Bulletin of the American Physical Society
2006 Division of Nuclear Physics Annual Meeting
Wednesday–Saturday, October 25–28, 2006; Nashville, Tennessee
Session GC: Nuclear Structure IV |
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Sponsoring Units: DNP Chair: Jolie Cizewski, Rutgers University Room: Gaylord Opryland Tennessee B |
Saturday, October 28, 2006 9:00AM - 9:12AM |
GC.00001: Quadrupole moments of normal deformed and triaxial strongly deformed bands in $^{167}$Lu E. Ngijoi-Yogo, W.C. Ma, D.G. Roux, R.B. Yadav, Y. Zhang, G.B. Hagemann, C.R. Hansen, B. Herskind, G. Sletten, H. Amro, D.A. Meyer, G. G\"urdal, C.W. Beausang, D.J. Hartley, C. Engelhardt, H. H\"ubel, A. Neusser, P. Bringel, M.P. Carpenter, T.L. Khoo, T. Lauritsen, E.F. Moore $^{167}$Lu is the heaviest Lu isotope in which wobbling motion, a characteristic excitation mode of triaxial structures, has been identified [1,2]. In order to further quantify the quadrupole moments (Q$_{t}$) of these structures, lifetime measurements were performed at ANL using fusion-evaporation reaction and GAMMASPHERE. Previously, the Q$_{t}$ of TSD1 was analyzed [3]. In this work, the $\gamma$-ray lineshapes were analyzed with the Doppler-shift attenuation method and Q$_{t}$ extracted for bands TSD3, [404]7/2$^{+}$ and [541]1/2$^{-}$. The measured Q$_{t}$ of TSD3 is much smaller than the predicted Q$_{t}$=10.1 eb by CSM calculations using the Ultimate Cranker code, and supports the suggestion that the barrier between normal deformed and TSD potential energy minima is rather small [2]. ~*Work supported by US DOE and NSF.\\ $^{1}$ H. Amro et al., Phys. Lett. B 553, 197 (2003).\\ $^{2}$ H. Amro et al., Phys. Rev. C 71, 011302 (2005).\\ $^{3}$ G. G"urdal et al., J. Phys. G 31, S1873 (2005). [Preview Abstract] |
Saturday, October 28, 2006 9:12AM - 9:24AM |
GC.00002: Internal Conversion Coefficient Measurements of Transitions in $^{167}$Lu G. G\"urdal, C.W. Beausang, D.S. Brenner, H. Ai, R.F. Casten, A. Heinz, E. Williams, B. Crider, R. Raabe, D.J. Hartley, M. Carpenter, R.V.F. Janssens, T. Lauritsen, C.J. Lister, D. Seweryniak, S. Zhu, A.A. Hecht, J.X. Saladin Experimental internal conversion coefficients can be used to determine the multipolarities of electromagnetic transitions and thus are valuable for assigning or confirming spins and parities of excited states. The normal and highly deformed bands of $\mathrm{^{167}Lu}$ were populated by the $\mathrm{^{123}Sb}(\mathrm{^{48}Ca}$,4n) reaction. Gammasphere and ICEBALL spectrometers were used to detect the coincidences between $\gamma$ rays and electrons. $\gamma\gamma$ and $\gamma$e matrices as well as $\gamma\gamma\gamma$ and $\gamma\gamma$e cubes were produced to analyze the coincidence data. Internal conversion coefficients were measured for the transitions from the normal deformed bands and the triaxially deformed bands, which have the importance of testing the wobbling mode in $\mathrm{^{167}Lu}$. The preliminary results of the analysis will be presented. This work was supported by the U.S D.O.E grants DE-FG02-88ER40417, DE-FG02-91ER-40609, DE-FG52-06NA26206, DE-FG02-05ER41379, Contract No. W-31-109-ENG-38 and by the NSF grant number PHY-0300673. [Preview Abstract] |
Saturday, October 28, 2006 9:24AM - 9:36AM |
GC.00003: Four-, Six- and Eight-quasiparticle Isomers in $^{174}$Lu F.G. Kondev, I. Ahmad, M.P. Carpenter, R.V.F. Janssens, T. Lauritsen, C.J. Lister, D. Seweryniak, G.D. Dracoulis, G.J. Lane, A.P. Byrne, T. Kibedi, P. Chowdhury, S.K. Tandel We report on new studies of $^{174}$Lu using a $^{136}$Xe beam from the ATLAS accelerator at Argonne National Laboratory that was incident on targets of natural Lu and enriched (to 47\%) $^ {176}$Lu. The targets were 6 mg/cm$^{2}$ in thickness with 25 mg/cm$^{2}$ of Au backing. The beam was pulsed approximately 1 ns on/825 ns off at energies of 6 MeV per nucleon. The recoils were stopped at the target position in the focus of the Gammasphere spectrometer, comprised for this experiment of 96 Compton-suppressed Ge detectors. Several high-K isomers were discovered in $^{174}$Lu and their structure characterized, including K$^{\pi}$=13$^{+}$ ($\tau$=280 (25) ns), 14$^{-}$ ($\tau$=55 (6) ns), (21$^{+}$) ($\tau$=140 (15) ns) and (26$^{-} $) ($\tau$=350 (28) ns). The 13$^{+}$ isomer was found to decay to both low-K (K$^{\pi}$=0$^{+}$) and high-K (K$^{\pi}$=7$^{+} $) states originating from couplings of the $\pi$ $7/2[404]$ and $\nu$ $7/2[633]$ orbitals. The path via the K$^{\pi}$=0$^{+} $ band cannot be explained through conventional Coriolis mixings and the accidental mixing scenario between the isomer and a nearby collective level was invoked [1]. Details of these measurements will be presented, together with a comparison with predictions from multi-quasiparticle calculations. \\ $[1]$ G.D. Dracoulis et al., submitted to Phys. Rev. Lett. \\ [Preview Abstract] |
Saturday, October 28, 2006 9:36AM - 9:48AM |
GC.00004: Search for highly deformed rotational structures in tungsten isotopes S.K. Tandel, A.J. Knox, U.S. Tandel, C. Parnell-Lampen, P. Chowdhury, D.J. Hartley, Jing-Ye Zhang, M.P. Carpenter, R.V.F. Janssens, T.L. Khoo, T. Lauritsen, C.J. Lister, D. Seweryniak, X. Wang, S. Zhu Highly excited states in tungsten isotopes ($^{172-175}$W) in the vicinity of N=100 were populated using a 225 MeV (on target) $^{50}$Ti beam from the ATLAS accelerator at Argonne National Laboratory, incident on a 235 $\mu $g/cm$^{2} \quad ^{128}$Te target. The $\gamma $-rays emitted by the evaporation residues were detected using the Gammasphere array. New rotational structures have been identified and existing bands have been extended, in some cases, beyond the second nucleon alignment. Theoretical calculations [1, 2] predict that triaxial structures with large deformation become yrast at high spins in this region. Results from the ongoing analysis will be presented and discussed within the context of triaxial superdeformed structures observed and expected in this mass region. [1] T. Bengtsson, Nucl. Phys. A 512, 124 (1990). [2] M.K. Djongolov et al., Phys. Lett. B 560, 24 (2003). [Preview Abstract] |
Saturday, October 28, 2006 9:48AM - 10:00AM |
GC.00005: Nucleon Alignment and Shape Competition at High Spin in $^{180}$Hf U.S. Tandel, P. Chowdhury, S.K. Tandel, S. Sheppard, D. Cline, C.Y. Wu, M.P. Carpenter, R.V.F. Janssens, T.L. Khoo, T. Lauritsen, C.J. Lister, D. Seweryniak, S. Zhu In light even-N Hf isotopes (N = 96-106), the first i$_{13/2}$ neutron alignment occurs at h$\omega \quad <$ 0.3 MeV. In contrast, no alignment was observed up to $\sim $ 0.4 MeV in $^{180,182}$Hf (N = 108,110) [1]. Theoretical calculations predict that oblate collective rotation becomes yrast at high spins in $^{180}$Hf [2, 3]. In the present work, the yrast band of $^{180}$Hf has been extended to high spins, via inelastic excitation, using a 1300 MeV $^{180}$Hf beam incident on a thin $^{232}$Th target. The $\gamma $ rays were detected by Gammasphere, with event by event Doppler correction and Q-value selectivity provided by CHICO. The data reveal onset of the first nucleon alignment in $^{180}$Hf at h$\omega $ $\sim $ 0.43 MeV, which is significantly higher than predictions ($\sim $ 0.35 MeV). Interestingly, the $\gamma $-vibrational band is crossed by a band with apparent high moment-of-inertia at $\sim $ 0.25 MeV. This structure, which becomes near yrast at the highest observed spins will be discussed in the context of nucleon alignment and shape competition at high spin in $^{180}$Hf. [1] E. Ngijoi-Yogo, Ph.D. thesis, U.Mass. Lowell (2004) [2] R.R. Hilton and H.J. Mang, Phys. Rev. Lett. 43, 1979 (1979). [3] F.R. Xu et al., Phys. Rev. C62, 014301 (2000). [Preview Abstract] |
Saturday, October 28, 2006 10:00AM - 10:12AM |
GC.00006: Studies of Triaxial Rotor and Band Mixing in $^{186,\,188,\,190,\,192}$\textrm{Os} J.M. Allmond, J.L. Wood, A-M. Oros-Peusquens, R. Zaballa, W.D. Kulp The $E2$ matrix elements of the Osmium isotopes are studied in the framework of collective rotations [1]. The rotational model with $\Delta K=2$ mixing fits the data fairly well; but deviations at high spin suggest that the spin-dependance of $\Delta K=2$ mixing in the rotor model is too strong. This supports a finding in $^{166}$\textrm{Er} [2]. \\ \\ $[1]$ J.L.~Wood, A-M.~Oros-Peusquens, R.~Zaballa, J.M.~Allmond, and W.D. Kulp, Phys.~Rev.~C \textbf{70}, 024308 (2004). \\ $[2]$ W.D.~Kulp, J.M.~Allmond, P.~Hatcher, and J.L.~Wood, Phys.~Rev.~C \textbf{73}, 014308 (2006). [Preview Abstract] |
Saturday, October 28, 2006 10:12AM - 10:24AM |
GC.00007: ABSTRACT WITHDRAWN |
Saturday, October 28, 2006 10:24AM - 10:36AM |
GC.00008: Search for isomers in {\boldmath $^{199-203}$}Tl N. Fotiades, R.O. Nelson, M. Devlin, J.A. Becker, W. Younes The $^{203}$Tl($n,xn\gamma$) reactions were used to study excited states in $^{199-203}$Tl isotopes. The data were taken using the GEANIE spectrometer comprised of 26 high-purity Ge detectors. The pulsed neutron source of the Los Alamos Neutron Science Center's WNR facility provided neutrons in the energy range from 0.6 to 250~MeV. The time-of-flight technique was used to determine the incident neutron energies. Partial $\gamma$-ray cross sections were measured from the beam-on data while half-lives of isomers were determined from the beam-off data. A candidate for the unknown 9/2$^{-}$ isomer in $^{203}$Tl has been identified. The half-life of this state is less than the shortest half-life that could be determined in the present experiment (typically, the half-lifes that can be currently measured with GEANIE vary between a few $\mu$s to a few ms). The candidate state is located at 1484-keV excitation energy, in excellent agreement with the theoretical prediction for the excitation energy of the 9/2$^{-}$ state in Ref.~[1]. In $^{202}$Tl, for the previously known 7$^{+}$ isomer at 950-keV excitation energy a 592(4)~$\mu$s half-life was obtained in the present work, which differs by $\sim 4 \%$ from the value adopted in the literature [572(7)~$\mu$s]. In the lighter Tl isotopes, the life-time measurements yielded results in agreement with previously known values reported in the literature. This work was supported by the U.S. Department of Energy under Contracts No. DE-AC52-06NA25396 (LANL) and W-7405-ENG-48 (LLNL). \\ \noindent [1] G. E. Arenas Peris and P. Federman, Phys. Rev. {\bf C 38}, 493 (1988). [Preview Abstract] |
Saturday, October 28, 2006 10:36AM - 10:48AM |
GC.00009: Octupole Strength in the $^{238,240,242}$Pu isotopes X. Wang, S. Zhu, R.V.F. Janssens, M.P. Carpenter, I. Ahmad, J.P. Greene, T.L. Khoo, F.G. Kondev, T. Lauritsen, C.J. Lister, D. Seweryniak, S.J. Freeman, U. Garg, I. Wiedenh{\"o}ever, A. Bernstein, P. Wilson, E. Diffenderfer, C. Teal, A. Larabee, B. Meredith A series of so-called ``unsafe'' Coulomb excitation experiments as well as 1-neutron transfer measurements was carried out with Gammasphere at the ATLAS accelerator in order to investigate the collective properties of $^{238,240,242}$Pu. New experimental evidence has emerged that in $^{240}$Pu a transition from an octupole vibration to stable octupole deformation occurs at high spin. A similar situation may be present in $^{238}$Pu, but is clearly absent in the heavier $^ {242}$Pu. The data will be presented and discussed together with available information on neighboring nuclei in the region. [Preview Abstract] |
Saturday, October 28, 2006 10:48AM - 11:00AM |
GC.00010: Preliminary results on $^{241,243}$Am and $^{235}$U (n,$\gamma$) cross sections measured at DANCE M. Jandel, T.A. Bredeweg, M.M. Fowler, E.M. Bond, J.M. O'Donnell, R. Reifarth, R.S. Rundberg, J.L. Ullmann, D.J. Vieira, J.B. Wilhelmy, J.M. Wouters, R.A. Macri, C.-Y. Wu, J.A. Becker The Detector for Advanced Neutron Capture Experiments (DANCE) at Los Alamos National Laboratory (LANL) was used for neutron capture cross sections measurements. Its high granularity of 160 BaF$_{2}$ detectors allows for highly efficient detection of prompt gamma-rays following a neutron capture. DANCE is located on the 20.26 m neutron flight path 14 at the Manuel Lujan Jr. Neutron Scattering Center at the Los Alamos Neutron Science Center (LANSCE). The moderated production target provides neutrons in the 0.02 eV - 500 keV energy range. An analysis of neutron capture measurements on $^{241,243}$Am and $^{235}$U targets will be presented. The experiments were carried out using a customized Parallel-Plate Avalanche Counter (PPAC) detector installed in the center of the DANCE array. The PPAC was used as a fission-tagging detector to separate (n,$\gamma$) from (n,fission) events. Preliminary results of (n,$\gamma$) cross sections will be presented and compared with the available evaluated data for neutron energies from 0.02 eV to 1 keV. Additional neutron capture measurements with DANCE will be briefly discussed. [Preview Abstract] |
Saturday, October 28, 2006 11:00AM - 11:12AM |
GC.00011: $\gamma-\gamma$ Angular Correlations and g-factor Measurements from Spontaneous Fission of $^{252}$Cf with Gammasphere K. Li, C. Goodin, A.V. Ramayya, J.H. Hamilton, J.K. Hwang, A.V. Daniel, G.M. Ter-Akopian, N.J. Stone, J.O. Rasmussen, Y.X. Luo, S.J. Zhu Measurements of g-factors of excited states have been of interest for decades for the investigation of nuclear structures. The g-factors of excited states in several neutron-rich nuclei have been determined by measuring attenuated $\gamma$-ray angular correlations from spontaneous fission of $^ {252}$Cf with the Gammasphere detector array. A $^{252}$Cf fission source was sandwiched between two iron foils (10 mg/cm$^2$) and placed at the center of Gammasphere. For successive transitions in a cascade with the lifetime of the intermediate state much greater than the stopping time of the fission fragments, it is assumed that the fission fragments are implanted into the iron foils before emitting $\gamma$-rays. By measuring the time-integral attenuation coefficients, the mean Larmor precession angle of the intermediate state is obtained, which is proportional to the lifetime and g-factor of the state and the hyperfine field acting on the nucleus. Lifetimes of several states have been measured by using the triple $\gamma$ coincidence technique. We will present details of this technique and compare our results with previous measurements. [Preview Abstract] |
Saturday, October 28, 2006 11:12AM - 11:24AM |
GC.00012: Alpha decay of $^{257}$Rf* J. Qian, A. Heinz, R. Winkler, J. Vinson, A.B. Gamsworthy, R.V.F. Janssens, D. Peterson, D. Seweryniak, B. Back, M.P. Carpenter, G. Savard, A.A. Hecht, C.L. Jiang, T.L. Khoo, F.G. Kondev, T. Lauritsen, C.J. Lister, A. Robinson, X. Wang, S. Zhu, M. Asai In this work, we studied $^{257}$Rf by using the Fragment Mass Analyzer(FMA) at Argonne National Laboratory. In addition to $\alpha$-$\alpha$ correlations, the FMA measures the Mass/charge (A/Q) ratio of residues which helps to identify the origin of weak alpha lines. A 233 MeV $^{50}$Ti beam with an average intensity of 115 pnA impinged on a rotating $^{208}$Pb target wheel. The recoil positions at the FMA focal plane - which allow the measurement of the A/Q ratio - were detected with a Parallel Grid Avalanche Counter (PGAC) and subsequently implanted in a Double-sided Silicon Strip Detector (DSSD). Recoils are identified by their energy deposition inside the DSSD as well as their time-of-flight between the PGAC and the DSSD. Position and time of implant events and subsequent alpha decays were measured and correlated. The details of the data will be presented. These data can test model predictions concerning the next proton shell closure and the structure of heavy nuclei in the absence of a liquid-drop fission barrier. This work was supported by the U.S.Department of Energy, Office of Nuclear Physics, under contract No. W-31-109-ENG-38 and DE- FG02-91ER-40609, and Nexia Solutions Ltd. [Preview Abstract] |
Saturday, October 28, 2006 11:24AM - 11:36AM |
GC.00013: Microscopic study of Nuclear Spin Cut-off Parameter Aziz Behkami, Mehmet Kildir, Mehrdad Gholami Spin cut-off parameter and effective moment of inertia have been investigated within the microscopic approach based on BCS- Hamiltonian. The spin cut-off parameter has been calculated at neutron binding energies over a large range of nuclear mass A, using the BCS theory. The results are compared with their corresponding macroscopic values. It is found that the values of $_{ }\sigma ^{2}$ (E) do not increase smoothly with A as expected based on macroscopic theory. Instead, the values of $\sigma ^{2}$ (E) shows structure reflecting the angular momentum of the shell model orbitals near the Fermi energy. The spin cut-off parameter $\sigma ^{2}$ (E) has also been computed from the knowledge of nuclear level density, at neutron binding energy, B$_{n }$and the average S-wave neutron spacing, $\sigma ^{2}$ = 1/2$\rho $(B$_{n})<$D$_{1/2}>$ . These are also compared with their corresponding values from the model calculations. The results will be presented and discussed. [Preview Abstract] |
Saturday, October 28, 2006 11:36AM - 11:48AM |
GC.00014: Proton configuration and mass variations are observed in each of the 3036 isotopes studied. Eugene Pamfiloff The fission and decay transitions of unstable isotopes are studied with particular detailed analysis of nuclei masses, proton - neutron substructure, and the change in mass experienced by individual nucleons of parent, daughter and product isotopes. The data shows the 3036 isotopes studied contain nucleons of a mass unique to each isotope, and further, indicating 3036 proton variations, each differentiated by a distinct mass. Of these, 283 proton variations are further distinguished by belonging to stable benchmark isotopes. The same variations were found with bound neutrons. A direct correlation is observed between the nearest stable benchmark mass and the mass of the nucleon returning to ground state during the transition, indicating a mass dependence to nuclear stability. These findings indicate that a nucleus in an excited state cannot stabilize or return to the ground state until it adjusts mass to match the nearest $Z - N$ and mass per nucleon benchmark. These conclusions were further tested with the analysis of nucleon mass adjustments occurring within the natural and artificial alpha emitter nuclei. The developed system of analysis provided good results when tested against the incident and product particles of high and low energy interactions and events of nuclear transmutation. Every transition to a stable product demonstrates a strong correlation with a specific mass per nucleon benchmark as a third condition of nuclear stability. [Preview Abstract] |
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