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 HE: Nuclear Structure II (A=21-50) |
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Chair: Heather Crawford, Lawrence Berkeley National Laboratory Room: Junior Ballroom D |
Saturday, October 15, 2016 8:30AM - 8:42AM |
HE.00001: Toward charge radii measurements of neutron-deficient Ca A. J. Miller, K. Minamisono, A. Klose, E. P. Abel, D. Garand, G. Given, C. Sumithrarachchi, J. Kr{\"a}mer, B. Maa{\ss}, W. N{\"o}rtersh{\"a}user, D. M. Rossi, Y. Liu, P. F. Mantica, M. R. Pearson The charge radii, $\langle r^{2} \rangle$, of $_{20}$Ca isotopes serve as an important reference for the size and shape of neighboring nuclides, since this element has two naturally-occurring isotopes at the $N=20$ and 28 neutron shell closures. An experiment has been approved to measure $\langle r^{2} \rangle$ of Ca for neutron-deficient isotopes across $N = 20$. The $\langle r^{2} \rangle$ of $_{18}$Ar and $_{19}$K isotopes show a disappearance of the shell-closure signature at $N = 20$. The aim of this measurement is to gain a microscopic understanding of this abnormal behavior. Current developments at the BEam COoling and LAser spectroscopy (BECOLA) facility at NSCL/MSU in preparation for this measurement include offline production and spectroscopy of stable Ca$^{+}$ and simulations of a new photon detection system. [Preview Abstract] |
Saturday, October 15, 2016 8:42AM - 8:54AM |
HE.00002: Detailed Spectroscopy of 46Ca with the GRIFFIN Spectrometer Jennifer Pore The neutron-rich calcium isotopes are currently a new frontier for modern ab-initio calculations based on NN and 3N forces. Detailed experimental data from these nuclei is necessary for a comprehensive understanding of the region. Many excited states in $^{46}$Ca have been previously identified by various reaction mechanisms, most notably from $(p,p^\prime)$ and $(p,t)$ reactions, but many spins are only tentatively assigned or not measured and very few gamma-ray transitions have been placed in the level scheme. A high-statistics data set of the $^{46}$K decay into low-lying levels of $^{46}$Ca was taken with the new GRIFFIN spectrometer located at TRIUMF-ISAC. The level scheme of $^{46}$Ca has been greatly expanded to include 160 new gamma-ray transitions and 12 new excited states. Angular correlations between cascading gamma rays have been investigated to obtain information about the spins of the excited states. An overview of the experiment and a discussion of the results will be presented. [Preview Abstract] |
Saturday, October 15, 2016 8:54AM - 9:06AM |
HE.00003: $\beta$-Delayed Gamma Spectroscopy of Neutron-Rich Mg Isotopes in and around the Island of Inversion Andrea Richard, Heather Crawford The question of the immutability of "magic numbers" and structure of exotic nuclei near to shell closures has long been an area of interest. The neutron-rich Mg isotopes around N=20 and N=28 are examples of regions where the expected spherical shell gaps have narrowed or have disappeared entirely. The "Island of Inversion," centered around $^{32}$Mg, is a region where a narrowed N=20 shell gap and collective \emph{np-nh} excitations result in nuclei with deformed ground states. The N=28 region also exhibits a narrowed shell gap and large deformation. The nature of the Mg nuclei between these two shell closures, however, has only recently been studied. Accessing nuclei between N=20 and N=28 provides information about the evolution of single particle states, and the evolution of structure with the addition of neutrons can be indirectly probed. An experiment was performed in March 2015 at the NSCL to study the $\beta$-decay of neutron-rich Na, Mg, Al, and Si isotopes between N=20 and N=28. The details of the experiment and preliminary level schemes will be discussed along with the implications for the nuclear structure. [Preview Abstract] |
Saturday, October 15, 2016 9:06AM - 9:18AM |
HE.00004: The $\beta$ decay of $^{34,35}$Mg and the structure of $^{34}$Al Mustafa Rajabali Nuclei in the island of inversion, near the $N=20$ shell closure, exhibit a fascinating behavior where the nuclear ground states show deformed configurations dominated by particle-hole excitations across the neutron shell gap. The $^{31-35}$Mg nuclei are in or at the border of this island displaying intruder ground-state configurations, while the $^{31-35}$Al isotopes are suggested to have mixed ground- state configurations of normal and intruder type and thus serve as a transition from intruder dominated Mg isotopes to the normal ground-state configuration in Si isotopes. An experiment was performed at the TRIUMF-ISAC-I facility with the goal of populating states in $^{33-35}$Al via the beta decay of $^{33-35}$Mg. Mg ions were produced, transported and implanted onto a moving Mylar tape at the center of the GRIFFIN spectrometer. Results obtained from the analysis of the $^{34,35}$Mg decay data from this experiment will be presented. This includes the half-lives of $^{34,35}$Mg and $^{34,35}$Al which clarify current conflicting information in the literature. [Preview Abstract] |
Saturday, October 15, 2016 9:18AM - 9:30AM |
HE.00005: Investigation of the nuclear structure of 33Al through beta-decay of 33Mg to probe the island of inversion Tammy Zidar Away from the valley of stability, some nuclei have been found to have ground state properties that are different than those naively expected from the nuclear shell model. Around the "island of inversion", N = 20 closed shell nucleus ${}^{32}$Mg has large ground state deformations occur in association with intruder configurations from the $f_{7/2}$ shell. The nuclear structure of transitional nuclei, in which the normal and intruder configurations compete, can be used to inform theoretical models used to explain the inversion mechanism. ${}^{32}$Mg is known to have a deformed ground-state configuration, while ${}^{34}$Si displays a normal one. In the present work we studied the intermediate ${}^{33}$Al through the $\beta$-decay of ${}^{33}$Mg to clarify conflicting previous results regarding its structure. ${}^{33}$Mg was delivered to the GRIFFIN high-purity germanium $\gamma$-ray spectrometer coupled with the SCEPTAR plastic scintillator $\beta$ particle detector. High efficiency of the GRIFFIN detector provides new $\gamma$-$\gamma$ coincidences to elucidate the excited state structure of ${}^{33}$Al, and its capability to detect weak transitions has provided $\beta$-decay branching ratios for the ${}^{33}$Mg$\rightarrow$ ${}^{33}$Al$\rightarrow$ ${}^{33}$Si decay chain. [Preview Abstract] |
Saturday, October 15, 2016 9:30AM - 9:42AM |
HE.00006: Exploring the higher spin state structure of $^{31}$Si by $\gamma$-ray spectroscopy Pei-Luan Tai, Samuel Tabor, Peter Bender, L. Hamilton, V. Tripathi, C. Hoffman, R.M. Clark, P. Fallon, A. O. Macchiavelli, S. Paschalis, M. Petri, M.P. Carpenter, R.V.F. Janssens, T. Lauritsen, E. A. MacCutchan, D. Seweryniak, S. Zhu, C. Chiara, X. Chen, W. Reviol, D.G. Sarantites We present a comprehensive $\gamma$-ray spectroscopic study to the higher spin structure of $^{31}$Si. $^{31}$Si was produced through the $^{18}$O($^{18}$O,$\alpha$n) reaction at the beam energy of 25 MeV at Argonne National Laboratory, which preferentially populates the higher spin states. The particle-$\gamma$-$\gamma$ coincidence technique was used to build the energy level scheme. The Microball detector was used for selecting the reaction channel, and the multiple $\gamma$-ray coincidences were detected by GAMMASPHERE. The $^{31}$Si recoil energies and angles were kinematically reconstructed event-by-event, leading to a better Doppler correction and allowing us to discover 25 new states and 49 newly-observed $\gamma$ transitions in total. 15 $\gamma$-decaying states above the neutron separation energy at 6587 keV were identified. [Preview Abstract] |
Saturday, October 15, 2016 9:42AM - 9:54AM |
HE.00007: Resonant proton scattering on 46Ar using the Active-Target Time Projection Chamber J. Bradt, T. Ahn, Y. Ayyad Limonge, D. Bazin, S. Beceiro Novo, L. Carpenter, M.P. Kuchera, W. Lynch, W. Mittig, S. Rost, N. Watwood, J. Barney, U. Datta, J. Estee, A. Gillibert, J. Manfredi, P. Morfouace, D. Perez Loureiro, E. Pollacco, J. Sammut, S. Sweany A well-known technique for studying the single-particle properties of neutron-rich nuclei is to use resonant proton scattering on a parent nucleus to populate the isobaric analog states of the corresponding neutron-rich nucleus. The locations and amplitudes of these resonances are directly related to the structure of the nucleus of interest by isospin symmetry. We performed an experiment of this type at the National Superconducting Cyclotron Laboratory to commission the recently completed Active-Target Time Projection Chamber (AT-TPC). A 4.6-MeV/u radioactive beam of 46Ar was injected into the AT-TPC. The detector was filled with isobutane gas---which provided the protons for the reaction and served as the tracking medium---and placed inside a 2-T magnetic field. We will present preliminary results from this experiment and discuss the benefits of the active-target method for this type of measurement. [Preview Abstract] |
Saturday, October 15, 2016 9:54AM - 10:06AM |
HE.00008: Study of 22Ne and 28Mg excited states using fusion-evaporation and Doppler shift measurements Jonathan Williams Electromagnetic transition rate measurements serve as a fundamental probe of nuclear structure and provide a stringent test for theoretical models. Doppler shift lifetime measurements offer an opportunity to directly access information about electromagnetic transition rates and discriminate between model calculations. The TIGRESS Integrated Plunger device (TIP), constructed at SFU, supports Doppler shift lifetime measurements via gamma-ray spectroscopy with the TIGRESS segmented Ge array as part of the experimental program at the ISAC-II facility of TRIUMF. A recent study commissioning the TIP device employed the fusion-evaporation reaction of $^{18}$O + $^{12}$C at a beam energy of 48 MeV, with reaction channel selection provided via coincident charged particle detection using ancillary CsI(Tl) detectors. Transitions were identified belonging to the 2 alpha particle and 2 proton evaporation channels from the compound system $^{30}$Si, corresponding to $^{22}$Ne and $^{28}$Mg respectively. Lineshapes, from which lifetimes can be determined by comparison to simulated data, have been observed for these transitions. The experimental approach, analysis procedure, and a comparison of lineshapes to simulations obtained using the GEANT4 toolkit will be discussed. [Preview Abstract] |
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