Bulletin of the American Physical Society
2006 Division of Nuclear Physics Annual Meeting
Wednesday–Saturday, October 25–28, 2006; Nashville, Tennessee
Session DC: Nuclear Structure III |
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Sponsoring Units: DNP Chair: Elizabeth A. McCutchan, Yale University Room: Gaylord Opryland Tennessee B |
Friday, October 27, 2006 2:00PM - 2:12PM |
DC.00001: The Parity Dependence of Nuclear Level Densities Hanghui Chen, Yoram Alhassid, Angel Manzur We use a simple model to calculate the odd-to-even parity ratio of nuclear level densities as a function of excitation energy. The model is based on a deformed single-particle Hamiltonian with pairing interaction. It differs from the model introduced in Ref. [1] by including fluctuations in the pairing gap and using number-parity projection to account for odd-even effects in particle number. We compare the results of the simple model with microscopic shell model Monte Carlo calculations in the full $fp+g_{9/2}$ shell for nuclei in the iron region. \newline [1] Y. Alhassid, G.F. Bertsch, S. Liu, and H. Nakada, Phys. Rev. Lett. 84, 4313 (2000). [Preview Abstract] |
Friday, October 27, 2006 2:12PM - 2:24PM |
DC.00002: Time - Odd Mean Fields in Covariant Density Functional Theory Anatoli Afanasjev The role of the time-odd mean fields, their evidence in experiment, and an accurate description of these fields are subjects of current interest. In the covariant density functional theory they are related to nuclear magnetism: time-odd mean fields arise from the space-like parts of the vector mesons and Lorentz invariance requires that their coupling strength is identical to that of the time-like parts. The role of time-odd fields and their impact on physical observables will be presented. It will be shown that these fields modify the moments of inertia, effective alignments, alignment gains at the band crossings and other physical observables in rotating nuclei. In particular, the question whether the time-odd mean fields are related to the isoscalar proton-neutron pairing will be discussed. The time-odd mean fields also reveal themselves in non-rotating odd and odd-odd nuclei (nuclei with broken time-reversal symmetry). Their impact on the binding energies, odd -even mass differences, the structure of such nuclei along the neutron and proton-drip lines will be discussed. Magnetic properties of deformed nuclei and the impact of time-odd mean fields on such properties, the study of which is in progress, will be covered. Whenever it is possible, the results will be compared with the ones obtained in the nonrelativistic Skyrme density functionals. [Preview Abstract] |
Friday, October 27, 2006 2:24PM - 2:36PM |
DC.00003: Coupling of chiral and shape vibrations in the A=130 region Daniel Almehed, Stefan Frauendorf Several near degenerate $\Delta I=1$ bands with the same parity have been found in the $A=130$ and $A=105$ regions. Some of these bands have been interpreted as chiral rotational bands within the Tilted Axis Cranking (TAC) model [V. I. Dimitrov {\it et al.}, PRL {\bf 84}, 5732 (2000)]. Chiral rotation can appear in triaxial nuclei when proton and neutrons align along different principal axes and the collective rotation occurs along the third. Candidates for chiral partner bands generally show a slowly decreasing or nearly constant energy splitting of a couple of 100 keV. This observation has been interpreted as appearance of a chiral vibration, which is a vibration of the orientation of the principal axes of the nucleus with respect to the angular momentum vector [K. Starosta {\it et al.}, PRL {\bf 86}, 971 (2001)]. The TAC calculations of chiral bands give potential energy surfaces that are soft in both the orientation and the $\gamma$ deformation degree of freedom. This suggests that these collective vibrational excitations are in fact made up of a pure chiral vibration coupled with $\gamma$-vibration. To investigate the structure of these vibrations we performed RPA calculations on top of the planar TAC mean field solutions. This allows us studying the coupling of shape and orientation degrees of freedom. We will discuss how the different degrees of freedom contribute to the collective vibration and present energy systematics and transition rates. [Preview Abstract] |
Friday, October 27, 2006 2:36PM - 2:48PM |
DC.00004: Lifetime spectroscopy of $^{112}$Cd via the $(n,n^{\prime}\gamma)$ reaction P.E. Garrett, K.L. Green, H. Lehmann, J. Jolie, C.A. McGrath, Minfang Yeh, S.W. Yates Lifetimes of many levels up to 4 MeV in $^{112}$Cd have been measured using the Doppler shift attenuation technique following neutron inelastic scattering with monoenergetic neutrons. Using these lifetimes, reduced transition matrix elements are determined. The electromagnetic properties of $^{112}$Cd are outlined, and together with results from previous studies, levels are interpreted in terms of single-particle configurations and collective excitations. The separate roles of proton and neutron excitations in a subset of excited states are determined by combining the ground state electromagnetic transition rates with the hadronic transition rates. The collective states and their $\gamma$-ray decays are compared with IBM-2 model calculations that allow for the mixing between the normal phonon states and intruder configurations. [Preview Abstract] |
Friday, October 27, 2006 2:48PM - 3:00PM |
DC.00005: Probing the Pygmy Dipole Resonance in $^{112}$Sn and $^{124}$Sn Melissa Boswell, C. Angell, H.J. Karwowski, J. Engel, J.H. Kelley, A.P. Tonchev, W. Tornow A high-resolution nuclear fluorescence experiment of enriched $^{112,124}$Sn has been performed using the 100\% polarized photon beam at the High-Intesity Gamma-Ray Source (HI$\gamma$S). Four HPGe detectors were used to observe 66 dipole transitions with excitation energies between 6.4 MeV and 8.4 MeV. The parity of each of the 21 previously identified transitions in $^{124}$Sn was found to be J$^{\pi}$=1$^{-}$. In addition, 10 new levels in $^{124}$Sn were identified, as well as 5 new levels in $^{112}$Sn all of which are E1 excitations with the exception of a 6.917 MeV state in $^{124}$Sn excited by an M1 transition. Both nuclei exhibited considerable decay strength to the first excited state. We shall discuss the possible causes of these decays as well as their implications to r-process nucleosynthesis. The measurements will be compared with calculations using quasiparticle random-phase approximations. [Preview Abstract] |
Friday, October 27, 2006 3:00PM - 3:12PM |
DC.00006: Beta decay strengths from the decay of $^{116m1,m2,gs}$Ag J.C. Batchelder, H.K. Carter, E.H. Spejewski, J.-C. Bilheux, K.P. Rykaczewski, D.W. Stracener, C.R. Bingham, R. Grzywacz, M.N. Tantawy, Y. Larochelle, J.H. Hamilton, W. Krolas, D. Fong, A.V. Ramayya, J.K. Hwang, P.E. Garrett, D.J. Hartley, D. Kulp, J.L. Wood, A. Piechaczek, E.F. Zganjar, J.A. Winger An inconsistency with the published data on the decay of a 5+ $^{116m}$Ag has been the non-zero beta feeding strength for the decay of 116Ag to low-lying levels with spins of 2 and 3 [1]. Recently [2], we have shown that 116Ag has a third isomer. Through the use of conversion electron and gamma spectroscopy, we were able to show that the ground state must be 0- rather than the previously assigned 2- [3]. This results in the three beta-decaying levels in 116Ag having J$^{pi}$ of 0-, 3+, and 6-. Our results indicate that the feeding of the levels in $^{116}$Cd with spins of 2 and 3 arise from the 3+ isomer in $^{116}$Ag, which is perfectly consistent with allowed beta transitions from the 3+ $^{116m1}$Ag isomer. In this talk, a discussion of the beta strengths of the three isomers as well as the levels in Cd will be presented. [1] Y. Wang, et al., Phys. Rev. C 64, 054315 (2001). [2] J. C. Batchelder, et. al., Rev. C. 72, 044306 (2005). [3] T. Bj{\o}rnstad and J. Alstad, J. Inorg. Nucl. Chem., 36, 2159 (1974). [Preview Abstract] |
Friday, October 27, 2006 3:12PM - 3:24PM |
DC.00007: Multiphonon and Mixed-Symmetry States in 127I Sharmistha Mukhopadhyay The complex low-lying structure observed in odd-mass nuclei arises from the interplay of phonon, intruder, mixed-symmetry and single particle degrees of freedom. Multiphonon excitations in odd-mass nuclei may occur as a result of weak coupling of an unpaired particle with the core nucleus. Little is known, however about these collective vibrations in odd-mass nuclei. We have studied 127I using the (n, n$'$, $\gamma )$ and (n, n$'$, $\gamma \gamma )$ reactions at incident neutron energies ranging from 1.2 to 3.0 MeV. From excitation functions and angular distribution, branching and mixing ratios were measured and level lifetimes and transition probabilities determined. To construct and extended 127I decay scheme both excitation functions and coincidence data were used. New information on multiphonon and mixed-symmetry states built on the 5/2+ ground state and the 7/2+[404] and 3/2+[422] Nilsson orbitals will be presented. [Preview Abstract] |
Friday, October 27, 2006 3:24PM - 3:36PM |
DC.00008: Candidates for Quadrupole-Octupole Mult-iphonon Excitations Observed in the Te Isotopes S.F. Hicks, J.R. Vanhoy Excited levels in the even-even isotopes $^{120-130}$Te have been investigated to 3.3 MeV using $\gamma$-ray spectroscopy following inelastic neutron scattering. Level characterisitics including spins, multipole-mixing and branching ratios, and lifetimes in the fs to ps regime have been deduced from $\gamma$-ray angular distributions, excitation functions, and Doppler-shift measurements, as well as $\gamma$-$\gamma$ coincidence measurements. This consistent sent of measurements has enabled us to examine multi-phonon excitations across the isotopic chain. In particular, the quintet of negative-parity states (1$^{-}$-5$^{- }$) arising from the coupling of the lowest quadrupole- and octupole-phonon exitations have been examined and candidates have been identified. Results from these investigations across the Te isotopic chain will be presented. [Preview Abstract] |
Friday, October 27, 2006 3:36PM - 3:48PM |
DC.00009: Evolution of shape phase transitions as functions of energy, spin, and boson number in the Interacting Boson Model E. Williams, R. J. Casperson, V. Werner Shape phase transitions from spherical to deformed nuclei have been a subject of recent interest because explorations of such behavior have led to a greater understanding of the evolution of collectivity throughout the nuclear landscape. Two critical points in particular, X(5), a first order phase transition, and E (5), a second order phase transition, were identified in the geometrical model. Recent work within the context of the Interacting Boson Model (IBM) has explored these regions in the finite N limit corresponding to realistic nuclei. IBM calculations extending to large boson numbers provide powerful tools for relating transitional behavior observed in nuclei to phase transitions in macroscopic systems. A study of first and second order phase transitions in the large boson limit as functions of N, spin, and energy has been undertaken with the use of a variety of observables, including both electromagnetic transitions, and energies. The results of this investigation will be presented. Work supported by US DOE grant number DE-FG02- 91ER-40609. [Preview Abstract] |
Friday, October 27, 2006 3:48PM - 4:00PM |
DC.00010: Deformation shrinking of the $\beta$-band at the first order shape phase transition V. Werner, E. Williams, C. Scholl, P. von Brentano One main effect of the nuclear shape phase transition between spherical and deformed nuclei is the vast rise of the geometrical deformation parameter $\beta$ for the ground state towards deformed nuclei. This rise is reflected in a the rise of the model independent quadrupole shape invariant $q_2(0^+_1)$, from which an effecfive ground state $\beta$-deformation can be extracted. While one would naively expect similar behavior for the $q_2$ value of the first excited $0^+$ state, which corresponds to the head of the $\beta$-vibrational band, the IBM-1 predicts indeed a shrinking of its $\beta$-deformation when crossing the first order phase transition between vibrators and well-deformed nuclei. Absolute B(E2) values known for the ground-band and the $\beta$-band of $^{152,154}$Gd support this effect. This work is supported by USDOE under contract numbers DE-FG02-91ER-40609, and DFG under Br 799/12-1. [Preview Abstract] |
Friday, October 27, 2006 4:00PM - 4:12PM |
DC.00011: 135Ba - A First Test of the E(5/4) Bose-Fermi Symmetry M.S. Fetea, R.B. Cakirli, R.F. Casten, D.D. Warner, E.A. McCutchan, D.A. Meyer, A. Heinz, H. Ai, G. Gurdal, J. Qian, R. Winkler Very recently, the first case of a critical point Bose-Fermi symmetry for odd-mass nuclei, E(5/4) was developed. It describes analytically a gamma-soft critical point E(5) core coupled to a j = 3/2 particle, where E(5) represents a second order phase transition from a vibrator U(5) to a gamma-soft rotor O(6). Since 134Ba has been found to be an empirical manifestation of E(5), 135Ba - in which the last neutron can occupy the 2d3/2 orbit, is the natural initial test of E(5/4). To complement this test and provide a perspective, we performed shell model and interacting boson-fermion approximation calculations. We will present the work and discuss the results. We will show that E(5/4) can account for some of the observables in 135Ba but that it does not provide a fully satisfactory description. Specifically, many of the collective and forbidden B(E2) values of E(5/4) agree well with the data. [Preview Abstract] |
Friday, October 27, 2006 4:12PM - 4:24PM |
DC.00012: Identification of high spin states in $^{137,138}$Cs nuclei K. Li, Y.X. Luo, J.K. Hwang, A.V. Ramayya, J.H. Hamilton, H.L. Crowell, C. Goodin, J.O. Rasmussen, I.Y. Lee, S.C. Wu, G.M. Ter-Akopian, A.V. Daniel, J.D. Cole, A. Covello, A. Gargano, R. Donangelo, W.C. Ma, M.A. Stoyer, S.J. Zhu High spin states of $^{137,138}$Cs have been studied by measuring the $\gamma-\gamma-\gamma$ coincidences from the spontaneous fission of $^{252}$Cf with the Gammasphere detector array. The level scheme of the N=83 neutron-rich Cs (Z=55) isotope, $^{138}$Cs, has been established for the first time up to a 4626keV level assigned ($16^+$) and that of $^{137}$Cs has been expanded up to a 5495keV level assigned ($31/2^-$). Spins, parities and configurations are assigned based on shell model calculations and level systematics. Pronounced differences in low-lying yrast cascade patterns are seen between N=83 isotones $^{138}$Cs (Z=55), $^{134}$Sb (Z=51) and $^{136}$I (Z=53), and between Cs isotopes $^{138}$Cs and $^{137}$Cs. Significant similarity is observed in the N=82 isotones $^{137}$Cs and $^{135}$I up to $17/2^+$ but not above the spin nor with $^{133}$Sb, which indicates the important role played by interactions between the excitation of the $g_ {7/2}$ valence protons outside the Z=50 major shell, and the $f_{7/2}$ valence neutron outside the N=82 major shell. [Preview Abstract] |
Friday, October 27, 2006 4:24PM - 4:36PM |
DC.00013: Partial Cross-Sections of $^{140}$Ce(n,2n)$^{139}$Ce Reaction C.T. Angell, B. Fallin, A. Hutcheson, H.J. Karwowski, J.H. Kelley, A.P. Tonchev, W. Tornow The excitation function for the $^{140}$Ce(n,2n)$^{139}$Ce reaction has been studied in the TUNL Shielded Neutron Source area. A pulsed and quasi-monoenergetic neutron beam was produced via the D(d,n) reaction ($\Delta$E/E=3-5\%) with energies of 12, 13.3, 14.5, and 16 MeV with a beam flux of $\sim$10$^{4}$ n/(s*cm)$^{2}$. The target consisted of a mixture of natural Ce and Fe. Two clover and two planar HPGe detectors were used to make in-beam measurements of the $\gamma$-ray cascade deexciting $^{139}$Ce. The partial cross sections were normalized to the 847 keV transition in $^{56}$Fe. Statistical model calculations using code EMPIRE will be compared with the present data as well as with the previously obtained [1] transition amplitudes for the $^{140}$Ce($\gamma$,n)$^{139}$Ce reaction. \\ $[1]$ C.T.Angell et al. CGS 12. Notre Dame, IN, 2005, p. 363. [Preview Abstract] |
Friday, October 27, 2006 4:36PM - 4:48PM |
DC.00014: Fine structure in proton emission from 141gsHo and 141mHo K. Rykaczewski, M. Karny, R. Grzywacz, J. Batchelder, C. Bingham, C. Goodin, C. Gross, J. Hamilton, J. Hwang, A. Korgul, W. Krolas, S. Liddick, K. Li, K. Maier, C. Mazzocchi, A. Piechaczek, A. Ramayya, D. Shapira, D. Simpson, M. Tantawy, J. Winger, C. Yu, E. Zganjar Fine structure in proton emission from the 7/2-[523] ground state and from 1/2+[411] isomeric state in the deformed nucleus $^{141}$Ho was discovered at the HRIBF, Oak Ridge. Proton transitions to the 0$^{+}$ ground state and to the 202 keV 2$^{+}$ state in $^{140}$Dy were observed. The branching ratios of I$_{gs}^{2^{+}}$= 0.9(1)\% and I$_{m}^{2^{gs}2^{+}}$=1.7(4)\% were measured by means of fusion-evaporation reactions, Recoil Mass Separator and digital processing of the Double-sided Silicon Strip Detector recoil implantation and decay signals. The structure of the deformed wave functions of the 7/2-[523] and 1/2+[411] states will be discussed. The decay properties of $^{141gs}$Ho can be explained by postulating a small triaxial deformation, while the same model fails to explain observed decay properties of $^{141m}$Ho. The measured isomeric halflife and I$_{m}^{2^{+}}$ value suggest that the contribution of the $\pi$s$_{1/2} \otimes 0^{+}$ component in the 1/2+[411] wave function is reduced to about 3\% only. [Preview Abstract] |
Friday, October 27, 2006 4:48PM - 5:00PM |
DC.00015: Spin measurements for $^{147}$Sm$+n$ resonances: Further evidence for non-statistical effects Paul Koehler, J.L. Ullmann, T.A. Bredeweg, J.M. O'Donnell, R. Reifarth, R.S. Rundberg, D.J. Vieira, J.M. Wouters We have determined spins of resonances in the $^{147}$Sm ($n,\gamma $) reaction by using the DANCE detector at LANSCE to measure multiplicities of $\gamma $-ray cascades following neutron capture. These new spin assignments, together with previously determined resonance parameters, allowed us to extract separate level spacings and neutron strength functions for $J=3$ and 4 resonances. Furthermore, although all the evidence indicates that very few resonances of either spin have been missed below $E_{n}=700$eV, reduced-neutron-width and level- spacing distributions do not agree with the expected distributions for resonances in the energy range $350<$ $E_{n} <700$ eV. The new spin assignments also allowed us to reanalyze $^{147}$Sm($n,\alpha $) data and obtain more reliable $\alpha $ widths. Although our new $\alpha $ widths are somewhat different from previous work, recently reported non-statistical effects revealed by these widths remain. Taken together, the neutron- width, $\alpha $-width, and level-spacing data indicate the onset of some non-statistical effect near $E_{n}=350$eV. We will discuss possible explanations for these effects and their possible relation to similar effects previously observed in $^ {232}Th+n$ resonances. [Preview Abstract] |
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