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

Abstract

Angular correlations between \ensuremath{\gamma} 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 \ensuremath{\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 \ensuremath{\gamma}-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 \ensuremath{\gamma}-ray spectra are fitted consistently in terms of the statistical \ensuremath{\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 \ensuremath{\gamma}-ray angular distribution with respect to the compound nucleus spin axis for prolate and oblate shapes. At 69.5 MeV the \ensuremath{\gamma}-ray anisotropy relative to the compound nucleus spin axis is well described by a prolate shape with a deformation \ensuremath{\beta}=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.