Giant Dipole Resonance in Highly Excited Thorium: Evidence for Strong Fission Hindrance

The giant dipole resonance in ${}_{}{}^{224}{}_{}{}^{}\mathrm{Th}$ excited to 44, 64, and 82 MeV was observed following the fusion of ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$+${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$. The total $\gamma$ spectrum is analyzed in terms of prefission and postfission $\gamma$ decay. The data can only be fitted by our requiring an enhancement of the giant-dipole-resonance $\gamma$ rays from the prefission nuclei which is explained by a large fission hindrance in the early decay steps from the compound system. A measurement of the $\gamma$-fission angular correlation supports this conclusion and establishes a deformed shape of the compound nucleus.