{83}$Ge suggesting the existence of low energy E2 isomers in the N=51 $^{81}$Zn and $^{79}$Ni nuclei. The low-energy 3s$_{1/2}$ state may have a spatially extended wave function (halo) in a weakly bound N=53 isotone $^{81}$Ni. The extension of the HRIBF studies to even more neutron-rich nuclei at the recently completed Low-energy Radioactive Ion Beam Spectroscopy Station will also be discussed. [1] Ch.Engelmann et al., Zeit. Phys. A 352, 351 (1995) [2] P.T.Hosmer et al., Phys. Rev. Lett. 94, 112501 (2005) [3] T.Otsuka et al., Phys. Rev. Lett. 95, 232502 (2005) [4] J.Dobaczewski et al., Prog. Nucl. Part. Phys. 59,432(2007) [5] C. J. Gross et al., Eur. Phys. Jour. A25, s01, 115 (2005) [6] J. A. Winger et al., Acta Phys. Pol. B39, 525 (2008) [7] B. Pfeiffer et al., Prog. Nucl. Energy 41, 39 (2002) [Preview Abstract] |
Sunday, October 26, 2008 9:42AM - 10:18AM |
LA.00003: Ab initio many-body calculations of light nuclei neutron and proton scattering Invited Speaker: One of the greatest challenges of nuclear physics today is the development of a quantitative microscopic theory of low-energy reactions on light nuclei. At the same time, technical progress on the theoretical front is urgent to match the major experimental advances in the study of exotic nuclei at the radioactive beam facilities. We build a new {\em ab initio} many-body approach\footnote{S. Quaglioni and P. Navratil, arXiv:0804.1560.} capable of describing simultaneously both bound and scattering states in light nuclei, by combining the resonating-group method\footnote{Y. C. Tang et al., Phys. Rep. 47, 167 (1978); K. Langanke and H. Friedrich, Advances in Nuclear Physics, chapter 4., Plenum, New York, 1987.} with the {\em ab initio} no-core shell model.\footnote{P. Navratil, J. P. Vary, and B. R. Barrett, Phys. Rev. Lett. 84, 5728 (2000); Phys. Rev. C 62, 054311 (2000).}. In this way, we complement a microscopic-cluster technique with the use of realistic interactions, and a microscopic and consistent description of the nucleon clusters, while preserving Pauli principle and translational symmetry. I will present results for neutron and proton scattering on light nuclei, including $n$- and $p$-$^4$He phase shifts, and low-lying states of one-neutron halo $p$-shell nuclei, obtained using realistic nucleon-nucleon potentials. In particular, I will address the parity inversion of the $^{11}$Be ground state. [Preview Abstract] |
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