GIANT-DIPOLE RESONANCE DECAY FROM FUSION-FISSION AND QUASI-FISSION OF HOT THORIUM NUCLEI

Giant dipole resonance (GDR) gamma-rays were measured in kinematic coincidence with fission fragments in the reactions O-16 + Pb-208 at 140 MeV, and S-32 + (nat)W at 185, 215, and 230 MeV bombarding energy, leading to approximately Th-216, Th-224 at a temperature of T almost-equal-to 1.8 to 2.1 MeV. The experiment determined the gamma-ray spectrum and the gamma-ray-fission fragment angular correlation as a function of fragment mass, kinetic energy, and total kinetic energy release. The coincidence gamma-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 (gamma = 10) and, for the S-32 +(nat)W reaction, the GDR gamma-emission during the quasifission process is included. The gamma-ray-fission fragment angular correlation indicates a deformed compound system in 224Th of either strongly prolate (beta = 0.3) or noncollective oblate (beta = -0.1) shape. This is consistent with, but does not prove, a transition to a liquid drop shape having occurred at T almost-equal-to 1. 8 MeV. The quasifission process is successfully included using regular extra-push and extra-extra-push energies and a quasifission lifetime tau(QF) = (20-40) x 10(-21) Sec. This is about ten times shorter than the compound nucleus fission lifetime in 224Th at this temperature.