Bulletin of the American Physical Society
77th Annual Meeting of the Southeastern Section of the APS
Volume 55, Number 10
Wednesday–Saturday, October 20–23, 2010; Baton Rouge, Louisiana
Session FA: Pygmies, Superheavies, and Magic: The Exotica of Nuclear Structure |
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Chair: Paul Cottle, Florida State University Room: Nicholson Hall 119 |
Friday, October 22, 2010 8:30AM - 9:00AM |
FA.00001: Nuclear many-body problem, from reactions to structure Invited Speaker: Structure and reaction aspects, while usually discussed separately, are deeply entangled in the nuclear many-body systems. In this presentation we highlight the advances, as well as difficulties in the path toward building a unified approach. For model studies of reactions involving composite objects, we obtain exact solutions with the newly developed Variable Phase Method. We demonstrate some non-trivial aspects of the dynamics projected onto the intrinsic Shell Model space. We discuss the limitations of the projection methods as well as convergence properties of the solutions. We present the Time Dependent Continuum Shell Model (TDCSM) and demonstrate its application to realistic nuclear problem. In the case of $^{8}$B the observation of the $^{7}$Be(p,p') cross section, its angular dependence, and interference between resonances allows one to make in-depth conclusions about the many-body structure using TDCSM. We find that virtual excitations are important. Behavior of the spectroscopic amplitudes, and changes in the collective dynamics due to the presence of reaction continuum will be discussed. [Preview Abstract] |
Friday, October 22, 2010 9:00AM - 9:30AM |
FA.00002: Synthesis of a new element with Z=117 Invited Speaker: The synthesis of new elements with neutron number (N) approaching 184 provide important tests of nuclear structure models used to predict closed spherical shells in the heaviest elements. Earlier, elements with Z=113-116, and 118 were synthesized in reactions of $^{48}$Ca with actinide targets at JINR. The synthesis of previously unknown Z=117 can provide additional crucial tests of the shell structure near the predicted Island of Stability with N=184. Here we report the synthesis of $^{293,294}$117 (N=176,177) in the $^{48}$Ca + $^{249}$Bk 4n and 3n reactions. The $^{249}$Bk was produced at ORNL in the High Flux Isotope Reactor and chemically separated at the Radiochemical Engineering Development Center at ORNL. Six arc-shaped targets of 0.31 mg/cm$^{2}$ of $^{249}$Bk were made at the Research Institute of Atomic Reactors (Dimitrovgrad). The experiments were performed employing the Dubna Gas-Filled Recoil Separator and the heavy-ion cyclotron U-400 at JINR, Russia. Separated evaporation residues were registered by a time-of-fight system and implanted in a 4 cm x 12 cm Si-detector array with 12 vertical position-sensitive strips surrounded by eight 4 cm x 4 cm side detectors. Irradiation at 252 MeV for 70 days starting July 27, with a total beam dose of 2.4 x 10$^{19}$ yielded five position-correlated ($\le $1.2mm) decay chains of 3 $\alpha $'s followed by spontaneous fission. These were assigned to $^{293}$117 produced in the 4n reaction. At a $^{48}$Ca energy of 247 MeV a new decay chain was detected involving six consecutive $\alpha $-decays and ending in SF and assigned to $^{294}$117 (3n channel). The daughters of $^{293,294}$117 have one or two more neutrons than previously observed isotopes and have much longer half-lives. The decays of the eleven newly identified isotopes expand substantially our knowledge of odd-Z nuclei of the most neutron-rich isotopes of elements 105 to 117. These nuclei display increasing stability with increase in neutron number to strongly support the island of stability. Their longer half lives open up further studies of the chemistry of super-heavy elements and their place in the Periodic Table. [Preview Abstract] |
Friday, October 22, 2010 9:30AM - 10:00AM |
FA.00003: Single particle spectroscopy of 133Sn via the (d,p) reaction in inverse kinematics Invited Speaker: It is important, both for nuclear structure physics and understanding the synthesis of heavy elements in the cosmos, to determine how single-particle states change as we move away from the valley of stability, especially around shell closures. One powerful method to probe single-particle structure of nuclei is to use single-nucleon transfer reactions. With short-lived exotic nuclei, these reactions need to be performed in inverse kinematics, using a radioactive ion beam and light ion targets. A beam of 132Sn produced at ORNLs Holifield Radioactive Ion Beam Facility was used in a transfer reaction experiment to study single-particle states in 133Sn. The beam impinged on a target of CD2 with effective thickness of around 150ug/cm2. Charged ejectiles were detected in an array of position sensitive silicon detectors, mostly of the new ORRUBA type, with SIDAR detectors at very backward angles. At forward laboratory angles, telescopes of detectors were used to discriminate protons from heavier, elastically scattered particles. From the angles and energies of the protons, the energies of the states populated in the final nuclei were measured. The present work has determined the purity of the low-spin single-neutron excitations in 133Sn. A previously unobserved state in 133Sn has also been measured here for the first time. The simplicity of the structure of 132Sn, and the single-neutron excitations in 133Sn, provides a new touchstone needed for extrapolations to nuclei further from stability, in particular those responsible for the synthesis of the heaviest elements via the r-process. [Preview Abstract] |
Friday, October 22, 2010 10:00AM - 10:30AM |
FA.00004: Study of the Nuclear Electric and Magnetic Dipole Response using Monoenergetic and Polarized Photons Invited Speaker: In stable and weakly bound neutron-rich nuclei a resonance-like concentration of dipole strength is observed at excitation energies around the neutron separation energy. This clustering of strong dipole transitions has been named the pygmy dipole resonance (PDR) in comparison to the giant dipole resonance that dominates the E1 response. Microscopic nuclear models predict the existence of the PDR arising from an oscillation of a small portion of neutron-rich nuclear matter relative to the rest of the nucleus. In addition, the dipole strength distributions at the particle separation energies might affect the reaction rates in astrophysical scenarios where photo-disintegration reactions are important, i.e., in hot stars and stellar explosions. This talk is giving an overview of the high-sensitivity studies of E1 and M1 transitions in neutron closed-shell nuclei using the nearly monoenergetic and 100{\%} linearly polarized photon beams from the High-Intensity-Gamma-Ray Source facility. The fine and gross structure of the dipole-strength distribution of the PDR has been observed for the first time and novel information about the character of this mode of excitation has been obtained. The observations will be compared with calculations using statistical and quasiparticle random-phase approximation. [Preview Abstract] |
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