Bulletin of the American Physical Society
2015 Fall Meeting of the APS Division of Nuclear Physics
Volume 60, Number 13
Wednesday–Saturday, October 28–31, 2015; Santa Fe, New Mexico
Session JE: Nuclear Structure A~30-90 |
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Chair: Kei Minamisono, Michigan State University Room: Sweeney Ballroom D |
Friday, October 30, 2015 10:30AM - 10:42AM |
JE.00001: Shell model and intruder states in the middle of the``Island of inversion'': spectroscopy of the 32Mg nucleus Sergey Ilyushkin The beta decay of $^{32}$Na [t$_{1/2}=$ 13.2(4) ms] to levels in $^{32}$Mg has been investigated at TRIUMF with the GRIFFIN spectrometer, instrumented with the SCintillating Electron Positron Tagging ARray (SCEPTAR). The nucleus $^{32}$Mg is at the center of the so-called ``Island of Inversion,'' where the weakened N$=$20 shell gap yields a region of deformation dominated by intruder states. Despite many studies, the structure of the low-energy states in $^{32}$Mg remains mostly elusive. New transitions and levels are placed in the level scheme of $^{32}$Mg from an analysis of gamma-gamma and beta-gamma coincidences. Spins and parities of several states were deduced and interpreted based on the analysis of gamma gamma angular correlations. The motivation for this measurement along with the preliminary results will be presented. [Preview Abstract] |
Friday, October 30, 2015 10:42AM - 10:54AM |
JE.00002: Isomeric character of the 4$_{1}^{+}$ state in $^{44}$S: Mechanisms of breaking of the N$=$28 shell J. Parker IV, I. Wiedenhover, J. Baker, P. Cottle, D. McPherson, M. Riley, D. Santiago-Gonzalez, A. Volya, V. Bader, T. Baugher, D. Bazin, A. Gade, T. Ginter, H. Iwasaki, C. Loelius, C. Morse, F. Recchia, D. Smalley, R. Stroberg, D. Weisshaar, K. Whitmore, A. Lemasson, H. Crawford, A. Macchiavelli, K. Wimmer The N$=$28 nucleus $^{44}$S exhibits a rich structure of excitations which illustrates different mechanisms of breaking the N$=$28 shell. A Coulomb excitation measurement [1] and an implantation-decay experiment [2] established the coexistence of 2p2h-deformed and 0p0h-spherical configurations. A two-proton knockout reaction [3] indicated a 4$^{+}$ state which shell model calculations suggest is likely isomeric, prolate-deformed and formed from a 1p1h configuration. A recent two-proton knockout experiment measured the lifetime of this 4$^{+}$ state using the recoil distance method and the GRETINA array. Results for the lifetime of the 4$^{+}$ state will be presented and its implication for the mechanisms of breaking the N$=$28 shell will be discussed. \\[4pt] [1] T. Glasmacher etal., Phys.Lett. B 395 (1997), 163\\[0pt] [2] C. Force etal., Phys.Rev.Lett. 105, 102501 (2010)\\[0pt] [3] D. Santiago-Gonzalez et al., Phys.Rev. C 83, 061305 R (2012) [Preview Abstract] |
Friday, October 30, 2015 10:54AM - 11:06AM |
JE.00003: Nuclear Structure Between N=20 and N=28: Beta-Decay in the Neutron-Rich Mg and Al Isotopes Heather Crawford The structure of nuclei in the vicinity of expected nuclear shell closures away from stability has been, and continues to be, a cornerstone for nuclear structure study. The confirmation of certain ``magic numbers'' in exotic nuclei provides insight into the evolution of nucleon configurations with isospin, but perhaps even more light is shed into the structure of the atomic nucleus when expected shell closures are found to be weakened, or entirely disappear. Two instances where this has been the case are the N=20 and N=28 neutron shell closures in the neutron-rich Mg, Si and S nuclei. However, a question which is only beginning to be answered is the nature of the transitional nuclei between N=20 and 28. Recent experimental work in the Mg isotopes has suggested a chain of prolate-deformed nuclei at Z=12, but the nature of the Al and Si isotopes just above remains a question. An experiment was conducted at NSCL to study the $\beta$-decay of neutron-rich Na, Mg, Al and Si isotopes to provide additional, and in some cases, first information on the level structures of the daughter isotopes in the region between N=20 and N=28. First results from this work will be presented, and the implications for nuclear structural evolution in this region discussed. [Preview Abstract] |
Friday, October 30, 2015 11:06AM - 11:18AM |
JE.00004: Single-particle degrees of freedom and pairing properties as sources of theoretical uncertainties in the position of the neutron drip line Sylvester Agbemava, Anatoli Afanasjev, Debisree Ray, Peter Ring The sources of theoretical uncertainties in the prediction of the two-neutron drip line are analyzed in the framework of covariant density functional theory [1-3]. We concentrate on single-particle and pairing properties as potential sources of these uncertainties [3]. The major source of these uncertainties can be traced back to the differences in the underlying single-particle structure of the various covariant energy density functionals (CEDF's). It is found that the uncertainties in the description of single-particle energies at the two-neutron drip-line are dominated by those existing already in known nuclei. Only approximately one-third of these uncertainties are from the uncertainties in the isovector channel of CEDF's. Thus, improving the CEDF description of single-particle energies in known nuclei will also reduce the uncertainties in the prediction of the position of the two-neutron drip line. The predictions of pairing properties in neutron-rich nuclei depend on the CEDF. Although pairing properties affect moderately the position of the two-neutron drip line they represent only a secondary source for the uncertainties in the definition of the position of the two-neutron drip line. \\[4pt] [1] Phys. Lett.B 726,680(2013); [2] Phys. Rev.C 89,054320(2014); [3] Phys. Rev.C 91,014324(2015) [Preview Abstract] |
Friday, October 30, 2015 11:18AM - 11:30AM |
JE.00005: Single-neutron states and the role of the $\nu g_{9/2}$ orbital in $^{71}$Zn Simone Bottoni The high-spin structure of $^{71}$Zn has been investigated at ATLAS by means of the deep inelastic reaction $^{48}$Ca+$^{70}$Zn at 25\% above the Coulomb barrier, using GRETINA and CHICO-2. In conjunction with GAMMASPHERE data from a similar reaction with a $^{70}$Zn beam on a thick $^{197}$Au target, a level scheme associated with the 3.96 h, 9/2$^{+}$ isomer in $^{71}$Zn was delineated with the aim to achieve a better understanding of the nature of the neutron excitations close to N = 40. 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 modern effective interactions. [Preview Abstract] |
Friday, October 30, 2015 11:30AM - 11:42AM |
JE.00006: First total-absorption spectroscopy measurement on the neutron-rich Cu isotopes F. Naqvi, A. Spyrou, S.N. Liddick, A.C. Larsen, M. Guttormsen, D.L. Bleuel, L.C. Campo, A. Couture, B.P. Crider, A.C. Dombos, T. Ginter, R. Lewis, S. Mosby, G. Perdikakis, C.P. Prokop, S.J. Quinn, T. Renstrom, B. Rubio, S. Siem The first beta-decay studies of $^{73-71}$Cu isotopes using the Total Absorption Spectroscopy (TAS) will be reported. The Cu isotopes have one proton outside the $Z=28$ shell and hence are good candidates to probe the single-particle structure in the region.Theories predict weakening of the $Z=28$ shell gap due to the tensor interaction between the valence $\pi \nu$ single-particle orbitals. Comparing the beta-decay strength distributions in the daughter Zn isotopes to the theoretical calculations will provide a stringent test of the predictions. The experiment was performed at the National Superconducting Cyclotron Laboratory (NSCL) employing the TAS technique with the Summing NaI(Tl) detector, while beta decays were measured in the NSCL beta-counting system. The experimentally obtained total absorption spectra for the neutron-rich Cu isotopes will be presented and the implications of the extracted beta-feeding intensities will be discussed. [Preview Abstract] |
Friday, October 30, 2015 11:42AM - 11:54AM |
JE.00007: Mixing of Triaxial and Intruder Configurations in $^{72,76}$Ge Studied via Multistep Coulomb Excitation A.D. Ayangeakaa, R.V.F. Janssens The low-lying states in even-even Ge isotopes have been a subject of intense scrutiny for many years due to the inherent challenge of interpreting their low-energy structure. While several explanations such as vibrational-rotational coupling, 2p-2h intruder mixing and shape coexistence have been proposed, none have been able to satisfactorily reproduce the properties of these low-lying excitations. Recent theoretical calculations have, however, emphasized the importance of the triaxial degree of freedom and, indeed, $^{76}$Ge is proposed to exhibit static triaxiality. In this study, the electromagnetic properties of low-lying states in $^{72,76}$Ge were investigated via sub-barrier multiple Coulomb excitation with GRETINA and CHICO-2. In the case of $^{72}$Ge, the extracted matrix elements seem to agree with the shape coexistence interpretation between the $0_1^+$ and $0_2^+$ states. However, significant mixing between the wavefunctions of these two states and triaxiality are required to reproduce the overall data. These results and calculations based on a triaxial rotor model with configuration mixing will be presented, and the role of triaxiality will be discussed. Preliminary results for $^{76}$Ge will also be highlighted. [Preview Abstract] |
Friday, October 30, 2015 11:54AM - 12:06PM |
JE.00008: Commissioning of a Replication Experiment to Investigate Claims of Beta-Decay Rate Fluctuations Correlated with Solar Proximity and Solar Activity G.W. Hitt, B. Goddard, A.A. Solodov, D. Bridi, R. El-Khazali, A.F. Isakovic Exponential decay is considered an immutable feature of radioactivity. While quantum mechanics predicts departure from exponential decay, it does so only in extreme time regimes for measurements; (1) comparable to the coherence time for the decay ($\le $10$^{-20}$s) [1] or (2) much longer than the half-life ($\ge $100t$_{1/2})$ [2]. Yet, from 2009, a number of studies have presented evidence to suggest [3] or refute [4] departures from exponential decay during the intermediate regime. If true, this would be evidence of new physics. To this end, the authors of [3] have presented evidence that beta-decays slightly (0.1{\%}) but significantly accelerate with increasing proximity to and activity of the Sun and that this may be evidence of novel neutrino-nucleus interactions. Here, we present six dedicated counting experiments aimed at replicating and improving upon the approaches used in [3]. Each experiment is commissioned in a shielded, climate-controlled setting, with continuous temperature, pressure and humidity recording. Multiple detection schemes also provide additional experimental controls, relative to previous studies, against false positives.\\[4pt] [1] C.B. Chiu \textit{et al.}, \textit{PRD} \textbf{16}, 520 (1977).\\[0pt] [2] L.A. Khalfin, \textit{JETP} \textbf{6}, 1053 (1958).\\[0pt] [3] J.H. Jenkins \textit{et al.}, \textit{Astropart. Phys}. \textbf{32}, 42 (2009).\\[0pt] [4] J.C. Hardy\textit{ et al.}, \textit{App. Rad. Iso.} \textbf{70}, 1931 (2012). [Preview Abstract] |
Friday, October 30, 2015 12:06PM - 12:18PM |
JE.00009: Are there nuclear structure effects on the isoscalar giant monopole resonance near A=90? Yogesh Gupta, Umesh Garg, K. Howard, M. Senyigit, M. Itoh, S. Ando, A. Uchiyama, T. Aoki, C. Iwamoto, S. Adachi, A. Tamii, M. Fujiwara, C. Kadono, H. Akimune, Y. Matsuda, T. Nakahara, T. Kawabata, M. Tsumura, T. Furuno, M. Harakeh, N. Kalantar-Nayestanaki The excitation energy of the isoscalar giant monopole resonance (ISGMR) exhibits, in general, a very smooth behavior (E$_{x}\sim A^{1/3}$) over the periodic Table. In recent work\footnote{D. H. Youngblood \emph{et al.}, Phys. Rev. C \textbf{88}, 021301 (2013)} the Texas A\&M group has reported that ISGMR energies for $^{92}$Zr and $^{92}$Mo are appreciably higher than that for $^{90}$Zr, suggesting significant nuclear structure effects on ISGMR and, hence, on the nuclear compressibility. We have measured inelastic scattering of 385-MeV a particles on $^{90, 92}$Zr, $^{92}$Mo at extremely forward angles, including 0$^{\circ}$, using the ``Grand Raiden'' spectrometer at RCNP, Japan. Results of detailed multipole decomposition analyses to extract the ISGMR strength distributions in the three nuclei will be presented. [Preview Abstract] |
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