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

Abstract

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 $\ensuremath{\gamma}$ spectrum is analyzed in terms of prefission and postfission $\ensuremath{\gamma}$ decay. The data can only be fitted by our requiring an enhancement of the giant-dipole-resonance $\ensuremath{\gamma}$ 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 $\ensuremath{\gamma}$-fission angular correlation supports this conclusion and establishes a deformed shape of the compound nucleus.