Bulletin of the American Physical Society
2007 APS April Meeting
Volume 52, Number 3
Saturday–Tuesday, April 14–17, 2007; Jacksonville, Florida
Session Y8: Minisymposium on Gamma Ray Spectroscopy at the Limits II |
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Sponsoring Units: DNP Chair: Mark Riley, Florida State University Room: Hyatt Regency Jacksonville Riverfront City Terrace 4 |
Tuesday, April 17, 2007 1:30PM - 1:42PM |
Y8.00001: Search for excited states in $^{101}$Sn D. Seweryniak, M.P. Carpenter, S. Gros, R.V.F. Janssens, T.L. Khoo, T. Lauritsen, C.J. Lister, D. Peterson, A. Robinson, X. Wang, S. Zhu, G. Lotay, P.J. Woods, A.A. Hecht, N. Hoteling, W.B. Walters Single-particle excitations near closed shells are critical in understanding nuclear structure. Single-particle energies in the doubly-magic self-conjugate $^{100}$Sn nucleus are not known. Studies of nuclei around $^{100}$Sn are at the current sensitivity limit. A search for gamma-ray transitions in $^{101}$Sn, which contains only one neutron outside of the $^{100}$Sn core, was carried out at the Argonne Tandem-Linac Accelarator System. $^{101}$Sn nuclei were produced using the $^{46}$Ti($^{50}$Cr,3n)$^{101}$Sn reaction with a cross section of about 50 nb. Beta-delayed protons with energies and decay times consistent with previous $^{101}$Sn decay studies were observed in a Double-Sided Si Strip Detector at the focal plane of the Argonne Fragment Mass Analyzer. In-beam gamma rays were detected in the GAMMASPHERE array of Ge detectors and were correlated with $^{101}$Sn beta-delayed protons. Implications of the $^{101}$Sn gamma-ray spectrum for the structure of $^{101}$Sn, $^{100}$Sn and neighboring nuclei will be discussed. [Preview Abstract] |
Tuesday, April 17, 2007 1:42PM - 1:54PM |
Y8.00002: Gamma-ray Transition Matrix Elements in $^{21}$Na: First TIGRESS Radioactive Beam Experiment Greg Hackman Modern shell model calculations should be expected to reliably reproduce the properties of the deformed five-particle nucleus $^{21}$Na. However the lowest-lying B(E2) value deduced from lifetime and mixing ratio measurements disagrees with models by an unacceptably large factor of two. To measure the B(E2) values directly, a beam of $^{21}$Na at 1.7 MeV/u from the TRIUMF ISAC facility was directed upon a 0.5 mg/cm$^{2} \quad ^{nat}$Ti target. Gamma-ray yield in coincidence with inelastically scattered heavy ions was measured with two TIGRESS high energy- and position-resolution germanium detector units and the BAMBINO highly segmented silicon detector system. The result resolves the discrepancy between the shell model and prior measurements. This represents the first radioactive in-beam experiment with TIGRESS. [Preview Abstract] |
Tuesday, April 17, 2007 1:54PM - 2:06PM |
Y8.00003: Nuclear Structure with Gammasphere/CHICO and beyond Ching-Yen Wu The sensitivity and selectivity attained by Gammasphere in conjunction with CHICO has provided unique opportunities to explore exotic aspects of nuclear structure, such as the population of $K$ isomers and electromagnetic properties of the $S$ band in nuclei. Furthermore, the capability of Gammasphere/CHICO allows the nuclear structure study expended into the new territory of exotic nuclear with extreme neutron/proton ratios, populated in the fission processes and deep-elastic reactions. Selective examples to address specific aspects of nuclear structure will be given. In addition, the plan to further improve the ability of studying the nuclear structure of exotic nuclei by building SuperCHICO with the position resolution matching that of GRETINA and TIGRESS will be discussed. [Preview Abstract] |
Tuesday, April 17, 2007 2:06PM - 2:18PM |
Y8.00004: Coulomb Excitation of Odd-A Neutron-Rich Radioactive Beams Near $^{132}$Sn Chang-Hong Yu, C. Baktash, J. Beene, A. Galindo-Uribarri, C.J. Gross, P.A. Hausladen, J.F. Liang, J. Pavan, D.C. Radford, J.C. Batchelder, C. Bingham, M. Danchev, W. Krolas, E. Padilla Coulomb excitation of even-even neutron-rich nuclei near $^{132}$Sn has been a tremendous success at the Holifield Radioactive Ion Beam Facility (HRIBF). This paper reports the extension of such studies to odd-A neutron-rich nuclei using RIBs provided by HRIBF. By using a 400-MeV A=129 radioactive beam and $^{50}$Ti targets, excited states in $^{129}$Sb and $^{129}$Te were populated by Coulomb excitations. The HRIBF CLARION, Hyball, and a Bragg detector were used to detect gamma rays, charged particles, and to monitor the beam composition, respectfully. Doppler corrected (event-by-event) gamma rays were identified as mainly belonging to $^{129}$Sb and$^{ 129}$Te, the main components of the mixed beam. Data analysis resulted in B(E2) values for four transitions in $^{129}$Sb. These values were compared to B(E2)'s measured [1] in $^{123}$Sb, the heaviest odd-A Sb isotope for which B(E2) measurements exist prior to this study, and showed similar trends between $^{123}$Sb and $^{129}$Sb. Phenomena revealed in the comparison call for further theoretical investigations. [1] K.C. Jain, et al., Phys. Rev. \textbf{C40}, (1989) 2400. [Preview Abstract] |
Tuesday, April 17, 2007 2:18PM - 2:30PM |
Y8.00005: Rotation around the longest principle axis in $^{142}$Gd I. Ragnarsson, B.G. Carlsson, E.O Lieder, R.M. Lieder, A.A. Pasternak It is difficult to verify triaxial shape in nuclei and no case has been clearly established where a triaxial nucleus rotates around the longer axis ($\gamma < -60^{\circ}$). In view of this fact, the yrast band of $^{142}$Gd (Z=64, N=78), which has been observed to I=34, displays some interesting features. The general properties of the band show that it is built with no particles excited across the N=82 gap, but with four protons excited across the Z=64 gap, i.e. it has a $\pi$(h$_{11/2})^4\nu$(h$_{11/2})^{-2}$ high-$j$ configuration. This configuration leads to triaxial shape with well-developed potential energy minima calculated for rotation around all three principal axes but with rotation around the intermediate axis energetically favoured at $I \approx 20$. However, a change of rotation axis takes place, because it is only for rotation around the longer axis that it is possible to build $I>30$ states. This conclusion is based on general rules how high-$j$ particles and holes align their spin vectors, and is supported by our recent lifetime measurements indicating a small collectivity at high spins. Another interesting feature is that the yrast states around I=40 are predicted to be dominated by favoured terminations at prolate shape. Cranking calculations and particle-rotor calculations will be presented to support our general conclusions. [Preview Abstract] |
Tuesday, April 17, 2007 2:30PM - 2:42PM |
Y8.00006: Band Terminations and Density Functional Theory: Critical Analysis Anatoli Afanasjev It was recently suggested in Refs. [1,2] that the set of terminating states in the N$\sim $Z, A$\sim $44 mass region provides unique and reliable constraints on time-odd mean fields and the strength of the spin-orbit interaction in Skyrme and covariant (relativistic) density functionals. The authors of these references claim that the method which they suggest and that is based on the energies of terminating states is free from the drawbacks of standard methods of defining spin-orbit interaction based on the measuring the single-particle energies of the spin-orbit partner orbitals. Thus, they claim that it allows to define very accurately both isoscalar and isovector channels of spin-orbit interaction. However, the detailed analysis performed shows that this is not a case. The results of this analysis will be presented in the talk. [1] H.Zdunczuk et al, Phys. Rev. C71, 024305 [2] A.Bhagwat et al, reprint nucl-th/0605009 [Preview Abstract] |
Tuesday, April 17, 2007 2:42PM - 2:54PM |
Y8.00007: Wobbling Modes in Ta Nuclei? D.J. Hartley, W.H. Mohr, J.R. Vanhoy, M.A. Riley, A. Aguilar, C. Teal, R.V.F. Janssens, M.P. Carpenter, A.A. Hecht, F.G. Kondev, T. Lauritsen, E.F. Moore, S. Zhu, M.K. Djongolov, M. Danchev, L.L. Riedinger, G.B. Hagemann, G. Sletten, P. Chowdhury, S.K. Tandel, W.C. Ma, S.W. $\O$deg\"{a}rd Although the existence of asymmetric (or triaxial) shaped nuclei has been predicted for decades, acquiring experimental proof of this shape has been quite challenging. A triaxial nucleus should produce a collective wobbling mode when it is given significant angular momentum. Indeed, evidence of this phenomenon has been observed in a few odd-A Lu nuclei. A neutron shell gap is often cited for the stabilization of this rare shape in Lu. Theoretical calculations suggest triaxiality should also be observed in Hf nuclei; but despite several measurements, no wobbling has been confirmed. Recently, a high- spin, Gammasphere experiment aimed at identifying the wobbling mode in $^{171}$Ta was performed in order to determine whether triaxial nuclei can be observed in the N$\approx$100 region. In addition, $^{169}$Ta was found to be strongly populated. No wobbling was found in either nucleus and conclusions based on these results will be discussed. [Preview Abstract] |
Tuesday, April 17, 2007 2:54PM - 3:06PM |
Y8.00008: Two-proton knockout in neutron rich N=40 nuclei P. Adrich, A.M. Amthor, D. Bazin, M.D. Bowen, C.M. Campbell, J.M. Cook, A. Gade, D. Galaviz, T. Glasmacher, S. McDaniel, D. Miller, Y. Shimbara, K.P. Siwek, J.R. Terry, A. Oberteli, D. Weisshaar Two-proton knockout from intermediate energy beams of neutron-rich nuclei impinging on light targets has been shown recently to be a direct reaction capable of producing neutron-rich residues and populating their excited states [1]. In an experiment carried out at the National Superconducting Cyclotron Laboratory at Michigan State University this reaction was used to produce and study in-beam $\gamma$-spectroscopy of excited states up to spin 6$^{+}$ in N$\sim$40 isotopes of iron, $^{64,66,68}$Fe. So far, these are the heaviest nuclei produced by means of two-proton knockout. The study was motivated by the recently reported onset of deformation in N$\sim$40 Cr and Fe isotopes below $^{68}$Ni [2, 3]. In this contribution, the experimental technique, gamma spectra and inclusive one- and two-proton knockout cross sections will be presented and discussed. \newline (1) D. Bazin \emph{et al}., Phys. Rev. Lett. 91, 012501 (2003) \newline (2) M. Hannawald \emph{et al}., Phys. Rev. Lett. 82, 1391 (1999) \newline (3) O. Sorlin \emph{et al}., Eur. Phys. J. A 16, 55 (2003) [Preview Abstract] |
Tuesday, April 17, 2007 3:06PM - 3:18PM |
Y8.00009: Shape and structure of N=Z $^{64}$Ge; Electromagnetic transition rates from the application of the Recoil Distance Method to knock-out reactions. K. Starosta, A. Dewald Transition rate measurements are reported for the $2^+_1$ and $2^+_2$ states in the N=Z nucleus $^{64}$Ge. The measurement was done utilizing the Recoil Distance Method (RDM) and a unique combination of state of the art instruments at the National Superconducting Cyclotron Laboratory (NSCL). States of interest were populated via an intermediate energy single neutron knock-out reaction. RDM studies of knock-out and fragmentation reaction products hold the promise of reaching far from stability and providing lifetime information for intermediate-spin excited states in a wide range of exotic nuclei. The large-scale Shell Model calculations applying the recently developed GXPF1A interaction are in excellent agreement with the above results. Theoretical analysis suggests that $^{64}$Ge is a collective $\gamma$-soft anharmonic vibrator. [Preview Abstract] |
Tuesday, April 17, 2007 3:18PM - 3:30PM |
Y8.00010: Detailed study of $^{21}$Mg excited states in one neutron knockout. C. Aa. Diget, P. Adrich, D. Bazin, M.D. Bowen, B.A. Brown, C.M. Campbell, J.M. Cook, A. Gade, T. Glasmacher, S. McDaniel, A. Obertelli, K. Siwek, J.R. Terry, D. Weisshaar, K. Hosier, D. McGlinchery, L.A. Riley The identification of excited state properties of $^{21}$Mg has so far been directed by comparison to the mirror nucleus $^{21}$F. To obtain independent information on excited states in $^{21}$Mg this nucleus is investigated in the single neutron knockout reaction: $^9$Be($^{22}$Mg,$^{21}$Mg)X. Following the knockout, the $\gamma$-decays are studied using in-beam $\gamma$-ray spectroscopy. From the $\gamma$-ray data, the properties of bound states in $^{21}$Mg are identified. Similarly, spectroscopic factors for the $^{22}$Mg ground state are determined through the individual level feedings. [Preview Abstract] |
Tuesday, April 17, 2007 3:30PM - 3:42PM |
Y8.00011: The role of very weak gamma-ray transitions W. David Kulp The success of a nuclear model is generally judged with respect to how well the model reproduces the experimental excitation spectrum and the transition strengths of the strongest $\gamma$-ray transitions. When many models apparently describe the same nucleus, measuring the weakest transitions very well can provide insight into which models are preferred and can lead to new physics when a simple model appears to break down. Gamma rays at the limits of detection are presented and the implications of these measurements for nuclear models are discussed. [Preview Abstract] |
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