Bulletin of the American Physical Society
2010 Fall Meeting of the APS Division of Nuclear Physics
Volume 55, Number 14
Tuesday–Saturday, November 2–6, 2010; Santa Fe, New Mexico
Session FG: Nuclear Structure: 160<A<200 |
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Chair: James Vary, Iowa State University Room: Peralta |
Thursday, November 4, 2010 4:00PM - 4:12PM |
FG.00001: Lifetime Measurements in $^{162}$Dy S.R. Lesher, A. Aprahamian, R.C. de Haan, H.G. B\"{o}rner, M. Jentschel, D.D. Warner, R.F. Casten There has been great debate over the existence of low-lying vibrational excitations in deformed nuclei -- The original picture presented by Bohr \& Mottelson was one of collective vibrations superimposed on a deformed ground state shape, however empirically, it has been difficult to determine if the observed excitations are indeed vibrational in nature or excitations built on two-single particle states. One signature of collectivity is the extraction of absolute B(E2) transition probabilities from measured lifetimes. The complete spectroscopy of $^{162}$Dy was recently published up to 4 MeV [1], we have measured the lifetimes of several levels in the $^{162}$Dy nucleus using the GRID technique at The Institut Laue-Langevin in Grenoble, France. The Dy target was inserted into the core of a 60 MW high flux beam reactor and the line width was measured using a double flat crystal spectrometer (GAMS4) of perfect Silicon crystals. The GRID technique allows measurements of lifetimes from a few picoseconds and lower. Preliminary lifetime measurements will be presented.\\[4pt][1] A. Aprahamian, {\it et al.}, Nucl. Phys. A {\bf 764} 42 (2006). This work was supported by the NSF under contract number PHY07-58100. [Preview Abstract] |
Thursday, November 4, 2010 4:12PM - 4:24PM |
FG.00002: The ``tilted wobbler" in $^{167}$Ta D.J. Hartley, E.P. Seyfried, J.R. Vanhoy, S. Frauendorf, U. Garg, S. Mukhopadhyay, R.V.F. Janssens, M.P. Carpenter, C.J. Chiara, F.G. Kondev, T. Lauritsen, E.A. McCutchan, I. Stefanescu, S. Zhu, L.L. Riedinger, I.G. Darby, M.A. Riley, A. Aguilar, X. Wang, P. Chowdhury, S. Lakshmi, S.K. Tandel, U. Tandel, Q.A. Ijaz, W.C. Ma The exotic wobbling mode (which is direct experimental confirmation of asymmetric nuclear shapes) has now been observed in $^{163,165,167}$Lu and $^{167}$Ta. Particle-rotor model calculations have successfully described the transition strength ratios between the wobbling band and the $\pi i_{13/2}$ sequence upon which it is based [1]. However, assumptions made in this model suggest that the wobbling band should increase in energy (with respect to the $\pi i_{13/2}$ band) with spin. This is in direct contrast to what is systematically observed in experiments. Using a different set of assumptions within the particle-rotor model, the transition strength ratios can be reproduced while also predicting correctly the reduction in the energy splitting between the bands with increasing spin.\\[4pt] [1] I. Hamamoto and G.B. Hagemann, Phys. Rev. C 67, 014319 (2003). [Preview Abstract] |
Thursday, November 4, 2010 4:24PM - 4:36PM |
FG.00003: Quadrupole moment measurements for strongly deformed bands in $^{171,172}$Hf S. Mukhopadhyay, W.C. Ma, R.B. Yadav, J. Marsh, Q.A. Ijaz, D.M. Cullen, L.L. Riedinger, D.J. Hartley, S. Lakshmi, S.S. Hota, P. Chowdhury, C.J. Chiara, Y. Toh, M.P. Carpenter, R.V.F. Janssens, T.L. Khoo, F.G. Kondev, T. Lauritsen, S. Zhu A lifetime measurement using the DSAM technique has been performed at Gammasphere to study the nature of strongly deformed bands in $^{171,172}$Hf [1]. The measured quadrupole moments, $\sim $9.5 b for Band ED in $^{171}$Hf, and $\sim $13.5 b for Band SD1 in $^{172}$Hf, support our previous interpretation that Band ED is associated with near-prolate shape and an enhanced deformation w.r.t. the normal deformed bands, and the Band SD1 has a superdeformed shape with little triaxiality, contrary to the cranking calculations with the Ultimate Cranker code. The calculations successfully predicted the existence of triaxial strongly deformed (TSD) structures in Lu isotopes, but the predicted TSD bands in Hf isotopes remain elusive apart from the one in $^{168}$Hf. [1] Y. C. Zhang et al., Phys. Rev. C 76, 064321 (2007). [Preview Abstract] |
Thursday, November 4, 2010 4:36PM - 4:48PM |
FG.00004: New Structures in $^{178}$Hf and Coulomb Excitation of Isomers A.B. Hayes, D. Cline, C.Y. Wu, M.P. Carpenter, J.J. Carroll, D.M. Cullen, B. Detwiler, J.P. Greene, T. Harle, A.M. Hurst, R.V.F. Janssens, S.A. Karamian, T. Lauritsen, N.M. Lumley, P. Mason, I. Mills, S.V. Rigby, D. Seweryniak, T.P.D. Swan, G. Trees, P.M. Walker, S. Zhu Recent Coulomb excitation of a 985 MeV $^{178}$Hf beam (ATLAS) by a 500 $\mu $g/cm$^{2} \quad ^{208}$Pb target with CHICO+Gammasphere has yielded $\sim $3$\times $10$^{9}$ p-p-$\gamma $ events and approximately 368 $\gamma $-decay transitions involving 185 levels in 18 rotational bands, about 57 of which are newly identified. High-K isomer bands were populated at the 10$^{-3}$ level normalized to the ground-state band (GSB). A new ``tilted'' band crossing in the GSB provides a mechanism for the previously reported direct Coulomb excitation of the K$^{\pi }$=6$^{+}$ and 8$^{-}$ two-quasiparticle isomer bands. Direct $\gamma $-decay feeding to some of the isomer band states has been observed, which could confirm previously postulated breakdown of K conservation in the low-K bands. There is evidence of a new band connected with the 16$^{+}$, 31 year isomer band, which may be consistent with a $\gamma $-vibration built on the isomer. [Preview Abstract] |
Thursday, November 4, 2010 4:48PM - 5:00PM |
FG.00005: Lifetime measurement for the $2_1^+$ of $^{174}$W using fast timing with LaBr$_3$ scintillators N. Cooper, V. Werner, E. Williams, M. Bonett-Matiz, J.-M. R\'egis, T. Ahn, V. Anagnostatou, M. Elvers, A. Heinz, G. Ilie, J. Jolie, V. Mutai, D. Radeck, M. Rudigier, M.K. Smith Past experiment suggests nuclei in the rare-earth region exhibit a saturation of deformation as mid-neutron-shell is approached on the isotopic chain. Precision measurements of the B(E2:$2_1^+\rightarrow 0_1^+$) of these nuclei will help clarify the structure in this region and may serve as a test for theoretical models for well-deformed nuclei. Fast timing measurements after beta decay have been performed on $^{174}$W at WNSL, Yale University. The standard fast timing technique drew great benefits from LaBr$_3$ scintillators. The high energy resolution of these scintillators, with time resolution comparable to BaF$_2$ scintillators, allows for a much enhanced peak-to-background ratio, the placement of narrow energy gates, and accurate background subtraction. The result is a cleaner time difference spectra than possible with BaF$_2$ scintillators. Results will be presented. [Preview Abstract] |
Thursday, November 4, 2010 5:00PM - 5:12PM |
FG.00006: Spectroscopic studies on proton-rich nucleus $^{175}$Pt G. G\"urdal, M.P. Carpenter, F.G. Kondev, R.V.F. Janssens, I. Ahmad, T.L. Khoo, T. Lauritsen, C.J. Lister, D. Seweryniak, S. Zhu, D.J. Hartley, L.L. Riedinger Neutron-deficient nuclei around the $Z=82$ shell closure are known to exhibit shape coexsistence, due to the interplay between the occupation of specific intruder orbitals and core effects. It is challenging to study such nuclei experimentally, since fission dominates and, as a consequence, evaporation residue cross sections are small. The Recoil Decay Tagging (RDT) technique combined with the use of a large gamma-ray array is a powerful tool to study such nuclei. $^{175}$Pt was investigated with this approach to clarify the structure of this nucleus and to extend the systematics in this region. Excited states were populated by the $^{94}$Mo($^{84}$Sr,$2pn$) reaction at beam energies of 380 and 385 MeV provided by ATLAS. Prompt gamma rays were detected by Gammasphere and the recoiling nuclei were identified according to their $m/q$ ratio using the FMA. After $m/q$ identification, the recoiling nuclei were implanted in a 40x40 DSSD, where subsequent charge particle decays were measured. An array of four large-volume Ge detectors and one LEPS detector surrounded the DSSD. The results of this measurement will be presented. This work was supported by the U.S. Department of Energy, Office of Nuclear Physics, under Contract No. DE-AC02-06CH11357. [Preview Abstract] |
Thursday, November 4, 2010 5:12PM - 5:24PM |
FG.00007: Decay of the K$^{\pi}$=23/2$^{-}$ Isomer (T$_{1/2}$=160.44 d) in $^{177}$Lu Revisited F.G. Kondev, M.P. Carpenter, S. Zhu, I. Ahmad, C.J. Chiara, J.P. Greene, G. Gurdal, R.V.F. Janssens, T. Lauritsen, D. Seweryniak, S. Lalkovski, P. Chowdhury Studies of K isomers play an important role in understanding properties of deformed, axially-symmetric nuclei. While considerable progress has been made in the past 30 years to describe the excitation energy and quantum numbers of many isomers located in different areas of the nuclear chart, prediction of their lifetimes still remains a challenge for theory. We have recently studied the deexcitation of the K$^{\pi}$=23/2$^{-}$ isomer (T$_{1/2}$=160.44 d) in $^{177}$Lu by means of various Ge and scintillation detectors. The isomer was produced following neutron capture on $^{176}$Lu (using natural lutetium material as a target) at the University of Massachusetts Lowell research reactor facility. The radioactive source was prepared following radiochemical separation of $^{177}$Lu from $^{182}$Ta, the latter being produced from small tantalum impurities in the natural lutetium material. Singles measurements were carried out using high-purity Ge and LEPS detectors. Gamma-ray coincidence studies were also performed with Gammasphere. In addition, two LaBr$_{3}$(Ce) scintillation detectors were incorporated in Gammasphere, which allowed both gamma-ray coincidences and fast-timing measurements to be performed. Results from these studies will be presented. [Preview Abstract] |
Thursday, November 4, 2010 5:24PM - 5:36PM |
FG.00008: In-beam and decay properties of 180Tl C. Nair, C.A. Copos, M.P. Carpenter, F.G. Kondev, S. Zhu, I. Ahmad, B.B. Back, P.F. Bertone, C.J. Chiara, J.P. Greene, G. Gurdal, G. Henning, C.R. Hoffman, R.V.F. Janssens, B.P. Kay, T.L. Khoo, T. Lauritsen, C.J. Lister, E.A. McCutchan, A.M. Rogers, D. Seweryniak, M. Smith, D. Hartley Nuclear structure studies of the proton-rich nucleus $^{180}$Tl were carried out in order to investigate the electron-capture delayed fission, which is a rare decay mode. This nucleus was produced with the symmetric $^{92}$Mo($^{89}$Y, 1n) reaction using a 375 MeV beam delivered by the ATLAS accelerator at Argonne National Laboratory. Prompt $\gamma$ rays were detected with the Gammasphere array. The evaporation residues were transported through the Fragment Mass Analyzer (FMA) and dispersed according to their mass-to- charge (m/q) ratio. A position-sensitive parallel grid avalanche counter (PGAC) at the FMA focal plane provided the m/q information and the time of arrival of the recoils. The recoils were subsequently implanted into a double-sided silicon strip detector (DSSD) located behind the PGAC, where fission and $\alpha$ decays were measured. The DSSD was surrounded by an array of four Ge Clover detectors, which were used to measure $\gamma$ rays following electron-capture decay of $^ {180}$Tl. Results from these studies will be presented. [Preview Abstract] |
Thursday, November 4, 2010 5:36PM - 5:48PM |
FG.00009: Anomalous fluctuations of $s$-wave reduced neutron widths of $ ^{192,194}$Pt resonances P.E. Koehler, F. Be\v{c}v\'{a}\v{r}, M. Krti\v{c}ka, J.A. Harvey, K.H. Guber Neutron resonance parameters remain some of the most important information for testing random matrix theory (RMT), even more than fifty years after such data served as the original impetus for its creation. RMT implicitly assumes that reduced neutron widths $\Gamma _{\lambda \mathrm{n}}^{0}$ of $s$-wave resonances $\lambda $ follow a Porter-Thomas distribution (PTD). Currently, the overwhelming consensus is that $\Gamma _{\lambda \mathrm{n}}^{0}$ data agree with the PTD. However, there are problems with both the data and analysis techniques (mostly 1970's vintage) used in previous tests of the PTD that call those results into question, so it is still worthwhile to perform new tests. We obtained an unprecedentedly large number of \textit{s}-wave neutron widths through $\mathcal{R}$-matrix analysis of neutron cross-section measurements at the Oak Ridge Electron Linear Accelerator (ORELA) facility on enriched Pt samples. Careful analysis of these data rejects the validity of the Porter-Thomas distribution with a statistical significance of at least 99.997\%. To our knowledge, this result represents the most stringent test of the PTD to date, and the observed disagreement could have far-reaching consequences. [Preview Abstract] |
Thursday, November 4, 2010 5:48PM - 6:00PM |
FG.00010: $\beta -$delayed $\gamma $-ray spectroscopy of 196Hg C. Bernards, M. Elvers, D. Radeck, J. Jolie, T. Thomas, K.O. Zell, T. Ahn, A. Heinz, G. Illie, D. Savran, V. Werner, T. Ahmed, C. Deng, E. Jiang, R. Lee, N. Shenkov Recent experimental results on the nucleus 196Hg -- especially newly determined level spins and multipole mixing ratios of $\gamma $ transitions between low-energy states -- show a good agreement with supersymmetrical predictions, describing 196Hg as the two-fermion, five-boson member of the supermultiplet around 194Pt. To complete the data of a previous 194Pt($\alpha $,2n) experiment, and to search for further low-lying low-spin states that might not be populated by that reaction, we chose to perform a new experiment on 196Hg. This time we approached 196Hg via $\beta $ decay. We used a 35 MeV proton beam at the Wright Nuclear Structure Laboratory to induce the reaction 198Hg(p,3n)196Tl. Our target consisted of highly enriched 198HgS. One $\beta $ decay branch populates preferably the low-spin states we are interested in. The 196Hg $\gamma $ transitions were detected with the highly efficient YRAST Ball $\gamma $ spectrometer. We present first preliminary results and discuss the description of the even-even mercury isotopes in the context of the nuclear structure supersymmetry. Supported by DFG grants Jo391/2-3, Jo391/3-2, and by U.S. DOE grant DE-FG02-91ER40609. [Preview Abstract] |
Thursday, November 4, 2010 6:00PM - 6:12PM |
FG.00011: Scaling Relations for Hadronic Spectra from Carbon R.J. Peterson Scaling relations have been widely used to compare inclusive electron scattering spectra from complex nuclei, based upon a quasifree assumption and a range of detailed methods. These same methods may be applied to inclusive continuum hadron spectra at suitable beam energies and angles. All such schemes have now been used to compare all suitable hadron scattering spectra for the single example of carbon, including proton, pion and K+ quasifree data. Results will be shown for a scaling system based upon the ideas of a relativistic Fermi gas, using off-shell, in-medium, resonance-averaged hadron- nucleon methods. It will be shown these methods yield very similar responses for all beam probes and a wide range of momentum transfers, at beam energies from 392 to 1014 MeV, for energy losses less than for free scattering. A comparison will be made to experimental and theoretical methods successful for electron scattering. [Preview Abstract] |
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