Bulletin of the American Physical Society
APS April Meeting 2019
Volume 64, Number 3
Saturday–Tuesday, April 13–16, 2019; Denver, Colorado
Session G13: Nuclear Structure |
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Sponsoring Units: DNP Chair: Alexandra Gade, Michigan State University/ NSCL Room: Sheraton Plaza Court 2 |
Sunday, April 14, 2019 8:30AM - 8:42AM |
G13.00001: Level densities in deformed nuclei beyond the mean-field approximation Paul Fanto, Yoram Alhassid Nuclear level densities are important inputs to the Hauser-Feshbach theory of compound nucleus reactions. Microscopic calculations of level densities often rely on the mean-field approximation, which underestimates level densities in deformed nuclei [1]. The static-path plus random-phase approximation (SPA+RPA) includes large-amplitude thermal fluctuations and small-amplitude time-dependent quantal fluctuations beyond the mean field [2,3]. We investigate how much of the rotational enhancement of the level density is recovered in the SPA+RPA by benchmarking the level densities calculated with a quadrupole-quadrupole interaction against exact results calculated with the auxiliary-field Monte Carlo method [4]. [1] Y. Alhassid et al., Phys. Rev. C 93, 044320 (2016). [2] H. Attias and Y. Alhassid, Nucl. Phys. A 625, 565 (1997). [3] B. Lauritzen et al., Phys. Lett. B 246, 329 (1990). [4] Y. Alhassid, in Emergent Phenomena in Atomic Nuclei from Large-Scale Modeling: a Symmetry-Guided Perspective, ed. by K. D. Launey (World Scientific, Singapore, 2017), pp. 267-268. |
Sunday, April 14, 2019 8:42AM - 8:54AM |
G13.00002: In-beam γ-ray spectroscopy and lifetime measurements in the neutron-deficient phosphorus isotopes Brenden Longfellow, Alexandra Gade, B. Alex Brown, Werner A. Richter, Daniel Bazin, Peter C. Bender, Michael Bowry, Brandon Elman, Eric Lunderberg, Dirk Weisshaar, Scott J. Williams The properties of the neutron-deficient phosphorus isotopes have prompted a number of experimental and theoretical studies with focuses on both nuclear structure and nuclear astrophysics. Here we present results from in-beam γ-ray spectroscopy experiments performed at the National Superconducting Cyclotron Laboratory with the high-efficiency CsI(Na) array CAESAR and the high-resolution segmented Ge array SeGA. A new level in the drip-line nucleus 26P was observed, the energy of the dominant resonance in 27P for the (p,γ) reaction rate was confirmed, and lifetimes of excited states in 28P were measured for the first time. Shell-model calculations using the USDB-CDPN interaction were performed with the single-particle energy of the 1s1/2 proton orbital lowered to reproduce the observed Thomas-Ehrman shifts and electromagnetic transition probabilities in 28P and its mirror nucleus were compared. |
Sunday, April 14, 2019 8:54AM - 9:06AM |
G13.00003: Connecting Nuclear Structure to Stellar Astrophysics: Neutron Skin in Tin Isotopes Jack A Silano, Nicolas Schunck, Anton P Tonchev, Lee G Sobotka, Cole D Pruitt, Sean W Finch, FNU Krishichayan, Werner Tornow, Robert V F Janssens, Nadia Tsoneva The first observation of a neutron star merger by the LIGO-Virgo collaboration in 2017 highlights the need to improve our fundamental understanding of the equation of state of dense, neutron rich matter. The origin of heavy elements in the r-process and the structure of neutron stars are governed by the properties of neutron rich matter, for which experimental data is limited. Further analysis of this historic event and all future neutron star mergers relies on constraining the nuclear equation of state with experimental observables. We propose a novel method for systematically studying the evolution of the neutron skin in stable tin isotopes, by measuring the low-energy nuclear dipole strength over the broadest possible range of neutron-to-proton ratios in a single element. Nuclear resonance fluorescence with linearly polarized photons from the HIGS facility will be used to selectively measure the E1 photoabsorption strength of 112Sn and 124Sn at excitation energies from ~4 MeV up to neutron separation, where the Pygmy Dipole Resonance dominates. Progress on the measurement campaign will be presented. |
Sunday, April 14, 2019 9:06AM - 9:18AM |
G13.00004: Recent updates on the structure of 77Ga from high-resolution β decay study of neutron-rich 77Zn Durga Siwakoti, S.V. Ilyushkin, J.A. Winger, C.J. Gross, K.P. Rykaczewski, J.C. Batchelder, L. Cartegni, I.G. Darby, C. Goodin, Robert K. Grzywacz, J.H. Hamilton, A. Krogul, W. Krolas, S. Liddick, C. Mazzocchi, S. Padgett, A. Piechaczek, M.M. Rajabali, D. Shapira, E.F. Zganjar Study on the β decay of 77Zn into 77Ga is presented with updates on the decay scheme and structure of 77Ga. The present study utilized a more efficient detector setup along with high purity of the 77Cu beam in comparison to previous studies. The purity of beam used prevented any member of the decay chain from being dominant and allowed for comparisons of branching rations between the decays. The greater efficiency of the HPGe clover detectors array means more low energy γ-rays were detected in the decays. The γ-γ and β-γ coincidence data obtained from the experiment were used to develop a revised decay scheme using an objective method. In this method, the standard Gaussian function was used to fit each peak in the γ-γ spectra to determine the peak area on both a peak gate and an adjacent background gate. This enabled us to identify the statistically significant γ-γ coincidence peaks to be used in developing the decay scheme. |
Sunday, April 14, 2019 9:18AM - 9:30AM |
G13.00005: Sub-Barrier Coulomb excitation of 106Cd with the JANUS setup at ReA3 Daniel M Rhodes, Alexandra Gade, John Ash, Peter C Bender, Robert M Elder, Brandon Elman, Jack Henderson, Mara Grinder, Hironori Iwasaki, Brenden Longfellow, Tea Mijatovic, Mark-Christoph Spieker, Dirk W Weisshaar, Ching-Yen Wu Describing the evolution of the B(E2;0+→2+) transition strength in Sn isotopes from A=130 to 104 is challenging for shell-model calculations [1], making measures of collectivity near N=Z=50 100Sn quite interesting. We explore collectivity in Z=48, N=58 106Cd via Coulomb Excitation. Experimental results for 106Cd are contradictory: Recent lifetime measurements [2] disagree with NNDC values adopted from a 40 year old Coulomb excitation measurement [3]. B(E2) values derived from [2] disagree with large-scale shell model calculations, questioning the earlier good reproduction of quadrupole collectivity in 106Cd within the shell model. The results of a Coulomb excitation measurement of 106Cd will be presented. The measurement will clarify collectivity in 106Cd, extend the data to higher-lying states, and add understanding of collectivity towards N=Z=50 100Sn. [1] V. M. Bader et al., Phys. Rev. C 88, 051301(R) (2013) [2] N. Benczer-Koller et al., Phys. Rev. C 94, 034303 (2016) [3] M. T. Esat et al., Nucl. Phys. A 274, 237 (1976) |
Sunday, April 14, 2019 9:30AM - 9:42AM |
G13.00006: A new frontier: neutron-shell structure below Pb along N=127 isotone via the 206Hg(d,p) reaction. Tsz Leung Tang, Benjamin P Kay, Calem R Hoffman, David Sharp, Gaffney Liam, Peter A. Butler, Wilton N. Catford, Giacomo de Angelis, Freddy Flavigny, Sean John Freeman, Eleonora T. Gregor, Joonas Konki, Marc Labiche, Patrick T. MacGregor, Ismael Martel-Bravo, Robert D. Page, Zsolt Podolyak, Oleksii Poleshchuk, Riccardo Raabe, Alex A. Raj, Francesco Recchia, John P Schiffer, John F. Smith, Stuart V. Szwec, Jiecheng Yang The single-particle structure of the N=127 isotones below 209Pb has hitherto remained unexplored. In a first exploration of this region, single-neutron excitations in 207Hg have been measured via 206Hg(d,p) reaction in inverse kinematics. The 206Hg beam was produced at the ISOLDE facility at an energy of 7.4 MeV/u. The energy and position of outgoing protons were measured by the new ISOLDE Solenoidal Spectrometer (ISS) at a field of 2.5 T. Seven states have observed in 207Hg. Angular distributions suggest that these states are carry components of the 0g9/2, 2d5/2, 3s1/2, 2d3/2 and 0g7/2 strength. Aside from the 2d5/2 strength, which is strongly fragmented, the states represent close to the total single-neutron strength for each orbital. |
Sunday, April 14, 2019 9:42AM - 9:54AM |
G13.00007: Nuclear Two-Photon Decay with GRIFFIN C R Natzke, A B Garnsworthy, K G Leach Theoretical models predicting neutron star properties require rigorous bench-marking of calculated properties which are experimentally observable; for example the electric polarizability of nuclear matter and the difference in electric polarizability for excited nuclear matter. One possible way of extracting this quantity is through the second order electromagnetic process of nuclear two-photon decay between low-lying $0^+$ states where single photon emission is forbidden. The first excited state of $^{90}$Zr satisfies these conditions and has been observed to undergo two-photon decay. The GRIFFIN spectrometer at TRIUMF-ISAC is a powerful set-up for decay studies that has the angular sensitivity, energy resolution, and electronics required to make a precision measurement of $^{90}$Zr decay using a high-activity $^{90}$Sr source. Preliminary data has been taken using a $^{90}$Sr source to test GRIFFIN's sensitivity to $2\gamma$ decay and the findings of these investigations will be discussed; along with ongoing GEANT4 simulation work to understand the effects of source position, event filtering, and polarization response to $2\gamma$ decay in the GRIFFIN array. |
Sunday, April 14, 2019 9:54AM - 10:06AM |
G13.00008: Nuclear Excitation via Electron Capture with TITAN Jon B Ringuette, Kyle G Leach 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 we are making to the TITAN system. |
Sunday, April 14, 2019 10:06AM - 10:18AM |
G13.00009: Strong one-neutron emission from two-neutron unbound states in decays of neutron-rich Ga isotopes Rin Yokoyama, Robert K. Grzywacz, Bertis C Rasco, Nathan T Brewer, Krzysztof Piotr Rykaczewski, Iris Dillmann, Jose Louis Tain, Shunji Nishimura Beta-delayed one-neutron and two-neutron branching ratios (P1n and P2n) have been measured in the decay of neutron-rich Ga isotopes A = 84 to 87 at the RI-beam Factory at the RIKEN Nishina Center using a high-efficiency array of 3He neutron counters (BRIKEN). Two-neutron emission was observed in the decay of 84, 85, and 87Ga for the first time. P1n values are also obtained with more precision than previous measurements. The observation of the large P1n values compared to the P2n values in the Ga isotopes are interpreted as a signature of one neutron emission from the two-neutron unbound excited states in Ge daughters. We demonstrated that inclusion of Hauser Feshbach statistical model reproduces our experimental results better both on QRPA[P. Moller et al.] and Shell Model[M. Madurga et al.] calculations. This shows the relevance and importance of a statistical description of neutron emission for the prediction of the decay properties of multi-neutron emitters and that it must be included in the r-process modeling. |
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