Binary fragmentations of ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$${+}^{209}$Bi (${\mathit{E}}_{\mathit{p}}$=491 MeV) and ${}_{}{}^{64}{}_{}{}^{}\mathrm{Ni}$${+}^{165}$Ho (${\mathit{E}}_{\mathit{p}}$=778 MeV) leading to compound nuclei with ${\mathit{E}}^{\mathrm{*}}$≊400 MeV and similar masses were investigated by measuring fission coincident neutrons and α particles. The neutron field was decomposed into contributions from preequilibrium (PE), prescission, and the two fragment sources with a moving source analysis. It was performed for separate excitation energy classes deduced with the folding angle technique from the linear momentum transfer. The angular distribution of the PE neutrons from the ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$${+}^{209}$Bi reaction indicates a pronounced out-of-plane anisotropy that would result in an overestimation of the PE multiplicity by 37% for a measurement only in plane. The PE multiplicities and temperatures are interpreted by a delay in formation for the more symmetric entrance channel ${}_{}{}^{64}{}_{}{}^{}\mathrm{Ni}$${+}^{165}$Ho, which is typical for quasifission and in line with the enhancement of asymmetric mass splits. Prescission times derived from the neutron clock method extend from 30×${10}^{\mathrm{-}22}$ s for ${}_{}{}^{64}{}_{}{}^{}\mathrm{Ni}$${+}^{165}$Ho to 160×${10}^{\mathrm{-}22}$ s for ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$${+}^{209}$Bi. Here, a constant level density parameter of A/10 ${\mathrm{MeV}}^{\mathrm{-}1}$ was used which is an outcome of the simultaneous measurement of α emission temperatures. The prescission times are qualitatively discussed in terms of the reaction dynamics.

• Received 7 October 1994

DOI:https://doi.org/10.1103/PhysRevC.51.1908