Search for mass-symmetric ternary fission in the reactions 14N(53 A MeV)+197Au and 232Th

Ternary fission of heavy hot composite systems with excitation energies of 1.5-2.5 MeV/amu has been studied in the reactions of 14N(53 A MeV) with 197Au and 232Th. The ternary yields have been explored as a function of the charge ZL of the lightest fragment: while ZL increases from 6 to 25, the cross sections decrease from 5 to 0.08 mb for N+Au and from 15 to 0.8 mb for N+Th. The velocity vector v→L of the lightest fragment has been investigated in the rest frame of the other two heavier fragments. Two different components are observed: (i) an isotropic one with values of vL corresponding to the Coulomb repulsion from the combined heavier fragments before separation and, (ii) an anisotropic contribution with the lightest fragment emitted with lower vL perpendicular to the scission axis of the two heavier fragments. The latter component is distinguished from the isotropic one by an enhanced fraction of mass-symmetric ternary events and by up to 50 MeV lower total kinetic fragment energies. These features are indicative of a collinear stretched scission configuration, where the lightest fragment is positioned between the two heavier ones