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

Abstract

Giant dipole resonance (GDR) \ensuremath{\gamma} 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 $^{\mathrm{\ensuremath{\sim}}216,224}\mathrm{Th}$ at a temperature of T\ensuremath{\approxeq}1.8 to 2.1 MeV. The experiment determined the \ensuremath{\gamma}-ray spectrum and the \ensuremath{\gamma}-ray-fission fragment angular correlation as a function of fragment mass, kinetic energy, and total kinetic energy release. The coincidence \ensuremath{\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 (\ensuremath{\gamma}=10) and, for the $^{32}\mathrm{S}$${+}^{\mathrm{nat}}$W reaction, the GDR \ensuremath{\gamma} emission during the quasifission process is included. The \ensuremath{\gamma}-ray-fission fragment angular correlation indicates a deformed compound system in $^{224}\mathrm{Th}$ of either strongly prolate (\ensuremath{\beta}=0.3) or noncollective oblate (\ensuremath{\beta}=-0.1) shape. This is consistent with, but does not prove, a transition to a liquid drop shape having occurred at T\ensuremath{\approxeq}1.8 MeV. The quasifission process is successfully included using regular extra-push and extra-extra-push energies and a quasifission lifetime ${\mathrm{\ensuremath{\tau}}}_{\mathrm{QF}}$=(20--40)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}21}$ sec. This is about ten times shorter than the compound nucleus fission lifetime in $^{224}\mathrm{Th}$ at this temperature.