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

Abstract

Giant dipole resonance (GDR) {gamma} rays were measured in kinematic coincidence with fission fragments in the reactions {sup 16}O+{sup 208}Pb at 140 MeV, and {sup 32}S+{sup nat}W at 185, 215, and 230 MeV bombarding energy, leading to {sup {similar to}216,224}Th at a temperature of {ital T}{approx}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 {sup 32}S+{sup 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 {sup 224}Th of either strongly prolate ({beta}=0.3) or noncollective oblate ({beta}={minus}0.1) shape. This is consistent with, but does not prove, a transition to a liquid drop shape having occurred at {ital T}{approx}1.8 MeV. The quasifission process is successfully included using regular extra-push and extra-extra-push energies and a quasifission lifetime {tau}{sub QF}=(20--40){times}10{sup {minus}21} sec. This is about ten times shorter than the compound nucleus fission lifetime in {supmore » 224}Th at this temperature.« less