Bulletin of the American Physical Society
2016 Fall Meeting of the APS Division of Nuclear Physics
Volume 61, Number 13
Thursday–Sunday, October 13–16, 2016; Vancouver, BC, Canada
Session CE: Mini-symposium on New Results from GRETINA IMini-Symposium
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Chair: Mike Carpenter, Argonne National Laboratory Room: Junior Ballroom D |
Friday, October 14, 2016 8:30AM - 9:06AM |
CE.00001: An Overview of GRETINA and its Physics Program Invited Speaker: Augusto Macchiavelli GRETINA [1], a first implementation of a gamma-ray tracking array, combines unparalleled position resolution, large Ge efficiency, and good P/T to provide a powerful tool for in-beam gamma-ray spectroscopy. The commissioning in 2012 demonstrated the technical feasibility and unique capabilities of a gamma-ray tracking array, and successful physics campaigns have followed at NSCL/MSU(2013/14) and ATLAS/ANL (2014/15). New and exciting physic results have been shown in a broad range of topics, clearly confirming the expectations of excellent performance in both high- and Coulomb barrier- energy environments, and in multiple configurations. GRETINA is again operating at NSCL for a second campaign coupled to the S800 spectrometer. Following a brief status report of the array, I will present some selected highlights from the science campaigns, complementing the latest results to be discussed in this mini-symposium. Future plans, with an emphasis on the development and construction of the full 4$\pi$ GRETA, will also be discussed. [1] S. Paschalis, I.Y.Lee, et al. NIM A709 (2013) 44-55 [Preview Abstract] |
Friday, October 14, 2016 9:06AM - 9:18AM |
CE.00002: Coulomb Excitation of Radioactive Mo-Ru Isotopes J.M. Allmond The study of shapes in atomic nuclei has been a major focus of nuclear structure ever since the observation of large electric quadrupole moments in the first half of the 20th century. A leading challenge has been to experimentally establish regions of oblate deformation, which are very limited, and triaxial deformation. The neutron-rich Mo-Ru region is expected to exhibit triaxial deformation in the low-lying states, mediated by a relatively rare instance of prolate-to-oblate shape evolution. A survey of equipment, techniques, and preliminary results from recent Coulomb-excitation and beta-decay experiments in the neutron-rich Mo-Ru region will be presented. These experiments were conducted at the CARIBU-ANL facility using GRETINA-CHICO2. An emphasis will be placed on unique opportunities with 3-MeV/u beams. [Preview Abstract] |
Friday, October 14, 2016 9:18AM - 9:30AM |
CE.00003: Single-neutron states and the role of the $\nu$g$_{9/2}$ orbital in $^{71}$Zn Simone Bottoni The structure of $^{71}$Zn has been studied at ATLAS by means of transfer reactions between heavy ions at beam energies 25$\%$ above the Coulomb barrier. The $\gamma$ rays were detected by GRETINA while scattered charged particles were identified by using CHICO2. In conjunction with data from a Gammasphere stand-alone experiment using a thick target, level schemes associated with the long-lived, 9/2$^+$ isomers in $^{71}$Zn were delineated. The level sequences built on the g$_{9/2}$ neutron orbital all appear to be of single-particle character. The results will be presented and compared with shell-model calculations, using current effective interactions, which reveal a preferential oblate configuration for high-spin yrast states. The present experiment allowed to investigate, for the first time, the potential of transfer reactions between heavy ions using GRETINA and can serve as a benchmark for future experiments. [Preview Abstract] |
Friday, October 14, 2016 9:30AM - 9:42AM |
CE.00004: Shell Evolution in the Neutron-Rich Cu and Zn Isotopes Joe Belarge, Daniel Bazin, Alexandra Gade, Yassid Ayyad, Peter Bender, Robert Elder, Brandon Elman, Hiro Iwasaki, Nobuyuki Kobayashi, Charles Loelius, Brenden Longfellow, Eric Lunderberg, Pierre Morfouace, Chris Sullivan, Dirk Weisshaar, Kenneth Whitmore Recent shell model calculations predict a gradual reduction of the Z=28 shell gap in Ni isotopes as the $\nu1g_{9/2}$ orbital is filled from $^{68}$Ni to $^{78}$Ni [Otsuka et al. PRL 95,232502]. These predictions can be experimentally tested by measuring the spectroscopic strength of a given orbital in an isotopic chain. The neutron-rich Cu isotopes, with one proton outside of a filled $\pi1f_{7/2}$ orbital, are some of the best candidates to exhibit the effects of the underlying structure evolution in this region. The high luminosity provided by fast beam, thick target experiments performed at the NSCL, coupled with the high resolution, high efficiency gamma-ray array GRETINA, provide a unique opportunity to study the neutron-rich Cu isotopes. The current experiment aims to measure the strength of 2p-1h excitations in $^{69-77}$Cu, populated through one proton knockout from $^{70-78}$Zn beams on a Be target, thereby probing the effective single particle energy and spectroscopic strength of the $\pi1f_{7/2}$ orbital. Results from the ongoing analysis will be presented. [Preview Abstract] |
Friday, October 14, 2016 9:42AM - 9:54AM |
CE.00005: Shape coexistence and the role of axial asymmetry in $^{72,76}$Ge A. D. Ayangeakaa, R. V. F. Janssens, C. Y. Wu, J. M. Allmond, J. L. Wood, S. Zhu, M. Albers, S. Almaraz-Calderon, B. Bucher, M. P. Carpenter, C. J. Chiara, D. Cline, H. L. Crawford, H. M. David, J. Harker, A. B. Hayes, C. R. Hoffman, B. P. Kay, K. Kolos, A. Korichi, T. Lauritsen, A. O. Macchiavelli, A. Richards, D. Seweryniak, A. Wiens The structure of low-lying states in medium-mass germanium isotopes $^{72}$Ge and $^{76}$Ge is investigated via projectile Coulomb excitation with GRETINA and CHICO2. In both nuclei, complete sets of $E2$ matrix elements were determined and substantial evidence for triaxiality and shape coexistence, based on the model-independent shape invariants deduced from the Kumar-Cline sum rule, has been observed [3]. These observations are supported by results of a two-state mixing model as well as multi-configuration mixing calculations carried out within the framework of the triaxial rotor model. These observations and the role of axial asymmetry in the shape coexistence phenomena, as well as results pertaining to rigid triaxiality in $^{76}$Ge will be presented. [Preview Abstract] |
(Author Not Attending)
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CE.00006: Nature of Collective Dipole and Octupole Transitions in Neutron-Rich Barium Isotopes B. Bucher, S. Zhu Recently, a direct measurement of octupole strength in $^{\mathrm{144}}$Ba was carried out via Coulomb excitation with a radioactive beam from Argonne's CARIBU facility using GRETINA and CHICO2 [1]. The results verify the presence of enhanced octupole collectivity in this isotope, as predicted by theory [2]. In the neighboring isotope $^{\mathrm{146}}$Ba, however, the importance of octupole correlations is more uncertain. Specifically, the electric dipole strength, expected to be closely correlated with the octupole one, displays what is perhaps the most significant drop in strength between neighboring isotopes of any medium- to heavy-mass nuclei. To address this puzzling question, a Coulomb excitation experiment was also performed on $^{\mathrm{146}}$Ba under the same conditions. The new measurement yields an enhanced octupole strength of the same magnitude as that observed in $^{\mathrm{144}}$Ba. This supports the notion that the strong-weak dipole behavior in this region results from the unique single-particle structure characteristic of $Z\sim 56$ and $N\sim 90$ in the presence of a pear-shaped mean-field potential [2]. [1] Bucher \textit{et al.}, \textit{Phys. Rev. Lett.} \textbf{116}, 112503 (2016) [2] Butler and Nazarewicz, \textit{Rev. Mod. Phys.} \textbf{68}, 349 (1996) [Preview Abstract] |
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