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 DE: Mini-symposium on New Results from GRETINA IIMini-Symposium
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Chair: Hiro Iwasaki, Michigan State University Room: Junior Ballroom D |
Friday, October 14, 2016 10:30AM - 10:42AM |
DE.00001: New precision lifetime measurement of the first excited state of $^{\mathrm{12}}$Be C.J. Lister, C. Morse, P. Chowdhury, E. Merchan, V.S. Prasher, E.A. McCutchan, T.D. johnson, A. Sonzogni, H. Iwasaki, V.M. Bader, D. Bazin, S. Beceiro Novo, A. Gade, C. Loelius, E. Lunderberg, F. Recchia, D. Weisshaar, K. whitmore $^{\mathrm{12}}$Be presents an important opportunity for nuclear structure studies. It has a canonically magic number of neutrons, N $=$ 8, but on the other hand the beryllium isotopes are well-known for their $\alpha $-clustering behavior. $^{\mathrm{12}}$Be is at the limit of computationally feasible GFMC ab initio calculations, and is experimentally accessible for the purposes of making precision measurements. Although recent experiments indicate that $^{\mathrm{12}}$Be favors the development of clustering over magicity, the electromagnetic decay properties of this system are poorly constrained due to the single measurement (\textunderscore 30{\%} uncertainty) of the B(E2; 2$^{\mathrm{+}}$-0$^{\mathrm{+}})$ value. Here we present a new precise measurement of the 2$^{\mathrm{+}}$ state lifetime using GRETINA at NSCL. We find that the lifetime is about a factor of two shorter than previously reported, so even more collective and clustered then expected. The implications for the structure of $^{\mathrm{12}}$Be will be discussed. [Preview Abstract] |
Friday, October 14, 2016 10:42AM - 10:54AM |
DE.00002: GRETINA as a Compton Polarimeter P. C. Bender, D. Weisshaar, A. Gade, A. Wiens, A. O. Macchiavelli, C. M. Campbell, R. M. Clark, H. L. Crawford, M. Cromaz, P. Fallon, I. Y. Lee, J. Rissanen, S. L. Tabor, V. Tripathi, M. Albers, A. D. Ayangeakaa, M. P. Carpenter, H. M. David, T. Lauritsen, S. Zhu, P. Chowdhury, C. J. Lister, E. Merchan, V. S. Prasher, D. Miller Characterization of GRETINA as a polarimeter using the tracking technique has been done by examing the gamma-rays emitted from polarized states following the $^{24}$Mg($p$,$p'$) reaction[1]. Here we consider GRETINA as a traditional Compton polarimeter, where the intensity of the scattered radiation measured between physical detecting elements is used to determine its polarization sensitivity using techniques developed over the past decades. This provides a direct basic measure of the linear polarization of the array independent of the signal-decomposition and tracking algorithms, and directly comparable to traditional Compton polarimeters. The performance of GRETINA as a traditional Compton-polarimeter will be presented. [1] A. Wiens $et\ al.$ BAPS2014..HAW.DK.2 [Preview Abstract] |
Friday, October 14, 2016 10:54AM - 11:06AM |
DE.00003: Electromagnetic Transition Strengths in $^{\mathrm{27}}$Ne Charles Loelius, Hironori Iwasaki, Kenneth Whitmore, Mara Grinder, Robert Elder, Eric Lunderberg, Brandon Elman, Brenden Longfellow, Alexandra Gade, Daniel Bazin, Dirk Weisshaar, Peter Bender, Joe Belarge, Nobu Kobayashi, Marina Petri, Sebastian Heil, Michael Mathy, Ina Syndikus, Alexander Hufnagel Previous measurements have established that halo nuclei are well characterized by their electromagnetic properties, with the E1 transition strengths reflecting a large neutron radius, and M1 transition strengths characterizing the dominant $s$ wave strength.~The 1/2$^{\mathrm{+}}$ excited state of $^{\mathrm{27}}$Ne is close to the neutron separation energy and is expected to have a single valence neutron in the $s$ orbital, and therefore has the potential to exhibit halo effects. Furthermore, neighboring isotopes $^{\mathrm{26}}$Ne, $^{\mathrm{28}}$Ne demonstrate substantial deformation, so that $^{\mathrm{27}}$Ne should serve as an excellent benchmark for investigating the interplay between halo and deformation effects. We present here results of a new measurement of the lifetime of the $^{\mathrm{27}}$Ne 1/2$^{\mathrm{+}}$ excited state, performed at the NSCL using the Recoil Distance Method with the TRIPLEX Plunger in conjunction with GRETINA. In addition, a Coulomb-excitation measurement of $^{\mathrm{27}}$Ne has been performed using a novel application of the TRIPLEX plunger. The resulting transition strengths have been extracted and their implications are discussed. [Preview Abstract] |
Friday, October 14, 2016 11:06AM - 11:18AM |
DE.00004: One-proton knockout reaction of $^{20}\mathrm{N}$ K. Whitmore, H. Iwasaki, B.A. Brown, A. Gade, C. Loelius, C. Morse, S.R. Stroberg, D. Bazin, N. Kobayashi, F. Recchia, D. Smalley, D. Weisshaar, K. Wimmer, A. Lemasson, C.M. Campbell, P. Fallon, A.O. Macchiavelli, T. Otsuka, T. Suzuki, J.A. Tostevin Nuclear structure away from stability can change drastically due to the re-ordering of shell-model orbitals. In particular, near the neutron drip line, the neutron $1s_{1/2}$ orbital and $0d_{5/2}$ orbitals may become degenerate or even inverted. In order to study the trend of these orbitals across the $N=13$ isotones, a one-proton knockout reaction from $^{20}$N has been performed. The cross section is sensitive to states in $^{19}$C as well as the ground state in $^{20}$N. The experiment was performed at the NSCL with a beam of $^{20}$N at 70 MeV/nucleon. Gamma rays in coincidence with the $^{19}$C fragments were measured with GRETINA to determine exclusive cross sections, and the momentum of $^{19}$C recoils were recorded by the S800. Results will be compared to reaction calculations in the eikonal model. [Preview Abstract] |
Friday, October 14, 2016 11:18AM - 11:30AM |
DE.00005: A new 55Ni(p,g) rate and its implications on the rp-process Wei Jia Ong, Christoph Langer, Fernando Montes The low-lying energy levels of proton-rich $^{56}$Cu have been extracted using in-beam $\gamma$-ray spectroscopy with the state-of-the-art $\gamma$-ray tracking array GRETINA in conjunction with the S800 spectrograph at the National Superconducting Cyclotron Laboratory at Michigan State University. Excited states in $^{56}$Cu are resonances in the $^{55}$Ni(p,$\gamma$)$^{56}$Cu reaction, which is a part of the rp-process in type I x-ray bursts. To resolve existing ambiguities in the reaction Q-value, a more localized IMME mass fit is used resulting in $Q=639\pm82$~keV. We derive the first experimentally-constrained thermonuclear reaction rate for $^{55}$Ni(p,$\gamma$)$^{56}$Cu. We find that, with this new rate, the rp-process may bypass the $^{56}$Ni waiting point via the $^{55}$Ni(p,$\gamma$) reaction for typical x-ray burst conditions with a branching of up to $\sim$40$\%$. We also identify additional nuclear physics uncertainties that need to be addressed before drawing final conclusions about the rp-process reaction flow in the $^{56}$Ni region. [Preview Abstract] |
Friday, October 14, 2016 11:30AM - 11:42AM |
DE.00006: Understanding the Nature of the Low-energy Enhancement in the Photon Strength Function of $^{56}$Fe. Michael Jones A recent experiment designed to determine the multipolarity and electric or magnetic character of transitions in the region of the photon strength function (PSF) enhancement in $^{56}$Fe was performed at ANL using GRETINA in combination with the Phoswich wall\big[1\big]. A beam of 16 MeV protons impinged upon a 1 mg/cm$^{2}$ $^{56}$Fe target, inelastically exciting it to high energies. The scattered protons were then measured by the Phoswich wall, providing the entrance excitation energy, while the resulting $\gamma$-ray cascades were measured in GRETINA. The PSF can be extracted using two-step cascades from the quasicontiuum to specific low-lying levels by a model independent method first employed in $^{95}$Mo\big[2\big]. This method is being extended to take advantage of GRETINA as a polarimeter to obtain angular and polarization information in the region of the low-energy enhancement of the PSF. Preliminary results will be discussed. \\ \\ \big[1\big] D. G. Sarantites \emph{et al.} Nuclear Instruments and Methods A, 790, 42-55 (2015) \\ \big[2\big] M. Wiedeking \emph{et al.} Phys. Rev. Lett. 108, 162503 (2012) [Preview Abstract] |
Friday, October 14, 2016 11:42AM - 11:54AM |
DE.00007: Sensitivity of GRETINA position resolution to hole mobility V.S. Prasher, M. Cromaz, P. Chowdhury, E. Merchan, C.J. Lister, H.L. Crawford, C.M. Campbell, A.O. Macchiavelli, D.C. Radford, I.Y. Lee, A. Wiens The GRETINA array has been commissioned and has begun generating physics results. As the community moves towards the proposed full 4-$\pi$ national gamma-ray energy tracking array GRETA, optimizing algorithms that reconstruct gamma-ray interaction points continues to be an important task. To this end, the sensitivity of the position resolution of the GRETINA array to the hole mobility parameter has been investigated. The chi-square deviations from a ``superpulse" exhibit a shallow minimum for hole mobilities $\approx$15$\%$ lower than currently used values. Calibration data on position resolution is analyzed, together with simulations that isolate the signal decomposition dependence from electronics cross-talk. The results of this exercise will be presented and the effect of varying hole mobility on the inferred interaction points will be discussed. [Preview Abstract] |
Friday, October 14, 2016 11:54AM - 12:06PM |
DE.00008: ABSTRACT WITHDRAWN |
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