Bulletin of the American Physical Society
2007 APS April Meeting
Volume 52, Number 3
Saturday–Tuesday, April 14–17, 2007; Jacksonville, Florida
Session X8: Minisymposium on Gamma Ray Spectroscopy at the Limits I |
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Sponsoring Units: DNP Chair: Mark Riley, Florida State University Room: Hyatt Regency Jacksonville Riverfront City Terrace 4 |
Tuesday, April 17, 2007 10:45AM - 11:21AM |
X8.00001: Gamma Ray Spectroscopy: Some highlights from the past, present and future Invited Speaker: The early implementation stages of the current generation of large scale gamma-ray spectrometers, EUROGAM Phase 1 closely followed by Gammasphere Early Implementation, came online in the early 1990's. Last August the tenth anniversary of the full Gammasphere Array was celebrated. Large arrays of Compton suppressed Ge detectors, such as Gammasphere, Eurogam/Euroball/Jurosphere operated in both stand alone mode and, more recently, when coupled to highly selective and sensitive channel selection devices, such as the Fragment Mass Analyzer or RITU, or auxiliary detectors, such as Microball and Chico, have led to an unprecedented increase in our knowledge of the properties of the atomic nucleus when stressed by the application of high angular momentum, large proton or neutron imbalance, high temperatures etc. Gamma-ray spectroscopy is now routinely carried out at the limits of nuclear existence, either in terms of mass or in nuclei on, or beyond, the drip-lines. This talk will touch upon some of the classic results obtained with such arrays, will review the current state of the art in gamma-ray spectroscopy and consider some potentials for the future of the field with new arrays such as GRETA in the US and AGATA in Europe. This work is supported by the US Department of Energy under grant numbers DE-FG52-06NA26206 and DE-FG02-05ER41379. [Preview Abstract] |
Tuesday, April 17, 2007 11:21AM - 11:33AM |
X8.00002: Exotic low-energy vibrational modes Nico Orce, Esmat Elhami, Jason Holt, Andreas Linnemann, Shelly Lesher, Christoph Fransen, Norbert Pietralla, Sharmistha Mukhopadhyay, Sadia Choudry, Marcus Scheck, Volker Werner, Marcus McEllistrem, Tom Kuo, Steve Yates Nuclei near closed shells or subshells are often interpreted as exhibiting vibrational structures. Mixed-symmetry (MS) states are collective vibrational modes in which neutron and proton intrinsic g-factors are additive in the isovector part of the M1 magnetic dipole operator and may lead to large B(M1) values of $\sim$1$\mu_N^2$. The fundamental MS mode in nearly-spherical nuclei is a 2$^+$ excitation (2$_{1,MS}^+$) with a strong M1 transition to the one-phonon 2$^+_1$ level and, typically, a rather weak E2 transition to the ground state. Exotic cases of MS states have recently been identified in $^{93}$Nb and $^{94}$Zr. The former, the first case of a MS state in a nearly spherical odd-mass nucleus, arises from the weak coupling between the bosonic core, (2$^+_{1,MS}$, $^{94}$Mo), and the fermionic $\pi$ 2p$^{-1}_{1/2}$ proton hole. The latter presents an anomalous case of quadrupole vibrations, where the MS state, 2$_{1,MS}^+$, lies below the strongly anharmonic 2$^+$ two-phonon state (isoscalar), and the 2$^+_{1,MS}$ $\rightarrow$ 0$^+_1$ transition is observed to have a larger E2 transition strength than the 2$^+_1$ $\rightarrow$ 0$^+_1$ decay. This material is based upon work supported by the U.S. National Science Foundation under Grant No. PHY-0354656. [Preview Abstract] |
Tuesday, April 17, 2007 11:33AM - 11:45AM |
X8.00003: Detailed level scheme of $^{92}$Rh and its relevance to the decay of $^{94m}$Ag O.L. Pechenaya, C.J. Chiara, D.G. Sarantites, W. Reviol, R.J. Charity, M.P. Carpenter, R.V.F. Janssens, C.J. Lister, D. Seweryniak, S. Zhu, L.-L. Andersson, E.K. Johansson, D. Rudolph The level scheme of $^{92}$Rh was studied via the $^{40}$Ca($^{58}$Ni,$% \alpha pn$) reaction in an experiment utilizing the Gammasphere, Neutron Shell, and Microball arrays for detection of $\gamma $ rays, neutrons, and charged particles, respectively. \ The level scheme of $^{92}$Rh reported in [1] has been modified and extended to higher spins, and angular distributions have been measured for most of the $\gamma $ rays observed. This information is relevant to the likelihood of the reported two-proton decay from the 6.7-MeV 21$^{+}$ isomer in $^{94}$Ag to excited states in $% ^{92}$Rh [2]. \ The evidence for that decay mode hinges on the claimed observation of several $^{92}$Rh transitions in coincidence with two protons. Our new results for $^{92}$Rh place severe constraints on the amount of angular momentum removed by a possible two-proton decay of $^{94m}$% Ag. \newline [1] D. Kast \textit{et al}., Z. Phys. \textbf{A356}, 363 (1997). \newline [2] I. Mukha \textit{et al.}, Nature \textbf{439}, 298 (2006). [Preview Abstract] |
Tuesday, April 17, 2007 11:45AM - 11:57AM |
X8.00004: Determination of spin alignment in fusion-evaporation reactions with a 4$\pi$ charged-particle detector array C.J. Chiara, D.G. Sarantites, W. Reviol, R.J. Charity, O.L. Pechenaya, A.E. Stuchbery It has become fairly standard practice in the analyses of the decays of nuclei produced in fusion-evaporation reactions to measure the angular distributions (ADs) of the emitted gamma rays relative to the beam direction. In such analyses, the dependence on the orientation of the spin vector of the residue is integrated over all directions perpendicular to the beam. Greater sensitivity to the anisotropies of the ADs can be achieved, however, if one can measure these ADs relative to the spin direction on an event-by-event basis. Techniques for determining the spin alignment of residues through the detection of evaporated charged particles will be discussed. Specific examples from data using the Gammasphere and Microball arrays will be shown. Consequences of the segmentation of the charged-particle detector array on such measurements will also be addressed in light of the impact it will have on the development of future auxiliary detectors for Gretina/GRETA. [Preview Abstract] |
Tuesday, April 17, 2007 11:57AM - 12:09PM |
X8.00005: Short-lived excited-state $g$ factors of fast $^{38,40}$S fragments Kei Minamisono, Andrew Stuchbery, Andrew Davies, Paul Mantica The transient field technique for measuring short-lived excited-state magnetic dipole moments generally requires ion velocities comparable to $Zv_0$ ($v_0$ = $c$/137) or lower, which, at face value, would preclude the study of the wide range of isotopes available at fragmentation facilities. However, stringent testing of nuclear models far from stability can be realized with $g$-factor measurements of these isotopes, once the experimental challenges are overcome. Measurements performed at the NSCL's Coupled Cyclotron Facility have extended the transient field technique to radionuclides produced as in-flight fast fragments. The high velocity transient field technique was applied to intermediate-energy beams of $^{38}$S and $^{40}$S. The signs of their first-excited 2$^+$ state $g$ factors were obtained, and with a parametrization of the transient field strength at high velocities, the $g$-factor magnitudes were extracted. Results, experimental details, and future outlook will be presented. [Preview Abstract] |
Tuesday, April 17, 2007 12:09PM - 12:21PM |
X8.00006: Spectroscopy inside the ``Island of Inversion'': $^{30}$Na Vandana Tripathi, S.L. Tabor, Paul Mantica, P. Bender, J. Cook, C.R. Hoffman, Sangjin Lee, J. Pereira, M. Perry, K. Pepper, J. Pinter, J. Stoker, D. Weisshaar Nuclei with large excess of one type of nucleons are expected to have properties which are far different from their stable counterparts. The neutron rich N=20 isotones for Ne, Na and Mg show large deformation unexpected for closed shell nuclei, suggesting that the shell gap may be weakened for large isospin. These nuclei thus exhibit co-existence of spherical and intruder configurations at low excitation energy. However in most cases, it is difficult to experimentally classify the excited states into members of these two configurations, due to lack of precise data. In this paper we report on the spectroscopy of the N=19, $^{30}$Na, supposed to belong to the ``island of inversion''. Three $1^+$ states have been identified, two of which have dominant intruder configurations. Details of the level scheme obtained from the $\beta$ decay of $^{30}$Ne and a comparison with Monte Carlo Shell Model calculations which include the $sd$ and partial $fp$ shells as the valence space will be discussed. [Preview Abstract] |
Tuesday, April 17, 2007 12:21PM - 12:33PM |
X8.00007: Transition Rate Measurement in $^{18}$N Mathis Wiedeking, P. Fallon, A.O. Macchiavelli, L.W. Phair, D.L. Bleuel, R.M. Clark, M. Cromaz, M-A. Deleplanque, J.D. Gibelin, I-Y. Lee, M.A. McMahan, L.G. Moretto, E. Rodriguez-Vieitez, D. Ward, L.A. Bernstein, J.T. Burke, B.F. Lyles, A. Volya Previous information on excited states of $^{18}$N has been obtained from selective reaction mechanisms such as beta-decay and charge exchange reactions only. This work is the first to successfully utilize the non-selective $^{9}$Be($^{11}$B,2p)$^{18}$N fusion-evaporation reaction to extract structure information. The LIBERACE-STARS detector array -- an array of large-area segmented silicon detectors (E-$\Delta $E) and Compton suppressed HPGe Clover detectors -- was used to detect the charged particles and $\gamma $ radiation, respectively. New $\gamma $ transitions were added to the $^{18}$N level scheme and the B(M1) from the first excited state to the ground state was determined to be 0.01 W.u. $<$ B(M1) $<$ 3.6 W.u. Shell model calculations were used to study the low-lying configurations of $^{18}$N and its odd-A neighbors $^{17}$C and $^{19}$O (N=11 isotones). The role of proton holes in determining the evolution of ground state and low-lying excited state properties of these N=11 isotones will be discussed. [Preview Abstract] |
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