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Abstract
The giant dipole resonance (GDR) built on highly excited states was measured in neutron deficient Pb and Th isotopes in order to study nuclear deformation at high excitation energies and spins, and to search for the presence of a compound nucleus GDR even in the heaviest masses. This was achieved by measuring high energy $\gamma$-rays in heavy ion fusion reactions. The compound nuclei were formed with $\sp{16}$O and $\sp{19}$F beams on W, $\sp{181}$Ta, and $\sp{208}$Pb targets with excitation energies from 57 to 102MeV and mean spins from 14 to 44$\hbar$. In this mass region, fission is a major decay channel and the total $\gamma$-ray spectrum is a sum of $\gamma$-rays emitted from the compound system and $\gamma$-rays emitted from the excited fission fragments.
High energy $\gamma$-rays were measured in "singles" as well as in coincidence with fission fragments. The fission fragment direction determined the compound nuclear spin axis and allowed $\gamma$-ray angular distribution measurements with respect to this axis.
The data analysis was performed using the evaporation code CASCADE, which was extensively modified to calculate the decay of these heavy systems and to include the subsequent decay of the fission fragments.
In the Pb isotopes, which are spherical in the ground state, the dipole strength function showed two components indicating a strongly deformed nucleus ($\beta\sim$ 0.3). Large $\gamma$-ray anisotropies were measured for the compound nucleus GDR from which a deformed shape could be unambiguously established. At 105MeV the fission coincidence data yield a prolate deformation of $\beta\sim$ 0.43 in agreement with a predicted superdeformed shape. However, at 141MeV, the ambiguity between the prolate and noncollective oblate shape could not be resolved.
The $\gamma$-ray energy spectra in the heavy mass region of Th isotopes required an enhancement of the GDR $\gamma$-rays over the statistical model predictions from the pre-fission nuclei. This is explained by a reduction of the fission probability by a factor of ten in the early decay steps of the compound system. The $\gamma$-fission coincidences support this conclusion and the extracted deformation ($\beta\sim$ 0.3) is larger than the deformation of the ground state.
