The concept of barrier in nuclear fission

An internal fission barrier can exist in a heavy nucleus if its internal energy, resulting from its internal dissociation into a dinuclear system, is not great enough for inducing a rearrangement into fragment pairs. But there exists also an external fission barrier, which is defined for a fission into a given pair ``i''. The study of $^{258}$Fm (s.f.) has shown that B$_{c}^{f}$ (i), equal to B$_{c}$ (i) -- Q$_{tot}$ (i), i.e. to the difference between Coulomb barrier and fission energy of the pair ``i'', is still negative, after sphericity correction, for its most energy-rich pairs $^{128}$Sn-$^{130}$Sn and $^{126}$Sn-$^{132}$Sn;this explains the considerable fission yield of $^{258}$Fm at A $\sim $129. For the system $^{235}$U + n$_{th}$, the B$_{c}^{f}$ (i)'s are positive for all possible fragment pairs, since B$_{c}^{f}$ (i) is already positive, and equal to 2.73 Mev, for the most energy-rich pair $^{132}$Sn-$^{104}$Mo; but a sphericity correction of about 3 MeV is necessary for the presence of the tin nucleus: this suggests that the reported value of 5.80 MeV of the ``fission barrier'' of $^{235}$U + n$_{th}$ is nothing else but its smallest external fission barrier, after sphericity correction.