Bulletin of the American Physical Society
2007 Annual Meeting of the Division of Nuclear Physics
Volume 52, Number 10
Wednesday–Saturday, October 10–13, 2007; Newport News, Virginia
Session CG: Nuclear Structure II |
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Chair: R. Grzywacz, University of Tennessee - Knoxville Room: Newport News Marriott at City Center Pearl Salon III |
Friday, October 12, 2007 9:00AM - 9:12AM |
CG.00001: Yrast structures of neutron-rich $^{51}$Ca and $^{52}$Sc S. Zhu, R.V.F. Janssens, M.P. Carpenter, T. Lauritsen, D. Seweryniak, B. Fornal, R. Broda, W. Kr\'olas, T. Pawlat, J. Wrzesinski, N. Marginean, L. Corradi, G.De Angelis, M. Honma, P.F. Mantica, P. Mason, T. Otsuka An $N$=32 subshell closure in neutron-rich nuclei around doubly-magic $^{48}$Ca$^{ }$occurs due to the weakening of the strong $\pi $f$_{7/2 }$- $\nu $f$_{5/2}$ monopole interaction as protons are removed from the f$_{7/2}$ shell. With more proton removed, the splitting between the $\nu $p$_{1/2}$ and $\nu $f$_{5/2}$ states may be sufficient to produce a subshell closure at $N$=34. $\gamma \gamma $ coincidence events, from species produced in deep-inelastic collisions of a $^{48}$Ca beam on a thick $^{238}$U target, were collected with the Gammashere array at Argonne. The same system was investigated by employing the PRISMA spectrometer coupled with the CLARA $\gamma $-ray multi-detector array at the INFN, LNL Legnaro. Analysis of the combined data sets allowed us to identify the $\gamma $ transitions in $^{51}$Ca and $^{52}$Sc, and to construct extended level schemes. The energy of these states will be compared with the results of shell model calculations. [Preview Abstract] |
Friday, October 12, 2007 9:12AM - 9:24AM |
CG.00002: Precision Branching Ratio Measurement for the Superallowed $\beta^+$ Emitter $^{62}$Ga Paul Finlay, C.E. Svensson, R.A.E. Austin, G.C. Ball, D. Bandyopadhyay, A. Chaffey, R.S. Chakrawarthy, P.E. Garrett, G.F. Grinyer, G. Hackman, B. Hyland, R. Kanungo, J.R. Leslie, C. Mattoon, A.C. Morton, C.J. Pearson, J.J. Ressler, F. Sarazin, H. Savajols A high-precision branching ratio measurement for the superallowed $\beta^+$ emitter $^{62}$Ga has been made using the 8$\pi$ $\gamma$-ray spectrometer in conjunction with the SCintillating Electron-Positron Tagging ARray (SCEPTAR) as part of an ongoing experimental program in superallowed Fermi beta decay studies at the Isotope Separator and Accelerator (ISAC) facility at TRIUMF in Vancouver, Canada, which delivered a high-purity beam of $\sim10^4$ $^{62}$Ga/s in December 2005. The present work represents the highest statistics measurement of the $^{62}$Ga superallowed branching ratio to date. 25 $\gamma$ rays emitted following non-superallowed decay branches of $^{62}$Ga have been identified and their intensities determined. These data yield a superallowed branching ratio with 10$^{-4}$ precision, and our observed branch to the first nonanalogue 0$^+$ state sets a new upper limit on the isospin-mixing correction $\delta_{C1}^1$. By comparing our {\em ft} value with the world average $\overline{\mathcal{F}}t$, we make stringent tests of the different calculations for the isospin-symmetry-breaking correction $\delta_C$, which is predicted to be large for $^{62}$Ga. [Preview Abstract] |
Friday, October 12, 2007 9:24AM - 9:36AM |
CG.00003: Enhanced lifetime measurement of low-lying states in 66Ge and 68Ge Robert Luettke, Elizabeth Ricard-McCutchan, Volker Werner, Ho-Chiang Ai, Robert Casperson, Richard Casten, Andreas Heinz, Jing Qian, Babak Shoraka, Russ Terry, Elizabeth Williams, Ryan Winkler Lifetimes in the Z=N+2 nucleus $^{66}$Ge and its neighbor $^{68}$Ge have been measured after a fusion evaporation reaction at the Wright Nuclear Structure Laboratory at Yale University, using the New Yale Plunger Device in the SPEEDY array. $^{66}$Ge was produced through the reaction $^{58}$Ni($^{10}$B,2np)$^{66}$Ge at E$_{10B}$=28 MeV, and $^{68}$Ge through $^{58}$Ni(12C, 2p)$^{68}$Ge at E$_{12C}$=38 MeV. Techniques used include gamma-gamma-coincidences and the recoil distance method. By comparing both nuclei, the evolution along the neutron-axis away from N=Z will be discussed. [Preview Abstract] |
Friday, October 12, 2007 9:36AM - 9:48AM |
CG.00004: A g-factor measurement of the $4^+$ state in $^{76}$Ge G. G\"urdal, N. Benczer-Koller, G. Kumbartzki, H. Ai, R. Casperson, R.F. Casten, A. Heinz, E.A. Mccutchan, J. Qian, V. Werner, E. Williams, R. Winkler The $^{70-76}$Ge isotopes are transitional nuclei with $E_4^+/E_2^+$ ratios varying from 2.07 to 2.51. The systematic studies of g factors of $2^+_1$ and $4^+_1$ excited states provide an understanding of the microscopic structure of these nuclei since the g factors are very sensitive to the proton and neutron contributions to the wave functions. The measured results can be compared to the predictions of either shell model calculations or collective vibrational excitations. The measured g factors of $2^+_1$ in Ge nuclei, as well as in neighboring Zr and Se nuclei show reasonable agreement with $Z/A$, a signature of collective behavior. However, in this region the g factors of the $4^+_1$ states are somewhat larger than $Z/A$, albeit with large error bars. In this work, the first measurement of the g factor of the excited $4^+_1$ state of $^{76}$Ge using the Transient Field (TF) technique will be presented. The $4^+_1$ state of $^{76}$Ge was populated by Coulomb excitation on a C target of a beam of $^{76}$Ge (190-230 MeV) in inverse kinematics. Work supported by the U.S. National Science Foundation and U.S.D.O.E under grant DE-FG02-91ER-40609. [Preview Abstract] |
Friday, October 12, 2007 9:48AM - 10:00AM |
CG.00005: Low-energy Coulomb excitation of radioactive $^{70}$Se Aaron Hurst An isobarically pure beam of $^{70}$Se ions was post accelerated to an energy of 206~MeV using {\small REX-ISOLDE}. Coulomb-excitation yields for states in the beam and target nuclei were deduced by recording de-excitation $\gamma$ rays in the highly segmented {\small MINIBALL} $\gamma$-ray spectrometer in coincidence with scattered particles in a silicon detector. At these energies, the Coulomb-excitation yield for the $2^{+}_{1}$ state in $^{70}$Se is expected to be strongly sensitive to the sign of the spectroscopic quadrupole moment through the nuclear reorientation effect. Experimental evidence is presented here for a prolate shape for this state, using an earlier published lifetime measurement, reopening the question over whether there are deformed oblate shapes close to the ground state in the neutron-deficient selenium isotopes. [Preview Abstract] |
Friday, October 12, 2007 10:00AM - 10:12AM |
CG.00006: Decay spectroscopy of $^{76-79}$Cu and $^{83,84}$Ga C.J. Gross, K.P. Rykaczewski, D. Shapira, J.A. Winger, S.V. Ilyushkin, R.K. Grzywacz, C.R. Bingham, S.N. Liddick, C. Mazzocchi, S. Padgett, M.M. Rajabali, E.F. Zganjar, A. Piechaczek, J.C. Batchelder, J.H. Hamilton, C. Goodin, A. Korgul, W. Krolas We have developed a technique to enhance beta decay studies of neutron-rich fission products by ranging-out high Z isobars or tagging all isobars in a transmission ionization chamber (IC). Neutron-rich radioactive ion beams are accelerated to 2.5-3 MeV/u and pass through a microchannel-plate-thin-foil detector (MCP) and the IC. With high gas pressure in the IC, isobars with low Z, pass through the IC and are deposited on a moving tape while those with higher Z are stopped inside the gas volume or mylar exit window. The tape transports the sample to the measuring station. With lower gas pressure all isobars are implanted at the measuring station and the tape is used to remove the daughters. For very short-lived isotopes, time correlations with the MCP can be used to enhance selectivity. Among our results, we have observed for the first time the 730 keV $\gamma$ ray following beta-delayed-neutron emission from $^{79}$Cu. In the decay of $^{84}$Ge, our data suggest that the $2^+$ state in $^{84}$Ge is 624~keV. Additional results on $^{76-79}$Cu and $^{83,84}$Ga beta decay will be presented. [Preview Abstract] |
Friday, October 12, 2007 10:12AM - 10:24AM |
CG.00007: Beta decay studies of neutron-rich nuclei near $^{78}$Ni S.N. Liddick, M. Rajabali, R. Grzywacz, C.R. Bingham, I.G. Darby, T. Ginter, P. Mantica, K.P. Rykaczewski, C. Mazzocchi, J.C. Batchelder, S.V. Ilyushkin, J.A. Winger, M. Karny, K. Miernik, M. Pfutzner Experiments on nuclei near doubly magic $^{78}$Ni can provide a wealth of information on the underlying nuclear structure in this region. Neutron-rich nuclei in the $^{78}$Ni region were investigated through beta decay spectroscopy following their production from the fragmentation of a $^{86}$Kr beam, at 140 AMev, on a Be target. The experimental setup consisted of a thick Double-sided Silicon Strip Detector for the correlation of implanted ions with their subsequent beta decays and the NSCL Segmented Germanium Array (SeGA) to monitor the emitted gamma rays. All detectors were read out with a new digital data acquisition system based on the Pixie16 boards produced by XIA and further developed at the University of Tennessee and Oak Ridge National Laboratory. Preliminary results of the experiment and the performance of the digital acquisition will be presented. This work was supported in part by the NSF Grant PHY-06-06007 and by the DOE Grants DE-FG02-96ER40983, DE-AC05-00OR22725, DE-FG02-96ER41006 and in part by the NNSA through DOE Cooperative Agreement DEFC03-03NA00143 [Preview Abstract] |
Friday, October 12, 2007 10:24AM - 10:36AM |
CG.00008: Structure of Fe isotopes at the limits of the pf-shell Nathan Hoteling, W.B. Walters, R.V.F. Janssens In this paper, new data from the deep-inelastic reaction of $^{64}$Ni and $^{238}$U will be discussed with respect to new levels identified in the Fe isotopes near N=40. Results will be discussed within the framework of the shell model ``beyond the pf-shell'', and implications to the structural trends in this region will be assessed. Particular emphasis will be directed toward new level schemes that have been deduced for $^{61}$Fe and $^{64}$Fe, and new low-spin structure identified from beta-decay of $^{62}$Mn in ``delayed'' spectra. [Preview Abstract] |
Friday, October 12, 2007 10:36AM - 10:48AM |
CG.00009: Investigation of the Spin cut-off Parameter of nuclear Level Density Aziz N. Behkami The spin cut-off parameter has been investigated within the microscopic approach based on the BCS Hamiltonian. This parameter has been determined for a long range of even-even, odd-A and odd-odd, light, medium weight as well as heavy and spherical and deformed nuclei. The spin cut-off parameter $\sigma^{2}(E)$ has also be determined using the macroscopic methods and the results are compared with their corresponding microscopic values. It is found that the values of $\sigma^{2}$ at neutron binding energy $B_n$, show structure reflecting the shell model orbitals near the Fermi energy. The huge difference between $\sigma^{2}(B_n)$ deduced from the model calculations for nuclei near major shells indicates that the large angular momentum are responsible for their differences. In particular, the $1f_{7/2}$, $1g_{7/2}$ and $1i_{11/2}$ proton orbitals and the $1g_{9/2}$, $1h_{11/2}$ and $1i_{13/2}$ neutron orbitals play an important role in the $\sigma^{2}(B_n)$ values. [Preview Abstract] |
Friday, October 12, 2007 10:48AM - 11:00AM |
CG.00010: Parity measurements in $^{80}$Sr R.A. Kaye, C.S. Myers, J. D\"{o}ring, S.L. Tabor, T.D. Baldwin, D.B. Campbell, C. Chandler, M.W. Cooper, C.R. Hoffman, J. Pavan, M. Wiedeking, S.M. Gerbick, L.A. Riley, M.A. Hallstrom High-spin states in $^{80}$Sr were studied using the $^{54}$Fe($^{28}$Si,2$p$) reaction at 90 MeV and the $^{58}$Ni($^{28}$Si,$\alpha$2$p$) reaction at 110 MeV using the Tandem--Superconducting LINAC accelerator at Florida State University (FSU). Prompt $\gamma-\gamma$ coincidences were measured in the first reaction using the FSU array of ten Compton-suppressed Ge detectors. $\gamma$-ray linear polarizations were measured in both reactions using three Clover detectors as Compton polarimeters. As a result of these measurements, the parities of three non-yrast band structures have been determined, confirming the interpretations of the most recent in-beam and $\beta$-decay studies. In particular, the most strongly populated non-yrast band has been assigned negative parity, in agreement with most other neighboring even-even nuclei. Another high-spin sequence, with a parity assignment left open by the most recent in-beam study, has been tentatively assigned negative parity. [Preview Abstract] |
Friday, October 12, 2007 11:00AM - 11:12AM |
CG.00011: High-spin states in $^{88}$Kr N. Fotiades, A.F. Lisetskiy, J.A. Cizewski, R. Kr\"{u}cken, R.M. Clark, P. Fallon, I.Y. Lee, A.O. Macchiavelli, J.A. Becker, W. Younes High-spin states in $^{88}$Kr have been studied following the fission of the $^{226}$Th compound nucleus formed in a fusion-evaporation reaction ($^{18}$O at 91~MeV on $^{208}$Pb). The Gammasphere array was used to detect $\gamma$-ray coincidences. High-spin states up to spin (14$^{+}$) and $\sim$8~MeV excitation energy have been established. The level scheme reported for $^{88}$Kr in the spontaneous fission of $^{248}$Cm~[1] has been enriched and extended to higher spin and excitation energies. Differences between the level scheme reported in~[1] and that obtained in the present work will be discussed. The observed experimental states are also compared with theoretical shell-model and interacting-boson-model-2 calculations. This work has been supported by the U.S. Department of Energy under Contracts No. DE-AC52-06NA25396 (LANL), W-7405-ENG-48 (LLNL) and AC03-76SF00098 (LBNL) and by the National Science Foundation (Rutgers). \\ \noindent [1] T. Rz\c{a}ca-Urban {\it et al.}, Eur. Phys. J. A {\bf 9}, 165 (2000). [Preview Abstract] |
Friday, October 12, 2007 11:12AM - 11:24AM |
CG.00012: Microscopic formation of mixed-symmetry states V. Werner, N. Benczer-Koller, P. Boutachkov, G. Kumbartzki, E. Stefanova, J. Holt, N. Pietralla, M. Perry, H. Ai, R.F. Casten, S. Eckel, A. Heinz, C. Lambie-Hanson, E.A. McCutchan, D.A. Meyer, J. Qian, A. Schmidt, E. Willams, R. Winkler, G. G\"urdal, M. Chamberlain, C.R. Fitzpatrick, A.B. Garnsworthy, N.J. Thompson, R.B. Cakirli One-phonon fully symmetric (FS) and mixed-symmetry (MS) states have been found to have pure F-spin near the N=50 shell closure. In Zr isotopes the proton and neutron contributions forming FS and MS states are isopolarized. Hence, the $2^+_1$ states in $^{92,94}$Zr have negative g factors due to dominant neutron configurations. The isopolarization was experimentally proven measuring the g factors of the $2^+_2$ states in $^{92,94}$Zr to be large and positive. Final values will be presented, and brought into context of the formation of FS and MS states near shell closures. [Preview Abstract] |
Friday, October 12, 2007 11:24AM - 11:36AM |
CG.00013: Study of mixed-symmetry states in $^{94}_{40}$Zr with (n,n$'\gamma$) reaction Esmat Elhami, S.N. Choudry, B. Crider, S. Mukhopadhyay, J.N. Orce, M. Scheck, M.T. McEllistrem, S.W. Yates, A.P. Tonchev group, D. Weisshaar The low-spin structure of $^{94}_{40}$Zr has been studied with the $(n,n'\gamma)$ reaction at the University of Kentucky 7 MV Van de Graaff acelerator facility and at TUNL at Duke University. Branching ratios, lifetimes, multipolarities and spin assignments have been determined. The 2$^{+}_{2}$ state at 1671.4 keV has been identified as the lowest mixed-symmetry state in $^{94}$Zr; $B(M1; 2^{+}_{1,ms} \rightarrow 2^{+}_{1})$ = 0.33(5) $\mu^{2}_{N}$. This state has an anomalous decay behavior, i.e., $B(E2; 2^{+}_{1,ms} \rightarrow 0^{+}_{1})$ = 8(1) W.u. is unusually larger compared to the $B(E2; 2^{+}_{1} \rightarrow 0^{+}_{1})$ = 4.9(11)W.u. The analysis of angular distribution data reveals even more anomalies in the states above the mixed-symmetry, 2$^{+}_{1,ms}$, state. For example, the 4$^{+}_{2}$ state, at 2330 keV decays strongly to the 2$^{+}_{1}$ state, $B(E2; 4^{+}_{2} \rightarrow 2^{+}_{1})$ = 19(2) W.u. compared to the 4$^{+}_{1}$ state at 1470 keV, $B(E2; 4^{+}_{1} \rightarrow 2^{+}_{1})$ = 0.88(2) W.u. Some results from the angular distributions data at $E_{n}$=2.8 and 3.5 MeV will be presented to address this issue. [Preview Abstract] |
Friday, October 12, 2007 11:36AM - 11:48AM |
CG.00014: Low-lying States of $^{53}$Sc and $^{55}$Sc from Shell Model Erdal Dikmen, Sunduz Korkmaz, Michel Vallieres We have carried out the shell model calculations for the odd-even $^{53}$Sc and $^{55}$Sc isotopes. We use three different two-body effective nucleon-nucleon (NN) interactions, KB3, FPD6, GXPF1, in the calculations. We obtain the low-lying energy spectra of $^{53}$Sc and $^{55}$Sc isotopes in the full pf-shell model space. [Preview Abstract] |
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