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$\text{/}$amu has been studied in the reactions of ¹⁴N(53 $\text{A}$ MeV) with ¹⁹⁷Au and ²³²Th. The ternary yields have been explored as a function of the charge $\text{Z}{}_{\mathrm{<mtext>L</mtext>}}$ of the lightest fragment: while $\text{Z}{}_{\mathrm{<mtext>L</mtext>}}$ increases from 6 to 25, the cross sections decrease from 5 to 0.08 mb for N$\text{+}$Au and from 15 to 0.8 mb for N$\text{+}$Th. The velocity vector $\text{v}\text{→}{}_{\mathrm{<mtext>L</mtext>}}$ 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 $\text{v}{}_{\mathrm{<mtext>L</mtext>}}$ corresponding to the Coulomb repulsion from the combined heavier fragments before separation and, (ii) an anisotropic contribution with the lightest fragment emitted with lower $\text{v}{}_{\mathrm{<mtext>L</mtext>}}$ 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.