Symmetric and asymmetric ternary fission of hot nuclei.

Abstract

Emission of \ensuremath{\alpha} particles accompanying fusion-fission processes in the $^{40}\mathrm{Ar}$ ${+}^{232}$Th reaction at E${(}^{40}$Ar) = 365 MeV was studied in a wide range of in-fission-plane and out-of-plane angles. The exact determination of the emission angles of both fission fragments combined with the time-of-flight measurements allowed us to reconstruct the complete kinematics of each ternary event. The coincident energy spectra of \ensuremath{\alpha} particles were analyzed by using predictions of the energy spectra of the statistical code cascade . The analysis clearly demonstrates emission from the composite system prior to fission, emission from fully accelerated fragments after fission, and also emission during scission. The analysis is presented for both symmetric and asymmetric fission. The results have been analyzed using a time-dependent statistical decay code and confronted with dynamical calculations based on a classical one-body dissipation model. The observed near-scission emission is consistent with evaporation from a dinuclear system just before scission and evaporation from separated fragments just after scission. The analysis suggests that the time scale of fission of the hot composite systems is long (about 7\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}20}$ s) and the motion during the descent to scission almost completely damped.