Bulletin of the American Physical Society
2006 Division of Nuclear Physics Annual Meeting
Wednesday–Saturday, October 25–28, 2006; Nashville, Tennessee
Session CC: Nuclear Structure II |
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Sponsoring Units: DNP Chair: Mirela Fetea, University of Richmond Room: Gaylord Opryland Tennessee B |
Friday, October 27, 2006 9:00AM - 9:12AM |
CC.00001: The $K$ quantum number in the Shell Model---$^{50}$Cr Shadow Robinson, Alberto Escuderos, Larry Zamick It was suggested~[1,2] that the $10^+_1$ state in $^{50}$Cr at 6.340~MeV does not belong to the $K=0^+$ g.s. band. In~[1] it is noted that the static quadrupole moments of the $J=2^+_1$--$8^+_1$ states are all negative, but that of $10^+_1$ is positive. While Ref.~[1] suggested that the $10^+_1$ state belongs to a high $K$ prolate band, in Ref.~[2] they assign it as $K=10^+$. There is a nearby second $10^+$ state. However, the $B(E2)_{10^+_2 \rightarrow 10^+_1}$ was not quoted by either group. In this work, we performed full $fp$ shell model calculations using four different interactions: FPD6, KB3, GXPF1, and GXPF1A. The results for $B(E2)_{10^+_2 \rightarrow 10^+_1}$ are robust around 135~e$^2$fm$^4$ and suggest strong $K$ mixing. It is not clear what the $K$ value for the $10_2^+$ state is. With FPD6, $Q(10^+_2)$ is negative, suggesting it is a member of the $K=0^+$ band. But it is hard to understand how to get strong mixing of $K=0^+$ and $K=10^+$. With the other interactions, $Q(10^+_2)$ is positive and thus inconsistent with a $K=0^+$ (prolate) band. If we assume that the $10^+_1$ state has $K=10^+$ and the $8^+_1$ state has $K=0^+$, then the $B(E2)_{10^+_1 \rightarrow 8^+_1}$ would vanish. However, for the last three interactions, the corresponding $B(E2)$ is about 75~e$^2$fm$^4$, which implies substantial $K$ mixing. Thus, while a $K=10^+$ assignment for the $10^+_1$ states makes the most sense in terms of energy systematics, in detail the situation is more complicated. [1] L.~Zamick et al., Phys. Rev. C {\bf 53}, 188 (1996); Phys. Rev. C {\bf 54}, 956 (1996). [2] F.~Brandolini et al., Phys. Rev. C {\bf 66}, 021302(R) (2002). [Preview Abstract] |
Friday, October 27, 2006 9:12AM - 9:24AM |
CC.00002: Update on the structure of n-rich 52-56Ti S. Zhu, R.V.F. Janssens, M.P. Carpenter, S. Freeman, B. Fornal, A. Deacon, B. Kay, J. Kozemczak, A. Larabee, T. Lauritsen, A. Robinson, D. Seweryniak, J. Smith, D. Steppenbeck, X. Wang Neutron-rich nuclei above $^{48}$Ca are presently the subject of much theoretical and experimental focus because of the presence of a N=32 sub-shell gap [1]. The spectroscopy of these hard-to-reach nuclei has been carried out with a number of reactions including deep inelastic processes and fusion- evaporation utilizing extremely weak channels. An example of the latter is the use of the $^{9}$Be($^{48}$Ca,2p) reaction with a $\sim$1 $\mu$b cross section to investigate $^{55}$Ti. In this case, advantage is taken of the resolving power of the combination of the Argonne Fragment Mass Analyzer and the Gammasphere array. Recent experimental progress in the structure of the odd and even Ti isotopes will be presented and compared with shell model calculations carried out with recently developed effective interactions. \newline [1] R.V.F Janssens et al., Phys. Let. B 546, 55 (2002). [Preview Abstract] |
Friday, October 27, 2006 9:24AM - 9:36AM |
CC.00003: A g-factor puzzle for the N=38 nuclei:First measurement of the $^{70}$Ge 4$_{1}^+$ magnetic moment. Plamen Boutachkov, G. Kumbartzki, N. Benczer-Koller, S. Robinson, A. Escuderos, E. Stefanova, Y. Sharon, L. Zamick, E. McCutchan, V. Werner, H. Ai, G. Gurdal, A. Heinz, J. Qian, E. Williams, R. Winkler, A. Garnsworthy, N. Thompson, P. Maier-Komor The transient field technique in inverse kinematics allows $g$-factor studies of short-lived states. This method gives information on both the sign and the magnitude of the $g$ factor. In a recent experiment, the $g$ factor of the 4$^+_1$ state of $^{68}_{30}$Zn$_{38}$ was measured to be -0.37(17) suggesting a significant neutron g$_{9/2}$ contribution to the wave function[1]. However, shell model calculations in the 0f$_{5/2}$,1p$_{3/2}$,1p$_{1/2}$,0g$_{9/2}$ space [1] predict a positive, nearly zero $g$ factor. To obtain more information on this region we measured the magnetic moment of the 4$^+_1$ in $^{70}_{32}$Ge$_{38}$. The measurement was performed at WNSL, Yale, using a 275 MeV $^{70}$Ge beam and a multilayered C+Gd+Cu target. A positive $g$ factor was obtained. The measured magnetic moment was compared to full $fp$ shell model calculations which we performed with the code ANTOINE using several effective interactions. The results were in good agreement with the experiment. The experiment and the implications of the new results will be discussed.\\ 1. J. Leske {\it et al.}, Phys. Rev C {\bf 72}, 044301 (2005). [Preview Abstract] |
Friday, October 27, 2006 9:36AM - 9:48AM |
CC.00004: High Spin Structure in Neutron Rich Zn Isotopes A.A. Hecht, N. Hoteling, W.B. Walters, M.P. Carpenter, R.V.F. Janssens, T. Lauritsen, D. Seweryniak, X. Wang, S. Zhu, B. Fornal, R. Broda, W. Krolas, J. Wrzesinski, A. Woehr, N.J. Stone, J. Stone The neutron rich region near doubly-magic $^{78}$Ni is significant for both nuclear structure and nuclear astrophysics: as input for models on shell structure near the neutron drip-line and as the seed region for the beginning of the rapid neutron capture process of nucleosynthesis. This region is not easily accessible and most of the data towards the drip line are on low spin states. To expand this knowledge to high spin states, a deep inelastic scattering (DIS) experiment was performed at Argonne National Laboratory. Pulsed beams of $^{82}$Se and $^{64}$Ni impinged on a target of $^{238}$U and the gamma rays emitted from the DIS products were observed using the Gammasphere detector array. $\gamma -\gamma -\gamma $ coincidence matrices were made with both prompt and delayed data and excited states of $^{68 - 78}$Zn were observed. Angular correlations and new high spins states were measured for $^{74}$Zn and several other Zn isotopes. This work was supported in part by the US DOE under Contract Nos. W-31-109-ENG-38, DE-FG02-94-ER40834 and Polish Sci. Committee Grant No. 1PO3B-059-29. [Preview Abstract] |
Friday, October 27, 2006 9:48AM - 10:00AM |
CC.00005: Transition strengths and degree of deformation in $^{79}$Sr R.A. Kaye, Y.K. Ryu, S.L. Tabor, T. Baldwin, D.B. Campbell, C. Chandler, M.W. Cooper, C.R. Hoffman, J. Pavan, M. Wiedeking, J. D\"{o}ring, Y. Sun, S.M. Gerbick, O. Grubor-Urosevic, L.A. Riley High-spin states in $^{79}$Sr were studied using the $^{54}$Fe($^{28}$Si, 2$pn$) reaction at 90 MeV with a thick 14 mg/cm$^2$ $^{54}$Fe target used to stop all recoils. Prompt $\gamma$--$\gamma$ coincidences were detected using the FSU Compton-suppressed Ge array, and allowed for a verification of the $^{79}$Sr level scheme in three separate band structures. Lifetimes of 34 excited states were measured using the Doppler-shift attenuation method, with the experimental line shapes obtained at two separate observation angles and by gating from above the transition of interest whenever possible. Transition quadrupole moments $Q_t$ inferred from the lifetimes indicate a high degree of collectivity and deformation over a rather wide range of spins in all three observed bands. These results will be interpreted within the framework of the projected shell and cranked Woods-Saxon models. [Preview Abstract] |
Friday, October 27, 2006 10:00AM - 10:12AM |
CC.00006: M1 transitions between low-lying states in the sdg-IBM-2 Robert Casperson, Volker Werner The interplay between collective and single-particle degrees of freedom for nuclei in the A=90 region have recently been under investigation. In Molybdenum and Ruthenium nuclei, collective symmetric and mixed-symmetric structures have been identified, while in Zirconium, underlying shell-structure plays an enhanced role. Collective symmetric structures appear when protons and neutrons are in phase, whereas mixed-symmetric structures occur when they are not. The one-phonon 2$^{+}$ mixed-symmetric state was identified from strong M1 transitions to the 2$^{+}_{1}$ state. Similar transitions were observed between higher-spin states, and are predicted by the shell model. These phenomena will be investigated within the sdg Interacting Boson Model 2 in order to obtain a better understanding about the structure of the states involved, and results from first model calculations will be presented. Work supported by US DOE under grant number DE-FG02-91ER-40609. [Preview Abstract] |
Friday, October 27, 2006 10:12AM - 10:24AM |
CC.00007: Low spin structure of $^{94}_{40}$Zr from (n,n$^{\prime}\gamma$) measurements E. Elhami, J.N. Orce, S. Mukhopadhyay, S.N. Choudry, M. Scheck, M.T. McEllistrem, S.W. Yates Recent measurements of negative g-factors for the 2$^+_1$ and 4$^+_1$ states in $^{92}$Zr and $^{94}$Zr have established the dominant role of 2d$_{5/2}$ neutron configurations between the N=50 closed shell and the N=56 subshell closure. Moreover, further studies of mixed-symmety (MS) states in $^{92}$Zr supported a weaker p-n interaction for the 2$^+_2$ MS state, as compared with the 2$^+_3$ MS state in $^{94}_{42}$Mo, which results in a partial decoupling of proton and neutron excitations. The strong M1 transition with B(M1)=0.37(4) $\mu_N^2$ connecting the 2 lowest 2$^+$ states indicates, however, that both proton and neutron configurations are still important parts of their wavefunctions. Following the previous discussion, we have analysed the low-lying structure of $^{94}$Zr at the University of Kentucky. The nuclide was studied through the (n,n$^{\prime}\gamma$) reaction at energies of 2.3, 2.8 and 3.5 MeV. A 98\% enriched $^{94}$Zr sample was used and angular distribution information yields to the measurement of branching and mixing ratios of $\gamma$-ray transitions, and determination of level lifetimes and transition strengths. For the purpose of this work, only the results for the 2.3 MeV data will be presented. [Preview Abstract] |
Friday, October 27, 2006 10:24AM - 10:36AM |
CC.00008: First Experimental Value of a mixed-symmetry G Factor: $^{94}$Zr M. Perry, V. Werner, P. Boutachkov, E. Stefanova, N. Benczer-Koller, G. Kumbartzki, N. Pietralla, H. Ai, R.F. Casten, A. Heinz, E.A. McCutchan, D.A. Meyers, J. Qian, E. Williams, R. Winkler, M. Chamberlain, C.R. Fitzpatick, A.B. Garnsworthy, N.J. Thompson, R.B. Cakirli, X. Liang, P. Maier-Komor, G. G\"urdal, A.E. Stuchbery, K.-H. Speidel Zr isotopes have the characteristic of a neutron(n)-dominated and a proton(p)-dominated $2^+$ state, resembling the building blocks of collective quadrupole excited states, i.e. one-phonon pn symmetric and mixed-symmetric $2^+$ states. The pn configuration mixing in these states can be tested by measuring their g factors. The g factors predicted by different models vary due to the characterization of the symmetry breaking. The g factor of a mixed symmetric state has been measured for the first time in $^{94}$Zr using the transient magnetic field technique at WNSL. The result, proving dominant p character in the $2^+_2$ state, will be presented and compared with theory. Work supported by USDOE under contracts DE-FG02-91ER-40609, DE-FG52-05NA25929, and DE-FG02-88ER40417 and th US NSF. [Preview Abstract] |
Friday, October 27, 2006 10:36AM - 10:48AM |
CC.00009: Identification of high spin states and nearly degenerate $\Delta$I=1 bands in $^{100}$Zr J.K. Hwang, A.V. Ramayya, J.H. Hamilton, J.O. Rasmussen, Y.X. Luo, D. Fong, K. Li, C. Goodin, S.J. Zhu, S.C. Wu, M.A. Stoyer, R. Donangelo, X.-R Zhu, H. Sagawa Eight new high spin states and twenty-three new $\gamma$ transitions have been identified in $^{100}$Zr from studies of $^{252}$Cf spontaneous fission with Gammasphere. A near-spherical excited band in $^{100}$Zr based on the 331.1 keV 0$^+$ state is extended from 4$^+$ up to 12$^+$. A $\Delta$I = 1 band with band-head energy of 2316.1 keV is extended. This band now forms $\Delta$I=1 doublet bands with the previously known $\Delta$I=1 band begining at 2259.8 keV. The energy difference between the bands with the sam spins in the two bands are nearly degenerate, $\Delta$E= 25.2(8$^+$), 8.6(9$^+$), 4.8(10$^+$) and 12.1(11$^+$) keV. Our theoretical calculations indicate the coexistence of prolate and oblate shapes for the two 0$^+$ bands in $^{100}$Zr. We propose the possible coexistence of prolate, oblate and triaxial shapes for the highly excited 2 quasiparticle bands. [Preview Abstract] |
Friday, October 27, 2006 10:48AM - 11:00AM |
CC.00010: Identification of band structures and proposed one- and two- phonon $\gamma$ -vibrational bands in $^{105}$Mo H.B. Ding, S.J. Zhu, J.H. Hamilton, A.V. Ramayya, J.K. Hwang, K. Li, Y.X. Luo, J.O. Rasmussen, I.Y. Lee, C.T. Goodin, X.L. Che, Y.J. Chen, M.L. Li High spin band structures in neutron-rich $^{105}$Mo were studied by measuring prompt $\gamma$-rays emitted by the spontaneous fission fragments of $^{252}$Cf with the Gammasphere detector array. The yrast band has been confirmed and five new collective bands are observed. The three bands based on the 246.3, 332.0 and 310.0 keV levels are proposed as the single-neutron excitation bands built on the 3/2$^+$[411], 1/2$^+$[411] and 5/2$^+$[413] Nilsson orbitals, respectively. The other two bands with band head levels at 870.5 and 1534.6 keV are candidates for one-phonon K=9/2 and two-phonon K=13/2 $\gamma$-vibrational bands, respectively. Systematic comparison of these bands with bands in $^{104,106}$Mo are discussed. [Preview Abstract] |
Friday, October 27, 2006 11:00AM - 11:12AM |
CC.00011: Identification of $^{109}$Xe and $^{105}$Te S. Liddick, J.C. Batchelder, R. Grzywacz, C. Mazzocchi, C.R. Bingham, G. Drafta, A. Korgul, M.N. Tantawy, R.D. Page, I.G. Darby, D.T. Joss, J. Thomson, K.P. Rykaczewski, C. Gross, C. Goodin, J.H. Hamilton, J.K. Hwang, K. Li, S. Ilyushkin, J.A. Winger, K. Lagergren, W. Krolas, A.A. Hecht The existence of a region of alpha emitting nuclei above $^{100}$Sn is due to the presence of the Z=50 shell closures. The region is a fertile area to investigate possible enhanced correlations between neutrons and protons filling the same single-particle orbits and could lead to the observation of superallowed alpha decay as an approach is made towards $^{100}$Sn. The new isotope $^{109}$Xe was produced at the HRIBF at Oak Ridge National Laboratory. The lightest mass $\alpha$-radioactivity identified to date, $^{105}$Te, was detected through the $^{109}$Xe$\rightarrow ^{105}$Te $\rightarrow ^{101}$Sn alpha decay chain. This marks the closest approach to the N = Z line above $^{100}$Sn. The half-life and Q$_{\alpha}$ value for $^{105}$Te were used to determine the reduced $\alpha$-decay width, $\delta^{2}$. The ratio $\delta^{2}_{^{105}Te}$/$\delta^{2}_{^{213}Po}$ of 2.7 indicates a superallowed character of the $\alpha$-emission from $^{105}$Te. Fine structure in the millisecond alpha decay of $^{109}$Xe to $^{105}$Te was identified and the energy difference between the $\nu$d$_{5/2}$ ground state and the $\nu$g$_{7/2}$ first excited state was determined to be around 150 keV in $^{105}$Te. Prospects for reaching the superallowed alpha decay chain $^{108}$Xe$\rightarrow ^{104}$Te $\rightarrow ^{100}$Sn will also be discussed. [Preview Abstract] |
Friday, October 27, 2006 11:12AM - 11:24AM |
CC.00012: Identification of 0$^{+}$ States in Transitional Nuclei R. Winkler, A. Heinz, R.F. Casten, C. Lambie-Hanson, J. Qian, R. Kruken, T. Faestermann, H.-F. Wirth, R. Graeger, J. Jolie, P. von Brentano, C. Scholl, S. Christen The (p,t) pickup reaction was used to populate excited 0$^{+}$ levels in the transitional nucleus $^{108}$Pd at the MLL (Maier-Leibnitz Laboratory at LMU and TU Munich) MP tandem accelerator laboratory. The use of a Q3D spectrometer with a high-resolution focal plane detector was instrumental in the assignment of level energy, spin and parity of the nuclei of interest. Angular distribution and cross section measurements are compared to calculated values from the DWBA. The shape of the L = 0 angular distribution is used to unambiguously identify any 0$^{+}$ state populated below an excitation energy of 3.5 MeV. These experiments are the continuation of the survey of transitional and well-deformed nuclei performed in the hope of providing information into the nature of the excited 0$^{+}$ states of nuclei in the vicinity of the rare earth region. First experimental results will be presented. This work has been supported by US DOE grant DE-FG02-91ER-40609. [Preview Abstract] |
Friday, October 27, 2006 11:24AM - 11:36AM |
CC.00013: Discovery of the alpha decay of 109I C. Mazzocchi, R. Grzywacz, C.R. Bingham, D. Simpson, C.J. Gross, K.P. Rykaczewski, J.C. Batchelder, S.N. Liddick, R.D. Page, A. Korgul, W. Krolas, S. Ilyushkin, J.A. Winger, J.H. Hamilton, J.K. Hwang, K. Li Alpha emission is a rich source for nuclear-structure information [1]. The alpha-particle energies E$_{\alpha}$, corrected for the recoil effect, yield the difference between the ground-state masses of parent and daughter nuclides (Q$_\alpha$). Far from stability the determination of Q$_\alpha$ often represents the only way to determine the masses of ground and isomeric states. The evolution of Q$_\alpha$ values along an alpha-decay chain are also a probe for shell effects. In the region above $^{100}$Sn an alpha-decay island occurs, its presence is related to the strong Z=50, N=50 double shell-closure. In an experiment performed at the Recoil Mass Separator of the HRIBF at Oak Ridge National Laboratory, the first evidence for the alpha-decay branch of the proton-emitter $^{109}$I was obtained. The results and the consequences for nuclear masses in this region will be discussed. \newline [1] E. Roeckl, Alpha decay, in: {\it Nuclear Decay Modes}, ed. D.N. Poenaru, IoP Publishing, 1996, p. 237. [Preview Abstract] |
Friday, October 27, 2006 11:36AM - 11:48AM |
CC.00014: Dipole strength distributions in stable odd-mass nuclei in the vicinity of the N=82 isotones Marcus Scheck The low-lying dipole strength distributions in the odd-mass nuclei $^{135}$Ba, $^{137}$Ba, $^{139}$La and $^{141}$Pr were studied in nuclear resonance fluorescence (NRF) experiments performed at the Stuttgart Dynamitron facility. experiments used bremsstrahlung beams with endpoint energies of 4.1~MeV. The spin selective NRF reaction allowed the excitation of states through dipole transitions, up to 4~MeV. A special focus is the fragmented E1-strength of transitions connecting the ground state to states of the $[2^+ \otimes 3^-] \otimes particle/hole$ coupling. The summed strength of the odd-mass nuclei is compared with the E1-strength of the $[2^+ \otimes 3^-]_{1^{-}}$ two-phonon states of the neighboring even-even core nuclei. [Preview Abstract] |
Friday, October 27, 2006 11:48AM - 12:00PM |
CC.00015: Systematic Features Indicative of Shape Coexistence in the Doubly-Even N=90 Isotones W.D. Kulp, J.L. Wood, P.E. Garrett The $N=90$ nuclei near stability ($^{150}$Nd, $^{152}$Sm, $^{154}$Gd, and $^{156}$Dy) are at the onset of stable nuclear deformation and have often been labeled as ``soft'' nuclei. Results from recent experimental and theoretical studies of these nuclei challenge this interpretation, however, and indicate instead strong mixing of near-degenerate coexisting quasi-rotational bands with different deformations. Details of these studies and data from recent experiments focused on $^{156}$Dy, $^{158}$Er, and $^{160}$Yb using the $8\pi$ Spectrometer at TRIUMF/ISAC will be presented. [Preview Abstract] |
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