  In the exothermic process of fission decay, an atomic nucleus splits into two
or more independent fragments. Several aspects of nuclear fission are not
properly understood, in particular the formation of the neck between the
nascent fragments, and the subsequent mechanism of scission into two or more
independent fragments. Using an implementation of time-dependent density
functional theory, based on a relativistic energy density functional and
including pairing correlations, we analyze the final phase of the process of
induced fission of $^{240}$Pu, and show that the time-scale of neck formation
coincides with the assembly of two $\alpha$-like clusters (less than 1 zs =
10$^{-21}$ s). Because of its much larger binding energy, the dynamical
synthesis of 4He in the neck predominates over other light clusters, e.g.,
$^3$H and $^6$He. At the instant of scission the neck ruptures exactly between
the two $\alpha$-like clusters, which separate because of the Coulomb repulsion
and are eventually absorbed by the two emerging fragments. The newly proposed
mechanism of light charged clusters formation at scission provides a natural
explanation of ternary fission.
