Elastic photon scattering below neutron emission threshold in Bi, 208Pb, 207Pb, 206Pb, Tl, Hg, Ce, and Ba
Average elastic photon scattering cross sections were measured for Bi 208Pb 207Pb, 206Pb, Tl, Hg, Ce, and Ba at excitations between 4.5 MeV and the respective neutron emission thresholds. The incident photon energy resolution ranged from 50 to 150 keV. Ground state transition widths were determined from the scattering data for 7 strong individual levels in 208pb , and total photon absorption cross sections were inferred for Tl, Hg, Ce, and Ba using reasonable assumptions about level densities and total radiative widths. The cross sections of the nuclei in the Pb region were found to be characterized by two distinct concentrations of transition strength centered at about 5.5 and 7 MeV. The strength near 5.5 MeV in doubly magic 208Pb amounts to .5% of the dipole sum, and that near 7 MeV to about 1%. In the neighboring isotopes this strength is distributed among many more levels than in 208pb , but it remains reasonably localized and the total amount of strength seems to stay about constant. Structure observed in the Ce and Ba cross sections was found to be neither as localized nor as pronounced as that in the nuclei near Pb. In addition, the inferred total absorption cross sections of Ba and Ce fall well below what would be expected from a low energy extrapolation of the giant dipole resonance Lorentzian. For the nuclei of the Pb region, the total transition strength seems to be in excess of the value expected from the Lorentz extrapolation. For 208pb the excess is about 25% in the energy range from 4.8 to 7.4 MeV.
elastic photon scattering, neutron emmision threshold, bismuth, lead, mercury, thallium, cerium, barium
University of Illinois - Urbana-Champaign
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