Bulletin of the American Physical Society
2011 Fall Meeting of the APS Division of Nuclear Physics
Volume 56, Number 12
Wednesday–Saturday, October 26–29, 2011; East Lansing, Michigan
Session FF: Nuclear Structure III |
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Chair: Volker Werner, Yale University Room: 104AB |
Thursday, October 27, 2011 4:00PM - 4:12PM |
FF.00001: Extreme ground-state deformation of the N=Z nucleus $^{76}$Sr A. Lemasson, H. Iwasaki, C. Morse, T. Baugher, D. Bazin, J. Berryman, A. Gade, S. McDaniel, A. Ratkiewicz, S. Stroberg, D. Weisshaar, K. Wimmer, R. Winkler, A. Dewald, C. Fransen, A. Nichols, R. Wadsworth The shape of the atomic nucleus is determined by the interplay of macroscopic and microscopic effects within this quantum mechanical many-body system. Self-conjugate nuclei give an opportunity to study the role of $np$ correlations in deformation and have attracted a great interest due to drastic shape evolution along the N=Z line. Strong ground-state deformation is expected to occur for N=Z nuclei above Z=36 from the 2$^+$ energy systematic as well as from theoretical predictions. Reduced transition strengths B(E2) can guide our understanding of the onset of collectivity along N=Z line. Here, we report on the first determination of B(E2; 2$^+ \to 0^+$) for the N=Z=38 nucleus $^{76}$Sr obtained from the measurement of the 2$^+$ state lifetime using a line shape technique. $^{76}$Sr nuclei were produced at the NSCL in charge exchange reaction from fast secondary $^{76}$Rb beam. $\gamma$-rays emitted at the reaction target position were measured with the SeGA HPGe array in coincidence with reaction residues detected in the S800 spectrometer. Results will be discussed in the light of available data and theoretical predictions to provide insight into the evolution of shell structure and deformation in this region. [Preview Abstract] |
Thursday, October 27, 2011 4:12PM - 4:24PM |
FF.00002: A=10 charge independence and the T=1, J=2-0 electromagnetic transition in $^{10}$B Christopher Lister, E.A. McCutchan, J.P. Greene, R.B. Wiringa, Steven Pieper, M. Elvers, A. Heinz, D. Savran, T. Ahn, G. Illie, R. Radeck, V. Werner, N. Cooper, T. Ahmed, R.O. Hughes, P. Pauerstein, N. Shenkov Electromagnetic transition matrix elements have proven to be a very sensitive probe of the wave functions predicted by modern ab-initio theories based on realistic two- and three-body forces. We have carefully measured the electric quadrupole decays in $^{10}$Be ($T$=1, $T_{z}$=+1) and $^{10}$C ($T$=1, $T_{z}$=-1) [1,2] and found them to be very similar. Thus the isovector component of the E2 matrix element, which has contributions from both charge dependence of the wave functions and the E2 operator, is small. Isotensor components can come only from charge independence breaking in the wave functions; and this can be tested by determining the gamma decay matrix element from the unbound analog transition in $^{10}$B. We populated the 5164 keV state in $^{10}$B using the $^{10}$B(p,p') reaction at 10 MeV using a proton beam from the ESTU accelerator at Yale University. Gamma decays were detected in the ``YrastBall'' array. We determined the branch for this transition to be 0.16(4){\%}. This indicates a significant isotensor term.\\[4pt] [1] E.A. McCutchan et al., Phys. Rev. Lett. \textbf{103,} 192501 (2009) \newline [2] C.J. Lister et al., Bull. Am. Phys. Soc. \textbf{55,} DNP.MG5 (2010) [Preview Abstract] |
Thursday, October 27, 2011 4:24PM - 4:36PM |
FF.00003: Low energy states in $^{124,126}$Cd Populated via the Beta Decay of $^{124,126}$Ag J.C. Batchelder, S.-H. Liu, N.T. Brewer, J.H. Hamilton, A.V. Ramayya, C.J. Gross, M. Karny, A.J. Mendez II, K. Miernik, K.P. Rykacewski, D.W. Stracener, R. Grzywacz, M. Madurga, D.T. Miller, S.W. Padgett, S.V. Paulaskas, A. Kuzniak, M. Wolinska-Cichocka The lowest lying levels in the neutron-rich even-even Cd isotopes have structures that resemble an anharmonic vibrator coupled to 2-proton intruder states. Deviations from this picture have been shown to occur in $^{112-120}$Cd isotopes [1,2]. To determine the systematics of these states across the neutron shell we have begun to measure the beta decays of the heavier even-mass Ag isotopes. $^{124,126}$Ag ions were produced via the proton-induced fission of U at the HRIBF at ORNL. 15 microA of 50 MeV p bombarded on a UCx target, and the fission products were then separated by a high-resolution magnetic isobar separator and deposited on a moving tape collector directly in the center of the LeRIBSS (Low-Energy RIB Spectroscopy station) array. Many new levels in $^{124,126}$Cd have been observed. These results and the systematics of the even-even Cd isotopes will be presented and discussed.\\[0pt] [1] K. L. Green,et al., Phys. Rev. C 80, 032502(R) (2009). [2] J. C. Batchelder,et al., Phys Rev C 80, 054318 (2009). Work supported by USDOE contract DE-AC05-76OR00033 and others. [Preview Abstract] |
Thursday, October 27, 2011 4:36PM - 4:48PM |
FF.00004: Low-lying Level Structure of $^{150}$Nd A. Chakraborty, F.M. Prados-Est\'{e}vez, S.W. Yates, S.N. Choudry, B.P. Crider, A. Kumar, M.T. McEllistrem, S. Mukhopadhyay, J.N. Orce, M.G. Mynk, E.E. Peters, P.E. Garrett, W.D. Kulp, J.L. Wood To address the issue of whether the $^{150}$Nd nucleus represents an example of a phase transition in the shape degree of freedom or a complex example of shape coexistence, its level structure, up to about 2 MeV excitation and 6$\hbar$, has been explored via the ($n,n'\gamma$) reaction at the University of Kentucky accelerator facility. Level lifetimes, in the sub-picosecond regime, were extracted with a Doppler-shift attenuation analysis. A significant extension of the level scheme was possible, and the observed low-lying level structure of $^{150}$Nd indicates a close resemblance to its neighboring $^{152}$Sm isotone. Results from the ongoing analysis will be presented. [Preview Abstract] |
Thursday, October 27, 2011 4:48PM - 5:00PM |
FF.00005: Low-lying Structure of $^{132}$Xe from Inelastic Neutron Scattering E.E. Peters, A. Chakraborty, B.P. Crider, A. Kumar, F.M. Prados-Est\'{e}vez, S.F. Ashley, M.T. McEllistrem, S.W. Yates The stable isotopes of xenon span a region which exhibits the transition from spherical vibrators to gamma-soft nuclei and could thus provide some insight into this lesser understood shape transition. Many measurements to examine the nuclear structure of the xenon isotopes are constrained, however, as xenon is a gas under ambient conditions. Recently, highly enriched samples of $^{132}$Xe and $^{134}$Xe were converted to solid XeF$_{2}$ and were studied at the University of Kentucky 7-MV Van de Graaff accelerator facility using inelastic neutron scattering with gamma-ray detection. Lifetimes for some of the low-lying levels were determined via the Doppler-shift attenuation method and reduced transition probabilities were determined. First results of the experiments on $^{132}$Xe will be presented. [Preview Abstract] |
Thursday, October 27, 2011 5:00PM - 5:12PM |
FF.00006: Triple configuration coexistence in $^{44}$S Daniel Santiago-Gonzalez, I. Wiedenhoever, V. Abramkina, M.L. Avila, T. Baugher, D. Bazin, B.A. Brown, P.D. Cottle, A. Gade, T. Glasmacher, K.W. Kemper, S. McDaniel, A. Rojas, A. Ratkiewicz, R. Meharchand, E.C. Simpson, J.A. Tostevin, A. Volya, D. Weisshaar The persistence or breaking of the N=28 shell closure at the drip line is of central importance to understand the modifications of the nuclear mean field in very exotic nuclei. We studied the nucleus $^{44}$S by two-proton knockout from $^{46}$Ar at the NSCL using the SeGA gamma-array at the S800 spectrograph. We analyzed the gamma-recoil and gamma-gamma-recoil coincidence events to study the excitations of $^{44}$S. The logitudinal momentum distribution of reaction residues was used to find angular momentum assignment to the excited states. We report the observation of four new excited states, of which one is a strongly prolate deformed $J^{\pi}=4^+$ state, as indicated by a shell-model calculation. Its deformation originates in a neutron configuration which is fundamentally different from the ``intruder" configuration producing the ground state deformation. Consequently, we do not have three coexisting shapes in $^{44}$S, but three coexisting configurations, corresponding to zero, one and two neutron particle-hole excitations. [Preview Abstract] |
Thursday, October 27, 2011 5:12PM - 5:24PM |
FF.00007: Coulomb Excitation of 78,80Se and the radioactive 84Se (N=50) isotopes A. Galindo-Uribarri, E. Padilla-Rodal, R.F. Garcia-Ruiz, J.M. Allmond, J.C. Batchelder, J.R. Beene, K.B. Lagergren, P.E. Mueller, D.C. Radford, D.W. Stracener, J.P. Urrego-Blanco, R.L. Varner, C.-H. Yu Coulomb excitation is a purely electromagnetic excitation process of nuclear states due to the Coulomb field of two colliding nuclei. It is a very precise tool to measure excitation probabilities and provide insight on the collectivity of nuclear excitations and in particular on nuclear shapes. We have measured the B(E2) value of various nuclei in the mass A$\sim $80 region using particle-gamma coincidences with the HyBall and Clarion arrays at HRIBF. The Coulomb excitation of various projectile-target combinations (ASe on 12C, 24Mg, 27Al and 50Ti) allow the use of consistency cross checks and the systematic study of isotopic and isotonic chains using both stable and radioactive nuclei under almost identical experimental conditions.We present new results for 78Se, 80Se and the radioactive nucleus 84Se (N=50). [Preview Abstract] |
Thursday, October 27, 2011 5:24PM - 5:36PM |
FF.00008: $\alpha$-Clustering in $^{18}$O Melina Avila, Grigory Rogachev, Eric Johnson, Amy Crisp, J. Giles, Bert Green, Kirby Kemper, K. Lee, Donald Robson, Brian Roeder, Vladilen Goldberg, Changbo Fu, Robert Tribble, Simon Brown The $\alpha$-cluster structure of $N\neq Z$ nuclei is poorly known and is a subject of intense theoretical discussion. $\alpha$-clusters have been used to explain various nuclear effects including quasi-rotational bands of states with large $\alpha$-particle widths, which were observed in light 4N nuclei, $^{8}$Be, $^{12}$C, $^{16}$O and $^{20}$Ne. We report on the observation of the $\alpha$-cluster structure in the $N\neq Z$ nucleus $^{18}$O. The $\alpha$-cluster structure of $^{18}$O was studied through $\alpha+^{14}$C elastic scattering using the Thick Target Inverse Kinematics (TTIK) technique. The analysis was performed using a multi-level, multi-channel \textbf{R}-Matrix approach. It was found that $^{18}$O has an elaborate $\alpha$-cluster structure, including two unusual states with $\alpha$ widths exceeding the formal single particle limit. A comparison of the observed $^{18}$O $\alpha$-cluster structure with the predictions of potential model approach was performed. A more detailed description of the two very broad states can be given using this potential model approach. [Preview Abstract] |
Thursday, October 27, 2011 5:36PM - 5:48PM |
FF.00009: In-beam and decay properties of the proton-rich nucleus $^{179}$Tl C. Nair, F.G. Kondev, M.P. Carpenter, S. Zhu, I. Ahmad, B.B. Back, P.F. Bertone, C.J. Chiara, C.A. Copos, J.P. Greene, G. Gurdal, G. Henning, C.R. Hoffman, R.V.F. Janssens, B.P. Kay, T.L. Khoo, T. Lauritsen, C.J. Lister, E.A. Mccutchan, A. Rogers, D. Seweryniak, M.L. Smith, D.J. Hartley Nuclear structure studies of proton-rich Tl nuclei are important in order to elucidate their shape evolution with neutron number as well as to better interpret the rare decay modes in this region, such as electron-capture delayed fission. The $^ {179}$Tl nucleus was produced via the symmetric $^{92}$Mo($^{89}$Y, 2n) reaction using a 375 MeV beam delivered by the ATLAS accelerator at Argonne National Laboratory. The recoiling nuclei were implanted into a double-sided silicon strip detector, located at the focal plane of the Argonne Fragment Mass Analyzer. The Recoil Decay Tagging technique in conjunction with the Gammasphere spectrometer helped identify the decays of the $^{179}$Tl ground state (I$^ {\pi}$=1/2$^{+} $) and a shorter-lived isomeric state (I$^{\pi}$=11/2$^{-}$). The observed $\alpha$-decay correlations allowed an unambiguous identification of the previously unassigned ground state of the daughter nuclide $^{175}$Au. [Preview Abstract] |
Thursday, October 27, 2011 5:48PM - 6:00PM |
FF.00010: Model-Independent Calculation of Radiative Neutron Capture on Lithium-7 Gautam Rupak, Lakma Fernando, Renato Higa The radiative neutron capture on lithium-7 is calculated model independently using a low energy halo effective field theory. The cross section is expressed in terms of scattering parameters directly related to the $S$-matrix element. The capture through E1 and M1 transitions is considered. At low energy the cross section depends on the poorly known $p$-wave effective range parameter $r_1$. This constitutes the leading order uncertainty in traditional model calculations. It is explicitly demonstrated by comparing with potential model calculations. A single parameter fit describes the low energy data extremely well and yields $r_1\approx -1.54$ fm$^{-1}$. The contribution from two-body currents is also discussed. [Preview Abstract] |
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