Bulletin of the American Physical Society
2008 Annual Meeting of the Division of Nuclear Physics
Volume 53, Number 12
Thursday–Sunday, October 23–26, 2008; Oakland, California
Session HE: Nuclear Structure: Rare Earth Region |
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Chair: Libby McCutchan, Argonne National Laboratory Room: Simmons Ballroom 1 |
Saturday, October 25, 2008 2:00PM - 2:12PM |
HE.00001: Identification of levels in $^{159}$Sm and high spin states in $^{157}$Sm and $^{89,91}$Kr Jae-Kwang Hwang, A.V. Ramayya, J.H. Hamilton, K. Li, C. Goodin, Y.X. Luo, J.O. Rasmussen, S.J. Zhu The high spin excited states of neutron-rich nuclei $^{157,159} $Sm and $^{89,91}$Kr were identified for the first time from the spontaneous fission of $^{252}$Cf. Six excited states and six gamma transitions in $^{159}$Sm are reported here for the first time. Four, five, and two new gamma transitions were identified in $^{157}$Sm, $^{89}$Kr, and $^{91}$Kr, respectively. Level schemes in $^{89,91}$Kr are interpreted as the 2d$_{5/2}$ neutron hole configuration weakly coupled to the neighboring even-even Kr nuclei. Yrast spin sequences of 11/2$^-$ - 9/2$^+$ - 5/2$^+$ is observed in $^{89,91}$Kr. Rotational bands are identified in $^{157}$Sm and $^{159}$Sm with the proposed configurations of $\nu$5/2[523] and $\nu$3/2[521], respectively. [Preview Abstract] |
Saturday, October 25, 2008 2:12PM - 2:24PM |
HE.00002: Characteristics of phase transitions in electric monopole transitions strengths using the IBM-1 R.J. Casperson, E. Williams, V. Werner The Interacting Boson Model-1 (IBM-1) predicts a sudden rise in E0 transition strengths between the lowest two 0$^{+}$ states when crossing the phase transition from spherical to deformed nuclei. In addition, the E0 strength is predicted to remain large toward the deformed limits, which was recently supported by new data. In order to identify characteristics of phase transitions and minimize finite N effects, large configuration spaces must be used. Arbitrary precision arithmetic has allowed for calculations of up to 400 bosons using the full parameter space of the IBM-1. The calculations to be presented show that the peaking of the E0 strength at the first order phase transition and the large value in the deformed limit are two independent features. First calculations of isomer shifts for large boson numbers will also presented. Work supported by US DOE under grant number DE-FG02-91ER-40609. [Preview Abstract] |
Saturday, October 25, 2008 2:24PM - 2:36PM |
HE.00003: Probing asymptotic behavior of quantum shape phase transitional systems with angular momentum E. Williams, R.J. Casperson, V. Werner Scaling properties of quantum phase transitional (QPT) systems in atomic nuclei have been a subject of great interest in recent years, as the manner in which a system scales with system size is intimately connected to fundamental system dynamics, and provides a means of relating properties of finite, experimentally accessible QPT systems to their infinite size counterparts. In the present work, the scaling behavior of finite nuclear systems in the large boson limit is explored within the context of the Interacting Boson Model-1 for both first and second order QPT systems. The expected power law relationship between energies, transition strengths, and shape invariants with increasing boson number at the critical point in the continuous limit will be tested, and the effects of angular momentum on scaling properties of these observables will be investigated. The shape of the nuclear potential is found to strongly influence scaling behavior at the first order transition. [Preview Abstract] |
Saturday, October 25, 2008 2:36PM - 2:48PM |
HE.00004: Wobbling Beyond Lu: TSD Bands in $^{167}$Ta D.J. Hartley, E.P. Seyfried, J.R. Vanhoy, I.G. Darby, L.L. Riedinger, A. Aguilar, M.A. Riley, X. Wang, M.P. Carpenter, C.J. Chiara, R.V.F. Janssens, F.G. Kondev, T. Lauritsen, E.A. McCutchan, I. Stefanescu, S. Zhu, P. Chowdhury, S. Lakshmi, S.K. Tandel, U.S. Tandel, Q. Ijaz, W.C. Ma, U. Garg, S. Mukhopadhyay Perhaps the best indication of the rarely observed triaxial shape is the identification of the wobbling mode. This collective excitation occurs when an asymmetric nucleus is rotated at high spin. Currently, only $^{163,165,167}$Lu and perhaps $^{161}$Lu have displayed this exotic phenomenon. The fact that neighboring nuclei have not exhibited wobbling has led to the suggestion that the Lu proton Fermi surface is possibly the optimal location to observe wobbling and that no other isotope is likely to display this mode [1]. In order to test this theory, an experiment which populated high-spin states in $^{167}$Ta was performed. The $^{120}$Sn($^{51}$V,$4n$) reaction was used and Gammasphere detected the emitted $\gamma$ rays. A sequence based on the $i_{13/2}$ proton was identified for the first time, and a structure feeding into the $i_{13/2}$ band was also found. This latter structure is a strong candidate for the first wobbling band beyond Lu nuclei. Its characteristics will be compared with previously established wobbling sequences. [1] N.S. Pattabiraman et al., Phys. Lett. B 647, 243 (2007). [Preview Abstract] |
Saturday, October 25, 2008 2:48PM - 3:00PM |
HE.00005: Neutron damage tests of a highly segmented Germanium detector T.J. Ross, C.W. Beausang, I.Y. Lee, A.O. Macchiavelli, S. Gros, M. Cromaz, R.M. Clark, P. Fallon, Henrik Jeppesen, J.M. Allmond Gamma ray energy tracking arrays such as GRETINA/GRETA and AGATA are the latest evolution in gamma ray detection. By locating the interaction points, in 3-dimensions, of individual gamma ray interactions such arrays allow the energies of gamma rays to be reconstructed. This leads to excellent energy resolution, superior peak-to-total ratio and photo peak efficiency and resolving powers up to a thousand times superior to the best current generation array. The position information is extracted from the detailed pulse shapes recorded in each segment. It is anticipated that these tracking-detectors will experience significant neutron fluxes during in beam experiments. Thus it is important to test the response of highly-segmented Ge detectors when subjected to high-energy neutrons. In a one week test carried out at the 88-Inch Cyclotron at LBNL the P3 prototype detector for the GRETINA array was exposed to a neutron flux equivalent to at least one and a half years normal use. The detector was then successfully annealed. Preliminary results for the energy and position resolution, prior to and after neutron damage, and after annealing will be presented. [Preview Abstract] |
Saturday, October 25, 2008 3:00PM - 3:12PM |
HE.00006: Determination of the position resolution of a segmented HPGe detector using a collimated source M. Cromaz, I.Y. Lee, A.O. Macchiavelli, M. Wiedeking, R.M. Clark, M.A. Delepanque, P. Fallon, S. Gros, F. Stephens, H.B. Jeppesen, J.R. Pavan, D. Radford, K. Lagergren New techniques in the use of highly-segmented HPGe detectors enable the tracking of the path of a scattered gamma ray in the detector. This enables precision Doppler correction of gamma rays emitted from fast-moving sources for high-resolution spectroscopy at radioactive beam facilities. Critical to these applications is a knowledge of the position resolution to which the scattering points can be determined. We directly measured the position resolution of using a highly collimated Cs source. Signal decomposition was used to determine the position and charge deposition of the interaction points from the scattered gamma ray in the crystal, followed by tracking to identify the first interaction point. The set of first interaction points form a line through the detector and their dispersion gives the position resolution of the crystal in two dimensions. Such a measurement was performed with the 36-way segmented GRETINA P3 prototype detector the position resolution was found to be $\sigma_x$ = 1.5 mm and $\sigma_y$ = 1.7 mm. [Preview Abstract] |
Saturday, October 25, 2008 3:12PM - 3:24PM |
HE.00007: Recent advances in the study of hyperdeformation at high spin Hazem Abusara, A.V. Afanasjev The systematic investigation of hyperdeforation (HD) at high spin in the Z=40-58 part of nuclear chart has been performed in the framework of the cranked relativistic mean field theory. The properties of the HD bands such as quadrupole transition moments Q$_{t}$, dynamic J$^{(2)}$ and kinematic J$^{(1)}$ moments of inertia have been studied. These observables are affected by centrifugal stretching. Our self-consistent calculations suggest that necking degree of freedom should play an important role in some nuclei at hyperdeformation. It is especially pronounced in the proton density distribution due to the repulsive Coulomb force. The density of the HD bands is high in the spin range where they are yrast or close to yrast in the majority of cases. In these cases the observation of discrete HD bands will most likely be impossible because the feeding intensity will be redistributed among many bands, thus, dropping below the observational limit of the experimental facilities. The calculations indicate Cd isotopes as the best candidates for a search of discrete HD bands. The HD configurations become yrast at lower spins in neutron-deficient nuclei than in the ones of the valley of $\beta $-stability. [1] W.Koepf and P.Ring, Nucl. Phys. A511, 279(1990), [2] A.V.Afanasjev and H.Abusara (submitted to Physical Review C) [Preview Abstract] |
Saturday, October 25, 2008 3:24PM - 3:36PM |
HE.00008: Study of $0^+$ States and Collectivity in $^{154}$\textrm{Gd} by the (p,t$\gamma$) Reaction J.M. Allmond Recent experiments [1] have revealed an unusual number of low-lying $0^+$ states ($<$~3~MeV) in a number of rare-earth nuclei, including $^{154,156}$\textrm{Gd}. Indeed, the structure of these and neighboring ($N\sim90$) nuclei have been of recent interest [2]. To investigate the decay and population of these $0^+$ states, an experiment was conducted at the 88" cyclotron at LBNL using the STARS and LiBerACE detector arrays. A 25~MeV proton beam incident onto a $^{156}$\textrm{Gd} target was used to populate states in $^{154}$\textrm{Gd} by the (p,t$\gamma$) reaction and $^{156}$\textrm{Gd} by (p,p'$\gamma$). The exit channel of the reaction and the residual excitation energy of the nucleus were tagged by detecting scattered charged particles in a \textrm{Si} telescope array (STARS) while coincident $\gamma$ rays were detected using 6 \textrm{Ge} clovers and 1 \textrm{Ge} LEPS detector of the LiBerACE array. Branching ratios, population distributions, and particle-$\gamma$ correlations are used to probe the nature of $0^+$ states and collectivity in $^{154}$\textrm{Gd}.Preliminary results are presented. DE-FG52-06NA26206 (UR), DE-AC52-07NA27344 (LLNL), and DE-AC02-05CH11231 (LBNL).\\ {[1]}S.R.~Lesher, \textit{et~al.}, Phys. Rev. C \textbf{66}, 051305 (2002);D.A.~Meyer, \textit{et~al.}, Phys. Rev. C \textbf{74}, 044309(2006).\\ {[2]}W.D.~Kulp, \textit{et~al.}, Phys. Rev. Lett. \textbf{91}, 102501(2003);W.D.~Kulp, \textit{et~al.}, Phys. Rev. C \textbf{77}, 061301(2008). [Preview Abstract] |
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