Giant dipole resonance decay from fusion-fission and quasifission of hot thorium nuclei

Giant dipole resonance (GDR) γ rays were measured in kinematic coincidence with fission fragments in the reactions ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ ${+}^{208}$Pb at 140 MeV, and ${}_{}{}^{32}{}_{}{}^{}\mathrm{S}$ ${+}^{\mathrm{nat}}$W at 185, 215, and 230 MeV bombarding energy, leading to ${}_{}{}^{\mathrm{\sim }216,224}{}_{}{}^{}\mathrm{Th}$ at a temperature of T≊1.8 to 2.1 MeV. The experiment determined the γ-ray spectrum and the γ-ray-fission fragment angular correlation as a function of fragment mass, kinetic energy, and total kinetic energy release. The coincidence γ-ray spectra are fitted successfully and consistently in terms of the statistical decay of the hot compound system and of the fission fragments, when a large nuclear dissipation (γ=10) and, for the ${}_{}{}^{32}{}_{}{}^{}\mathrm{S}$ ${+}^{\mathrm{nat}}$W reaction, the GDR γ emission during the quasifission process is included. The γ-ray-fission fragment angular correlation indicates a deformed compound system in ${}_{}{}^{224}{}_{}{}^{}\mathrm{Th}$ of either strongly prolate (β=0.3) or noncollective oblate (β=-0.1) shape. This is consistent with, but does not prove, a transition to a liquid drop shape having occurred at T≊1.8 MeV. The quasifission process is successfully included using regular extra-push and extra-extra-push energies and a quasifission lifetime ${\mathrm{\tau }}_{\mathrm{QF}}$=(20–40)×${10}^{\mathrm{-}21}$ sec. This is about ten times shorter than the compound nucleus fission lifetime in ${}_{}{}^{224}{}_{}{}^{}\mathrm{Th}$ at this temperature.