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

Angular correlations between γ rays and fission fragments were measured in the reaction ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ ${+}^{181}$Ta at 105 and 141 MeV bombarding energy. These correlations are used to extract the probability of giant dipole resonance γ-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 γ-ray energy region of the compound nucleus giant dipole resonance demonstrate unambiguously a deformed shape of the ${}_{}{}^{200}{}_{}{}^{}\mathrm{Pb}$ compound system at excitation energies of 69.5 and 102.4 MeV. The singles and fission-coincidence γ-ray spectra are fitted consistently in terms of the statistical γ-ray decay of the compound system and excited fission fragments. The giant dipole resonance parameters of these fits are then used to compute the γ-ray angular distribution with respect to the compound nucleus spin axis for prolate and oblate shapes. At 69.5 MeV the γ-ray anisotropy relative to the compound nucleus spin axis is well described by a prolate shape with a deformation β=0.43 in general agreement with theoretical predictions of a superdeformed shape in ${}_{}{}^{200}{}_{}{}^{}\mathrm{Pb}$. However, the large observed deformation survives to much higher 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 ${}_{}{}^{200}{}_{}{}^{}\mathrm{Pb}$ nucleus is ambiguous, allowing both a collective prolate as well as a noncollective oblate shape.