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 GE: Nuclear Structure: A>170 |
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Chair: Con Beausang, Richmond University Room: Simmons Ballroom 1 |
Saturday, October 25, 2008 10:30AM - 10:42AM |
GE.00001: Prospects of Neutron Triggering of Isomer Decays Ian Thompson, Edward Hartouni, Mau Chen, Jutta Escher, Alex Loshak, Petr Navratil, Erich Ormand, Jason Pruet, Tzu Wang The nucleus $^{178}$Hf$^{m2}$, with a 16+ isomeric state at 2.46 MeV of a half-life of 31 years, has been proposed as a means of storing energy. Many attempts have been made to determine whether this energy can be released by low-energy X-rays, but few unambiguous experimental results have been found: for a review see [1]. Neutrons have been also proposed [3] as a trigger, since they can interact inelastically with the isomer state to produce exit channels with positive Q-value, in what is called `super-elastic scattering' or `inelastic neutron acceleration'. Super-elastic scattering has been observed for neutron scattering on $^{177}$Lu$^{m}$ [3] and $^{180}$Ta$^{m}$ isomeric states [4]. We report on results of direct-reaction and Hauser-Feshbach calculations for neutron super-elastic scattering, where we ignore K-hindrance factors in order to obtain an upper-bound on the energy that may be released by the outgoing neutrons. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [1] J.J. Carroll, Laser Phys. Lett. \textbf{1}, 275--281 (2004) [2] G.V. Muradian, unpublished report (Moscow, 2004). [3] O. Roig \textit{et al}, Phys. Rev. C \textbf{74}, 054604 (2006) [4] S. A. Karamian\textit{, et al}, Phys. Rev. C \textbf{59} (1999) 755 [Preview Abstract] |
Saturday, October 25, 2008 10:42AM - 10:54AM |
GE.00002: Lifetime Measurement of the 2$^{+}_{1}$ state in $^{180}$Os M. Bunce, V. Werner, Z. Berant, R.J. Casperson, R.F. Casten, A. Heinz, G. Henning, M. Marshall, J. Qian, A. Schmidt, M. Smith, J.R. Terry, E. Williams, R. Winkler The lifetime of the excited 2$^{+}_{1}$ state in $^{180}$Os, was measured using the delayed coincidence timing technique. The $^{169}$Tm($^{16}$O,5n)$^{ 180}$Ir reaction was used which subsequently $\beta $-decay to populate excited states in $^{180}$Os. The Moving Tape Collector (MTC) at the Wright Nuclear Structure Laboratory (WNSL) at Yale University was used to collect the $^{180}$Ir recoil nuclei and transport them to an arrangement of four Barium Fluoride (BaF$_{2})$ scintillator detectors and one clover HPGe detector. The gamma rays emitted in transitions from the 4$^{+}_{1}$ to 2$^{+}_{1}$ and 2$^{+}_{1}$ to 0$^{+}_{gs}$ states were observed using BaF$_{2}$ detectors. The detectors were used in conjunction with fast electronic scintillation timing (FEST) techniques. The 2$^{+}_{1}$ lifetime will be input for a future g-factor measurement. First results will be presented and discussed within the study of the systematics of 2$^{+}_{1}$ states [1] in the region. This work is supported by U.S. DOE Grant No. DE-FG02-91ER-40609. References 1. Bao-An Bian et al., Phys. Rev. C \textbf{75}, 014312 (2007) [Preview Abstract] |
Saturday, October 25, 2008 10:54AM - 11:06AM |
GE.00003: What can ($^{3}$He,d) tell us about the structure of $^{186,188}$Os A.A. Phillips, P.E. Garrett, G.A. Demand, P. Finlay, K.L. Green, K.G. Leach, M.A. Schumaker, C.E. Svensson, J. Wong, R. Hertenberger, T. Faestermann, R. Kr\"{u}cken, H.-F. Wirth, L. Bettermann, N. Braun, D.G. Burke The structure of Os nuclei are of interest for a number of reasons including a debate over the vibrational nature of the K$^{\pi}$=4$^{+}$ bands, and a shape transition from well-deformed prolate to $\gamma$-soft oblate as the number of neutrons increases. In order to investigate the structure of $^{186,188}$Os, we have performed a ($^{3}$He,d) reaction on targets of $^{185,187}$Re. The 30 MeV $^{3}$He beams were obtained from the LMU/TUM Tandem Accelerator facility, and the Q3D spectrometer was used to analyze deuterons with 13 keV energy resolution. The absolute cross sections were measured at 9 angles from 5$^{\circ}$ to 50$^{\circ}$ up to $\sim$3 MeV in excitation energy. Fingerprint patterns are used to identify orbitals coupled to the 5/2$^{+}$[402]$_{\pi}$ target configuration. [Preview Abstract] |
Saturday, October 25, 2008 11:06AM - 11:18AM |
GE.00004: In-Beam Spectroscopy of $^{215}$Ac R. Winkler, A. Heinz, J. Qian, J.R. Terry, R.J. Casperson, R.F. Casten, A. Schmidt, V. Werner, E. Williams, Z. Berant, M. Bunce, G. Henning Knowledge of the evolution of proton single particle energies beyond the Z=82 shell closure is a vital ingredient in predictions of the stability of superheavy elements. In contrast to relativistic mean-field model predictions, which are used to describe shell stabilization effects, recent experiments have shown no indication of a subshell closure at Z=92. In-beam gamma spectroscopy of $^{215}$Ac has recently been carried out to study the evolution of structure of the N=126 isotones. Previous study of this isotope has been limited to delayed-gamma and alpha-gamma techniques. The powerful technique of recoil-decay-tagging using the gas-filled separator SASSYER and a newly commissioned focal plane detection system was implemented to extract decays associated with the weak fusion channel production of $^{215}$Ac. Experimental results will be presented and interpretation will be discussed in the framework of the shell model. This work is supported by U.S. DOE Grant No. DE-FG02-91ER-40609. [Preview Abstract] |
Saturday, October 25, 2008 11:18AM - 11:30AM |
GE.00005: Nuclear Resonance Fluorescence States in $^{239}$Pu Micah Johnson, D.P. McNabb, E.B. Norman Nuclear Resonance Fluorescence (NRF) has been used to probe collective excitations in many deformed rare-earth and actinide nuclei. Two collective modes have been established below 3 MeV, magnetic dipole excitations and Octupole-Quadrupole excitations. We will present measurements of newly discovered NRF states in $^{239}$Pu. The measurements were performed at the HVRL at MIT using a bremsstrahlung source with an endpoint energy up to 3 MeV. Plans for future measurements of NRF states in $^{239}$Pu at higher energies will be presented. We will also briefly discuss current research at LLNL to use NRF as a method to isotopically map containers. [Preview Abstract] |
Saturday, October 25, 2008 11:30AM - 11:42AM |
GE.00006: Decay Study of $^{257}$Rf J. Qian, A. Heinz, R. Winkler, R.V.F. Janssens, T.L. Khoo, D. Seweryniak, B.B. Back, M.P. Carpenter, A.A. Hecht, C.L. Jiang, F.G. Kondev, T. Lauritsen, C.J. Lister, D. Peterson, A. Robinson, G. Savard, X. Wang, S. Zhu, A.B. Gansworthy, M. Asai Excited states in heavy odd-even nuclei allow for the measurement of single-particle energies of orbitals playing a major role in the shell stabilization of superheavy nuclei. In this work we report on decay spectroscopy of $^{257}$Rf. The excited states of $^{257}$Rf and its daughter $^{253}$No can provide information on the single-particle structure near the deformed neutron shell N=152. $^{257}$Rf was produced in the fusion-evaporation reaction $^ {50}$Ti + $^{208}$Pb at the Argonne Tandem Linac Accelerator System, using the Fragment Mass Analyzer. The mass/charge ratio of the recoils was used for the identification of the evaporation residues. The $\alpha$ decays and internal conversion electrons from $^{257}$Rf or its decay products were recorded in a Double-sided Silicon Strip Detector and the gamma rays coincident with the charged particles were detected in four HPGe detectors. The results are compared with those of N=153 isotones and analyzed in a theoretical Macroscopic- Microscopic framework using the universal Woods-Saxon single- particle potential. These data can test the validity of this potential for superheavy nuclei. This work was supported by the U.S. DOE under contract No. DE-AC02-06CH11357 and DE-FG02- 91ER40609. [Preview Abstract] |
Saturday, October 25, 2008 11:42AM - 11:54AM |
GE.00007: Nuclear Spin Cut-off Parameter Deduced from Average Level Spacing A.N. Behkami, Mehrdad Gholami Nuclear spin cut-off parameters have been investigated for a large range of nuclear mass from the knowledge of nuclear level density at neutron binding energy, B$_{n}$ and average S-wave neutron spacing $<$D$_{1/2}^{+}>$ which is $\sigma ^{2}$=1/2$\rho $(B$_{n}) \quad <$D$_{1/2}^{+}>$. Nuclear level densities at neutron binding energy have been computed using microscopic approach. The average S-wave level spacing were taken from various compilations. The deduced values of spin cut-off parameters from the above expression have been compared with their corresponding newly published results obtained on the basis of the BCS Hamiltonian. It is found that our results are much lower than their corresponding values obtained on the basis of microscopic calculations. However, it is shown that if the values of the spin cut-off parameter deduced from the average level spacing are multiplied by a factor, F= 0.25Z ((Z/N)+1) , the agreement between the two sets becomes satisfactory. The results from both approaches will be presented and compared. The overall trends obtained from different approaches will be discussed. [Preview Abstract] |
Saturday, October 25, 2008 11:54AM - 12:06PM |
GE.00008: Formation of a new state of nuclear matter in nuclear fission Genevieve Mouze, Sabet Hachem, Christian Ythier The mass distributions of asymmetric and symmetric fission of actinide nuclei can be explained. Fissile nuclei are internally clusterized into a $^{208}$Pb-like core and a cluster made of its valence nucleons. If the energy released by the dissociation is great enough, the superficial nucleons of the core can be transferred to the cluster in a kind of internal collision, occurring within 1.8 10$^{-25}$s,as can be demonstrated. This collision creates extreme conditions, and a new nucleon phase replaces the normal proton- and neutron-phases, but conserves their organization law. The transferred nucleons are statistically distributed between the valence shells of an A$_{H}$= 126 nucleon core and those of an A$_{L}$ = 82 nucleon core (or of an A$_{L}$ = 126 nucleon core in symmetric fission), with a distribution coefficient of 0.206. The closure of the A$_{L}$ = 126 nucleon shell separates the regions of asymmetric and symmetric fission. The great yield of the symmetric mode results from the appearance of fragment-pair Q$_{tot}$-values greater than their own Coulomb barrier, i.e. from barrier-free fission. [Preview Abstract] |
Saturday, October 25, 2008 12:06PM - 12:18PM |
GE.00009: The concept of barrier in nuclear fission Genevieve Mouze, Christian Ythier An internal fission barrier can exist in a heavy nucleus if its internal energy, resulting from its internal dissociation into a dinuclear system, is not great enough for inducing a rearrangement into fragment pairs. But there exists also an external fission barrier, which is defined for a fission into a given pair ``i''. The study of $^{258}$Fm (s.f.) has shown that B$_{c}^{f}$ (i), equal to B$_{c}$ (i) -- Q$_{tot}$ (i), i.e. to the difference between Coulomb barrier and fission energy of the pair ``i'', is still negative, after sphericity correction, for its most energy-rich pairs $^{128}$Sn-$^{130}$Sn and $^{126}$Sn-$^{132}$Sn;this explains the considerable fission yield of $^{258}$Fm at A $\sim $129. For the system $^{235}$U + n$_{th}$, the B$_{c}^{f}$ (i)'s are positive for all possible fragment pairs, since B$_{c}^{f}$ (i) is already positive, and equal to 2.73 Mev, for the most energy-rich pair $^{132}$Sn-$^{104}$Mo; but a sphericity correction of about 3 MeV is necessary for the presence of the tin nucleus: this suggests that the reported value of 5.80 MeV of the ``fission barrier'' of $^{235}$U + n$_{th}$ is nothing else but its smallest external fission barrier, after sphericity correction. [Preview Abstract] |
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