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Title:Giant multipole resonances in lead-208
Author(s):Bell, Zane William
Doctoral Committee Chair(s):Cardman, Lawrence S.
Department / Program:Physics
Subject(s):giant multipole resonances
bremsstrahlung monochromator
scintillation detector
Abstract:The differential photoneutron cross section of 208pb in the energy region 9.9 through 11.2 MeV was measured with gamma energy resolution of approximately 100 KeV using the University of 111inois bremsstrahlung monochromator. Neutrons from a 72% enriched 208pb target and an 88% enriched 206po target were detected with an NE213 scintillation detector having a neutron energy resolution of 11.3 at 4 MeV, a 200 cm flight 'path, and an intrinsic time resolution of 1.6 ns. Neutrons leaving 207pb in its ground state and first two excited states were resolved and angular distributions for these partial cross sections were measured at seven angles from 450 to 1350 inclusive. The total photoabsorption cross section as well as the cross sections for leaving 207pb in its ground st ate and first two excited states were extracted. The data confirm the fine structure seen in earlier photoneutron studies and this structure is found to be mainly electric dipole in character. This assignment is consistent with the results of a recent elastic photon scattering study but inconsistent with an interpretation that ma~es an isovector E2 assignment based on inelastic electron scattering data. The observed integrated g1 strength in the energy region under study is consistent with the amount implied by the tail of a Lorentz line fit to earlier photoneutron data. The present data show local concentrations. and depletions of dipole strength rather than additional strength. The partial cross sections for leaving 207pb in its ground state and first two excited states were analyzed using the statistical compound nucleus model. The data indicate that this model does not correctly describe the branching to the lowest states of 207~b and that the relative amplitudes of the particle-hole states that contribute to the excited states of 208pb are not equal. The 312-hole state is found to dominate the emission process. Only moderate agreement with the observed branching is obtained by allowing for variations among the eight one particle one hole mulitudes that contribute to the decay of 208pb . It is shown that there must be a small g-wave component in the decay to the first excited state of 207 pb . The data, however, cannot be used to determine these amplitudes because there are only three measured cross sections. The experiment can only act as a "necessary" condition for a microscopic description of the excited states of 20Spb in this energy region. Lastly, a calculation of the angular distributions using the Direct-Semi-Direct excitation model and including a broad ( = 2.6 MeV) E2 resonance is not inconsistent with previous suggestions of such a resonance at 10.S MeV.
Issue Date:1979
Genre:Dissertation / Thesis
Rights Information:1979 Zane William Bell
Date Available in IDEALS:2011-06-28
Identifier in Online Catalog:397412

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