Photonuclear cross sections for O18

J. G. Woodworth, K. G. McNeill, J. W. Jury, R. A. Alvarez, B. L. Berman, D. D. Faul, and P. Meyer
Phys. Rev. C 19, 1667 – Published 1 April 1979
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Abstract

All the major photonuclear cross sections for O18, including σ(γ,p), σ[(γ,n)+(γ,np)], and σ(γ,2n), were measured as a function of photon energy from threshold to 42 MeV. The photon energy resolution was between 200 and 300 keV. The source of radiation was the monoenergetic photon beam obtained from the annihilation in flight of fast positrons. The partial photoneutron cross sections were determined by neutron multiplicity counting, and the average neutron energies for both single- and double-photoneutron events were determined simultaneously with the cross-section data by the ring-ratio technique. The photoproton cross section was determined by counting the delayed neutrons from the β-decay of the residual N17 nuclei. Integrated cross sections were extracted from the data and compared with sum-rule predictions. Resonances in the pygmy-resonance region exhibit substantial decay branching to negative-parity states in O17. Significant differences in the branching of resonances in the giant-resonance region to the various partial cross sections, together with the photoneutron-energy data, allow isospin assignments to be made for all the resonances, with the consequent delineation of the isospin strength distribution for O18. Evidence was found for two weak but broad resonances above the giant-resonance region. Finally, comparison with the corresponding cross sections for O16 shows considerable differences between the two nuclei.

NUCLEAR REACTIONS O18(γ,n), Eγ=8.041.8 MeV; measured 4π prompt, delayed neutron yield for monoenergetic photons; σ(Eγ,1n), σ(Eγ, 2n), σ(Eγ,p), integrated cross sections; detailed distribution of isospin strength; comparison with O16.

  • Received 24 July 1978

DOI:https://doi.org/10.1103/PhysRevC.19.1667

©1979 American Physical Society

Authors & Affiliations

J. G. Woodworth, K. G. McNeill*, J. W. Jury, R. A. Alvarez, B. L. Berman, D. D. Faul, and P. Meyer

  • Lawrence Livermore Laboratory, University of California, Livermore, California 94550

  • *Permanent address: Department of Physics, University of Toronto, Toronto, Ontario, Canada M5S 1A7.
  • Permanent address: Department of Physics, Trent University, Peterborough, Ontario, Canada K9J 7B8.

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Vol. 19, Iss. 5 — April 1979

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