Bulletin of the American Physical Society
2019 Fall Meeting of the APS Division of Nuclear Physics
Volume 64, Number 12
Monday–Thursday, October 14–17, 2019; Crystal City, Virginia
Session LN: Nuclear Structure IV |
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Chair: Sam Tabor, Florida State University Room: Salon K |
Wednesday, October 16, 2019 10:30AM - 10:42AM |
LN.00001: Exploring the Puzzle of Decay Half-lives Southeast of $^{\mathrm{132}}$Sn Miguel Madurga, Robert Grzywacz, Zhengyu Xu, Aleksandra Fijalkowska, Andrea Gottardo, Maria Borge, Razvan Lica The region of nuclei around doubly magic $^{132}$Sn [1] offers a unique window to study nuclear structure. Calculations in the region are used to provide the nuclear parameters in models of isotope productions in stellar rapid neutron capture (r-process) [2]. However, recent measurements of the decay half-lives in the region show systematic discrepancies with shell model calculations[3]. The beta decay of $^{132}$Cd was studied at the ISOLDE facility, CERN. The large majority of the beta-decay strength was expected to populate the unbound lowest-energy 1$^+$ state [2]. The neutron time-of-flight array VANDLE [4,5] was installed at the ISOLDE decay station. Large Gamow-Teller strength was observed in the neutron time-of-flight spectra. Analysis of the data indicates several individual 1+ states are fed in $^{132}$In. Strength fragmentation offers a compelling explanation for the experimental half-life of $^{132}$Cd being longer than state-of-the-art calculations [2]. [1] K.L. Jones et al., Nature 465, 454 (2010). [2] P. Möller, B. Pfeiffer, and K.-L. Kratz, Phys. Rev. C67,055802 (2003). [3] G. Lorusso et al., Phys. Rev. Lett. 114, 192501 (2015). [4] W. A. Peters et al., Nucl. Inst. Meth. A836, 122 (2016). [5] S.V . Paulauskas et al., Nucl. Instrum Meth. A737, 22 (2014). [Preview Abstract] |
Wednesday, October 16, 2019 10:42AM - 10:54AM |
LN.00002: Beta-decay study on the neutron-unbound states in 133Sn at ISOLDE Decay Station Zhengyu Xu, Miguel Madurga, Robert Grzywacz In this contribution, we will present a recent experimental work studying the neutron-unbound states in 133Sn from the beta decay of 133In at ISOLDE Decay Station. The beta decay in this region ($Z<50$ and $N>82$) is characterized by a large beta-decay energy window $Q_{\beta}$ and low neutron separation energy Sn. Due to the valence proton and neutron orbitals having opposite parities, Gamow-Teller transitions create deep-neutron holes in the 132Sn core. The large N=82 shell gap makes these neutron-hole states in 133Sn neutron unbound. The neutron time-of-flight detector VANDLE was used to identify these states for the first time. Neutron resonances were observed at energies between 1.5 and 3.7 MeV corresponding to candidate 11/2- (h11/2), 3/2+ (d3/2), 1/2+ (s1/2), and 7/2+ (g7/2). The neutron h11/2 state had been previously identified in gamma-ray spectroscopy, suggesting a strong suppression of the neutron emission channel. In this experiment, for the first time in the 132Sn region, it was possible to observe the neutron emission from a gamma-decaying state, thus allowing to establish partial decay widths for the gamma and neutron channels. Partial decay widths can be useful to calculate (n,$\gamma$) capture rates for rapid neutron capture (r-process) abundance calculations. [Preview Abstract] |
Wednesday, October 16, 2019 10:54AM - 11:06AM |
LN.00003: Chiral Wobbling in $^{135}$Pr Nirupama Sensharma, Umesh Garg, Stefan Frauendorf, Joseph L Cozzi, Kevin B Howard, Shaofei Zhu, Michael P Carpenter, Filip G Kondev, Dariusz Seweryniak, Robert V F Janssens, Akaa D Ayangeakaa, Daryl Hartley, Sandeep S Ghugre, Rudrajyoti Palit Chirality and wobbling are the two unique signatures that help in the identification of the rare triaxial shape in nuclei. While both these modes have been separately established in a few limited regions of the nuclear chart, the coexistence of chirality and wobbling in a nucleus, a Chiral Wobbler, has never been observed so far. Using a high statistics Gammasphere experiment with the $^{123}$Sb($^{16}$O,4n)$^{135}$Pr reaction, the very first observation of a Chiral Wobbler in $^{135}$Pr has been made. In addition to the previously established n$_{\omega}$ = 1 and n$_{\omega}$ = 2 wobbling bands, two chiral-partner bands with the configuration $\pi h_{11/2}$ $\times$ $\nu h_{11/2}^{-2}$ have been observed in this nucleus. Angular distribution analyses of the $\Delta$I = 1 connecting transitions between the two chiral partners have revealed their characteristic M1/E2 nature. Tilted axis cranking (TAC) calculations are found to be in good agreement with the experiment. [Preview Abstract] |
Wednesday, October 16, 2019 11:06AM - 11:18AM |
LN.00004: Spectra of Heavy Nuclei in the Shell Model Monte Carlo Method Sohan Vartak, Yoram Alhassid, Marco Bonett-Matiz The shell model Monte Carlo (SMMC) method is a powerful technique for calculating the thermal and ground-state properties of nuclei in model spaces that are many orders of magnitude larger than those that can be treated in conventional diagonalization methods. However, extracting information about individual excited states poses a challenge in SMMC. Recently, a method was developed to extract the energies of several excited states for given values of the spin and parity [1]. The method is based on solving a generalized eigenvalue problem that is satisfied by the imaginary-time response matrices of one-body densities. We are applying this method to calculate the energies of low-lying collective states in chains of rare-earth isotopes that describe a crossover from vibrational to rotational collectivity. [1] Y. Alhassid, M. Bonett-Matiz and C.N. Gilbreth, to be published (2019). [Preview Abstract] |
Wednesday, October 16, 2019 11:18AM - 11:30AM |
LN.00005: Hexadecapole Vibration in $^{160}$Gd? D.J. Hartley, A.D. Ayangeakaa, F.G. Kondev, K. Auranen, M.P. Carpenter, J.A. Clark, J.P. Greene, C.R. Hoffman, T. Lauritsen, J. Li, G. Savard, D. Seweryniak, S. Stolze, J. Wu, S. Zhu, K. Villafana, M.A. Riley, J. Barron, R.V.F. Janssens, D. Little, A.J. Boston, J. Heery, E.S. Paul, J. Simpson, G.L. Wilson Excited states in $^{160}$Gd were populated via Coulomb excitation of a $^{160}$Gd beam (at 1000 MeV) bombarding thick targets of $^{154}$Sm and $^{164}$Dy. The Gammasphere spectrometer, located at Argonne National Laboratory, was used to detect the emitted $\gamma$ rays. A rotational band based on the $K^{\pi} = 4^+$ state at 1071 keV was extended to higher spin. This state has been associated with a hexadecapole vibration, and the band is found to gain alignment at an unusually constant rate, which is different from that observed for the sequences based on the ground state and on the $\gamma$ vibration. A discussion based on the proximity of this nucleus to the$ N=98$ shell gap will address this constant rate of alignment. [Preview Abstract] |
Wednesday, October 16, 2019 11:30AM - 11:42AM |
LN.00006: Spectroscopy above the 6-qp K-isomer in $^{\mathrm{\mathbf{176}}}$\textbf{Hf}$^{\mathrm{\mathbf{\thinspace \ast }}}$. P. Chowdhury, E. Doucet, A.Y. Deo, S.S. Hota, D.M. Cullen, G. Mukherjee, F.G. Kondev, P.T. Greenlees, P. Jones, S. Ketelhut, S. Rinta-Antila, P. Ruotsalainen, J. Saren, J. Sorri, S. Stolze, K. Hauschild, A. Lopez-Martens, P.M. Walker High-K yrast isomers are classic representations of the competition between independent particle and collective degrees of freedom in generating high angular momentum in a well-deformed nucleus. In $^{\mathrm{176}}$Hf, 2-, 4- and 6-quasiparticle (qp) isomers were reported in early studies of high-K isomerism. The 6-qp K$^{\mathrm{\pi }}=$22$^{\mathrm{-}}$ yrast isomer (t$_{\mathrm{1/2}}=$43 $\mu $s, E$_{\mathrm{x}}=$4864 keV), in addition to its primary decay, exhibits a small and anomalously fast decay branch [1]. A predicted 8-qp yrast trap at K$=$28 has remained elusive, primarily due to the lack of suitable beam-target combinations to bring in high angular momentum. A recoil-decay-tagging experiment using the $^{\mathrm{130}}$Te($^{\mathrm{48}}$Ca,2$n)$ reaction was performed at JYFL to search for transitions feeding the 6-qp isomer, using the JUROGAM-II plus GREAT setup. Prompt gamma rays were correlated with $^{\mathrm{176}}$Hf implants and subsequent decays in a Ge-DSSD, with decay transitions detected in the focal plane. Results from the analysis will be presented and discussed. *Supported in part by the U.S.D.O.E. Grants DE-FG02-94ER40848 (UML) and DE-AC02-06CH11357 (ANL). 1. G. Mukherjee et al., Phys. Rev. C. 82, 054316 (2010). [Preview Abstract] |
Wednesday, October 16, 2019 11:42AM - 11:54AM |
LN.00007: Shape Coexistence in Neutron Deficient Mercury Isotopes Andrew MacLean Neutron deficient nuclei near Z=82 exhibit one of the most extensive manifestations of shape coexistence across the nuclear chart. In the even-even mercury isotopes, $^{182-188}$Hg, Coulomb excitation experiments have provided a sensitive probe to determine the E2 matrix elements, giving information on the nature of the deformation. For transitions of $J^{\pi}\rightarrow J^{\pi}$ with $J \neq 0$, the determination of B(E2) values also depend on the E0 matrix element. One of the best methods to extract the mixing ratios is through $\gamma-\gamma$ angular correlation measurements following $EC/\beta$ decay where a very high sensitivity can be achieved. The GRIFFIN spectrometer, located at TRIUMF-ISAC is ideal for such measurements and has been used with the decay of $^{188-200m}$Tl. Also included was measurement was the PACES array, used for the detection of conversion electrons to determine E0 transition strengths. The extraction of E0 components of mixed transitions are of utmost importance as they may be enhanced if there are significant mixings between the shape-coexisting configurations. By combining measurements of mixing ratios, conversion electron intensities and lifetimes a direct measurement of the mixing between the shape coexisting structures will be presented. [Preview Abstract] |
Wednesday, October 16, 2019 11:54AM - 12:06PM |
LN.00008: Insight into $\alpha$ clustering of proton-rich nuclei via their $\alpha$ decay Yibin Qian We focus on the $\alpha$ clustering phenomenon of proton-rich nuclei in terms of the preformation probability of $\alpha$ cluster before its penetration. Through the experimental decay data, the $\alpha$ preformation factor $P_{\alpha}$ is extracted for a large range of nuclei in the neutron-deficient region. It is found that the present $\alpha$ preformation factor varies more smoothly towards the large neutron-proton ratio, as compared to those from the previous evaluations. This may come from the separate consideration of proton and neutron density distributions of related nuclei, while they are treated as the same form before. The similarity between the $P_{\alpha}$ value and the pairing gap is clearly demonstrated, indicating the crucial role of pairing correlation involved in the $\alpha$ decay process. As a further step, the correlation between the $\alpha$ preformation factor and the microscopic correction of nuclear mass, corresponding to the effect of shell and pairing plus deformation, is in particular investigated to pursue the valuable knowledge of $P_{\alpha}$ pattern over the nuclide chart. Owing to this, the systematical results on lifetimes of $\alpha$ emitters are obviously improved within the transfer matrix method. [Preview Abstract] |
Wednesday, October 16, 2019 12:06PM - 12:18PM |
LN.00009: Nuclear Excitation via Electron Capture with TITAN Jon Ringuette, Kyle Leach, Iris Dillmann, Ania Kwiatkowski, Zachary Hockenbery, Thomas Brunner, Corina Andreoiu Nuclear Excitation via Electron Capture (NEEC) is the inverse process of internal electron conversion, where a free electron is captured into an atomic vacancy simultaneously exciting the nucleus to a higher-energy state. This process occurs naturally in hot astrophysical environments, and can excite nuclei in these isomeric states to shorter-lived states that would decay at a much faster rate than under terrestrial conditions, thus affecting reaction flows or survival rate of nuclei. Since NEEC is a resonant process, experimental access in the lab to study these cases requires strong atomic charge-state control over the sample, as well as careful selection and preparation of nuclear states that may be compatible with efficient electron recombination. Using an open-geometry electron beam ion trap (EBIT) in the TITAN experiment at the TRIUMF facility we are able to perform these studies with a high level of control and sensitivity. In this talk I will discuss the experimental concept, cases that we plan on studying in the near future, as well as current and ongoing upgrades being made to the TITAN system. [Preview Abstract] |
Wednesday, October 16, 2019 12:18PM - 12:30PM |
LN.00010: Overlaps of Deformed and Non-Deformed Basis States for Large-scale Shell-model Calculations David Kekejian, Jerry Draayer, Thomas Dytrych, Kristina Launey We provide a systematic approach for expanding non-deformed harmonic oscillator basis states in terms of deformed ones, namely we present analytical results for calculating these overlaps, transformation brackets between deformed and non-deformed basis states in cylindrical, spherical and Cartesian coordinates. The overarching objective is to integrate these results into shell-model codes to reduce the dimensionality of model spaces. This, in turn, will allow one to probe more deeply into the structure of nuclei and to provide ab initio or microscopic descriptions of medium-mass and heavy nuclei, short cutting a need to await the development of evermore robust computational resources for carrying out advanced microscopic nuclear structure calculations. [Preview Abstract] |
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