Nuclear deformation in excited Pb isotopes from giant dipole gamma -ray-fission angular correlations.

Abstract

Angular correlations between {gamma} rays and fission fragments were measured in the reaction {sup 19}F+{sup 181}Ta at 105 and 141 MeV bombarding energy. These correlations are used to extract the probability of giant dipole resonance {gamma}-ray emission relative to the spin axis of the compound system which gives direct information about the projection quantum numbers of the split giant dipole resonance components in a deformed nucleus. Large anisotropies observed in the {gamma}-ray energy region of the compound nucleus giant dipole resonance demonstrate unambiguously a deformed shape of the {sup 200}Pb compound system at excitation energies of 69.5 and 102.4 MeV. The singles and fission-coincidence {gamma}-ray spectra are fitted consistently in terms of the statistical {gamma}-ray decay of the compound system and excited fission fragments. The giant dipole resonance parameters of these fits are then used to compute the {gamma}-ray angular distribution with respect to the compound nucleus spin axis for prolate and oblate shapes. At 69.5 MeV the {gamma}-ray anisotropy relative to the compound nucleus spin axis is well described by a prolate shape with a deformation {beta}=0.43 in general agreement with theoretical predictions of a superdeformed shape in {sup 200}Pb. However, the large observed deformation survives to much highermore » temperatures than predicted. At 102.4 MeV the data require a reduction of the fission probability in the very early decay steps of the compound system. At this excitation energy the extraction of the shape of the {sup 200}Pb nucleus is ambiguous, allowing both a collective prolate as well as a noncollective oblate shape.« less