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 JG: Nuclear Structure: A>200 |
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Chair: Anna Hayes, Los Alamos National Laboratory Room: Peralta |
Friday, November 5, 2010 2:00PM - 2:12PM |
JG.00001: Enhanced Sensitivity to the Iso-Vector Giant Quadrupole Resonance using Linearly Polarized $\gamma$-ray Beams at HI$\gamma$S S.S. Henshaw, M.W. Ahmed, N. Brown, B.A. Perdue, S. Stave, H.R. Weller, R. Prior, M. Spraker, R. Pywell, G. Feldman, A. Nathan, S. Whisnant Data have been collected at the High Intensity $\gamma$-ray Source (HI$\gamma$S) to investigate the Isovector Giant Quadrupole (IVGQR) Resonance region in $^{209}$Bi, $E_{\gamma}=11-30$ MeV. Linearly polarized $\vec{\gamma}$-rays were incident upon an isotopically pure ($>$99.9\%) $^{209}$Bi target. Angular distributions of the scattered $\gamma$-rays both parallel and perpendicular to the plane of polarization were detected using the HI$\gamma$S NaI Detector Array (HINDA). An enhancement in sensitivity to the IVGQR parameters is observed in the angular distribution of the polarization ratio of scattered $\gamma$-rays. During the 80 hour run, the nearly mono-energetic $\vec{\gamma}$-ray intensities were $1-5\times10^{7}\gamma$'s/sec on target and statistical accuracies of 1-3\% were achieved. Angular distributions of the polarization ratios and subsequent extraction of the IVGQR resonance parameters for the $^{209}$Bi nucleus will be reported. [Preview Abstract] |
Friday, November 5, 2010 2:12PM - 2:24PM |
JG.00002: Measuring the Half Life of $^{229m}$Th E. Swanberg, J.T. Burke, R.J. Casperson, S.A. Sheets, B.R. Beck, N.D. Scielzo, E.B. Norman $^{229}$Th has the lowest known nuclear excited state at 7.6 $\pm$ 0.5 eV.\footnote{B.R. Beck \textit{et al.} (2007). ``Energy Splitting of the Ground-State Doublet in the Nucleus 229Th.'' \textit{Phys. Rev. Lett.} \textbf{98}: 142501} Decay to the ground state has never been directly observed, and the half life is unknown. Excitation from the ground state could allow applications such as a nuclear clock or a quantum computer. The half life, and hence the line width, are needed to pursue these applications. Due to the low energy, internal conversion (IC) is expected to be the dominant decay mode. The current experiment is attempting to measure the half life by searching for IC electrons or photons emitted as a result of IC. We have measured the IC electron emission of $^{235m}$U (76 eV energy, 26 minute half life) to validate or setup. Results to date will be presented. Partially supported by the U.S. Department of Homeland Security. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [Preview Abstract] |
Friday, November 5, 2010 2:24PM - 2:36PM |
JG.00003: Discrete deexcitations in 235U from Nuclear Resonance Fluorescence E. Kwan, C.R. Howell, R. Raut, G. Rusev, A.P. Tonchev, W. Tornow, A.S. Adekola, S.L. Hammond, H.J. Karwowski, R. Pedroni, J.H. Kelley Systematics of the even-even rare-earth nuclei suggest a concentration of $M1$ excitations peaking around 3 MeV with a $\sum{B(M1)\uparrow}$ strength of $\sim3\mu_N^2$. In addition, a linear dependence on the square of the ground-state deformation was observed in the systematics of the $\sum{B(M1)\uparrow}$ strengths. The actinide region is interesting for investigation of the ``scissors'' mode of $M1$ excitations because it has neutron-rich nuclei with large deformations. Evidence of $M1$ resonances concentrated around 2.0-2.5 MeV were found in $^{238}$U \& $^{232}$Th. A research program has been initiated at TUNL to measure dipole transitions in the actinide using HI$\gamma$S. Nearly monoenergic $\&$ circular polarized $\gamma$-ray beams below 3.0 MeV was used to measure transitions in $^{235}$U. More than 20 transitions were observed. The integrated cross sections, $B(M1)$ strengths \& branching transitions intensities will be presented and compared with previous measurements. [Preview Abstract] |
Friday, November 5, 2010 2:36PM - 2:48PM |
JG.00004: A study of the ($\vec{\gamma}$,n) reaction on $^{235}$U, $^{238}$U, and $^{nat}$Pb between 11 and 15.5 MeV J.R. Tompkins, H.J. Karwowski, M.W. Ahmed, N. Brown, S.S. Henshaw, J.M. Mueller, B.A. Perdue, S. Stave, H.R. Weller, B. Davis, D.M. Markoff, G. Feldman, L. Myers Polarized asymmetries of neutrons produced by linearly polarized $\gamma$-ray beams have been investigated for $^{235}$U, $^{238}$U, and $^{nat}$Pb. These studies were carried out at the High Intensity $\gamma$-ray Source at TUNL at 6 $\gamma$-ray energies, E$\gamma$, between 11.0 and 15.5 MeV. The ratio formed between neutrons emitted parallel to the plane of polarization to those emitted perpendicular was measured at $\theta$ = 54, 90, and 126$^{\circ}$. At E$_{\gamma}$ = 15.5 MeV and $\theta$ = 90$^{\circ}$, the ratio is as large as 2.5, 2.9, and 8.0 for $^{235}$U, $^{238}$U, and $^{nat}$Pb, respectively. Basic angular momentum formalism and the assumption of dominant E1 absorption in the GDR region are being used along with the level schemes of the residual nuclei in an attempt to understand the energy dependence of the measured ratios. The experimental technique and preliminary results will be presented. [Preview Abstract] |
Friday, November 5, 2010 2:48PM - 3:00PM |
JG.00005: Photofission Neutron Yield Ratios on $^{238}{\rm U}$ near $E_{\gamma}=6.2$ MeV using Linearly Polarized $\gamma$ rays S. Stave, M.W. Ahmed, N. Brown, S.S. Henshaw, J.M. Mueller, B.A. Perdue, H.R. Weller, H.J. Karwowski, J.R. Tompkins, M.S. Johnson Neutron yields and the ratios of the yields measured parallel to the plane of $\gamma$-ray polarization over the yields perpendicular to the plane of polarization ($I_{par}/I_{perp}$) have been measured using a U-238 target for the first time near the ($\gamma,n$) threshold of $E_\gamma \simeq 6.2$ MeV. Measurements were performed at $\gamma$-ray energies of 5.7 MeV (near the photofission threshold) through the ($\gamma,n$) threshold up to 6.5 MeV. The $I_{par}/I_{perp}$ data taken with the nearly 100\% linearly polarized beams at HI$\gamma$S have values ranging from 3 to 4 in the pure fission region below the ($\gamma,n$) threshold to about 2 at energies just above the ($\gamma,n$) threshold. In an effort to understand these new data, a model has been developed where the neutrons are emitted isotropically in the center-of-mass frame of the fission fragments. The fission fragment angular distributions are taken from previous $\gamma$-ray and neutron induced fission data and are used to predict the values of $I_{par}/I_{perp}$ for both the fission fragments and the neutrons. Experimental results will be shown and compared with the results of these calculations. [Preview Abstract] |
Friday, November 5, 2010 3:00PM - 3:12PM |
JG.00006: Determining neutron-induced fission cross sections of picosecond states R.J. Casperson, J.T. Burke, I.J. Thompson, J.E. Escher, J.J. Ressler, N.D. Scielzo The excited state neutron-induced fission cross section of $^{239}$Pu is not directly measurable, due to the short lifetime of the 8 keV 3/2$^{+}$ excited state. We will use recent developments in transfer reaction theory to identify the angular momentum of excited states in the pre-fission nucleus $^{240}$Pu$^{*}$. This nucleus will be produced in a (d,p) reaction on $^{239}$Pu, and the angular momentum will be determined from the fission cross section as a function of outgoing proton angle. From this angular momentum information, it should be possible to predict the neutron-induced fission cross section of the low energy excited state in $^{239}$Pu. The experimental details for measuring the appropriate proton distributions and fission cross sections from the (d,p) reaction will be discussed, as well as the feasibility of identifying the angular momentum of the excited states in the product nucleus from the proton angular distribution. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [Preview Abstract] |
Friday, November 5, 2010 3:12PM - 3:24PM |
JG.00007: High-K Isomers and Band Structures in $^{244,245}$Pu S. Hota, P. Chowdhury, S. Lakshmi, S.K. Tandel, E. Jackson, T. Harrington, K. Moran, U. Shirwadkar, I. Ahmad, M.P. Carpenter, C.J. Chiara, J. Greene, C.R. Hoffman, R.V.F. Janssens, T.L. Khoo, S. Zhu, F.G. Kondev, T. Lauritsen, C.J. Lister, E.A. McCutchan, D. Seweryniak, I. Stefanescu In continuation of our study of high-K configurations in the heavier neutron-rich N$\sim $150 $^{246-249}$Cm and $^{247,249}$Cf nuclei, using deep-inelastic and transfer reactions [1], we report on new results from Gammasphere in the N=150,151 nuclei $^{244,245}$Pu. High-spin states in $^{244,245}$Pu were populated using a $^{208}$Pb beam incident on a $^{244}$Pu target. In $^{244}$Pu, a high-K band is observed, most likely built on top of a K$^{\pi }$ = 8$^{-}$ isomer, the half-life and decay of which was measured in an earlier experiment by our group. In $^{245}$Pu, we find new rotational bands in coincidence with a 194 keV transition, which is presumably the known 7/2$^{+}$[624] to 9/2$^{-}$[734] (ground state) transition [2]. The new results will be discussed in the context of 2qp K$^{\pi }$ = 8$^{-}$ isomers observed in N=150 and 152 even-even isotones, along with expected high-K configurations and their decay modes in this region.\\[4pt] [1] U. Shirwadkar, Ph.D. Thesis, UMass Lowell, 2009.\\[0pt] [2] H. Makii et al., Phys. Rev. C\textbf{76}, 061301(R) (2007). [Preview Abstract] |
Friday, November 5, 2010 3:24PM - 3:36PM |
JG.00008: The effect of gamma deformation on the height of the fission barrier in actinides Hazem Abusara, Anatoli Afanasjev, Peter Ring Systematic calculations of the fission barriers with allowance of triaxial deformation have been performed for the first time within the covariant density functional theory. Pairing is treated within the BCS approximation using seniority zero forces adjusted to empirical values of the gap parameters. Different pairing schemes and their impact on inner fission barriers have been studied in detail. The covariant density functional theory is successful in reproducing the height of the fission barriers at a level of accuracy comparable with the best non-relativistic phenomenological macroscopic+microscopic approaches. Our analysis of the results of calculations shows that allowing triaxial deformation reduces the height of the inner barrier by 1-3 MeV. The fission path between the first and second axially deformed minima goes through the valley characterized by $\gamma$-deformation close to 10$^{\circ}$. [Preview Abstract] |
Friday, November 5, 2010 3:36PM - 3:48PM |
JG.00009: Fission Barrier of $^{254}$No at High Spin G. Henning, T.L. Khoo, D. Seweryniak, B.B. Back, P.F. Bertone, M.P. Carpenter, J.P. Greene, G. G\"urdal, C.R. Hoffman, R.V.F. Janssens, B.P. Kay, F.G. Kondev, T. Lauritsen, C.J. Lister, E.A. McCutchan, C. Nair, A.M. Rogers, S. Zhu, C.J. Chiara, K. Hauschild, A. Lopez-Martens, A. Heinz, J. Piot, P. Chowdhury, S. Lakshmi Superheavy nuclei provide opportunities to study nuclear structure at the limits in charge, spin and excitation energy. These nuclei exist only because shell effects create a fission barrier $B_f$. Hence, it is important to determine $B_f$ and its spin dependence. For $^{254}$No, the maximum spin and energy were found [1] to be $I_{max} = 22\hbar$ and E* = 8 MeV in the reaction $^{208}$Pb($^{48}$Ca,2n) at a beam energy of 219 MeV. At 223 MeV, the maximum spin increases to $32\hbar$. In contrast, the spin in $^{220}$Th, produced [2] in the $^{176}$Yb($^{48} $Ca,4n) reaction at 206 and 219 MeV, saturates at $20\hbar$. A measurement of the entry distribution of $^{254}$No at 223 MeV has been performed to determine $B_f (I)$ and results will be reported.\\[4pt] [1] P. Reiter \textit{et al.}, Phys. Rev. Lett. 84, 3542 (2000).\\[0pt] [2] A. Heinz \textit{et al.}, Nucl. Phys. A682, 458c (2001) [Preview Abstract] |
Friday, November 5, 2010 3:48PM - 4:00PM |
JG.00010: ABSTRACT WITHDRAWN |
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