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Phys. Rev. C 67, 064309 (2003) [14 pages]

Low-lying E1 strength in 20O

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E. Tryggestad *, T. Baumann, P. Heckman, and M. Thoennessen
National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, Michigan 48824, USA
Michigan State University, Department of Physics and Astronomy, East Lansing, Michigan 48824, USA

T. Aumann †, D. Bazin, and Y. Blumenfeld ‡
National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, Michigan 48824, USA

J. R. Beene, T. A. Lewis, D. C. Radford, D. Shapira, and R. L. Varner
Physics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA

M. Chartier §
National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, Michigan 48824, USA
Physics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831

M. L. Halbert
University of Tennessee, Department of Physics and Astronomy, Knoxville, Tennessee 37996, USA

J. F. Liang
Oak Ridge Associated Universities, Oak Ridge, Tennessee 37831, USA

Received 27 February 2003; published 23 June 2003

Intermediate energy Coulomb excitation at 100 MeV/nucleon was used to probe the low-lying level structure of both 18O and 20O in the region between 1 and 8 MeV. Discrete 1- states with energies of 5.35(10) and 6.85(5) MeV were observed in 20O. The strong direct excitation and subsequent γ-ray decay of these states, along with B(Eλ) predictions for 18,20O levels in this energy region, established their dipole character. The extracted B(E1)↑ values of 0.062(16)e2 fm2 and 0.035(9)e2 fm2 for the 5.35- and 6.85-MeV states, respectively, are significantly larger than shell model calculations, though modification of the single-particle energies, in particular, the p-sd shell gap, improves the agreement. The summed B(E1)↑ value for these levels (in Weisskopf units) is consistent with that for other nuclei that have been discussed recently in the literature as potential Pygmy dipole resonance hosts.


©2003 The American Physical Society

URL: http://link.aps.org/doi/10.1103/PhysRevC.67.064309
DOI: 10.1103/PhysRevC.67.064309
PACS: 25.70.De, 25.20.Dc, 24.30.Gd, 24.10.Lx

* Present address: Institut de Physique Nucléaire, IN 2P 3-CNRS, 91406 Orsay Cedex, France. Email address: tryggest@ipno.in2p3.fr
† Present address: Gesellschaft für Schwerionenforschung, Planckstrasse 1, 64291 Darmstadt, Germany
‡ Present address: Institut de Physique Nucléaire, IN 2P 3-CNRS, 91406 Orsay Cedex, France.
§ Present address: University of Liverpool, Department of Physics, Oliver Lodge Laboratory, Oxford Street, Liverpool L69 7ZE, United Kingdom.

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