  Following upon our earlier paper [1] containing some initial results, we
present detailed discussion and complete results in this paper, which provide
the first direct evidence for the validity of isospin as a nearly good quantum
number in neutron-rich systems. The evidence comes from the reproduction of the
general features of the partition-wise relative yields of neutron-rich fission
fragments produced in two heavy-ion induced fusion fission reactions, namely
$^{208}$Pb ($^{18}$O, f) and $^{238}$U ($^{18}$O, f), by using the concept of
isospin conservation. To fix the isospin values and use the isospin algebra, we
invoke what we term as Kelson's conjectures. We present a consistent scheme for
isospin assignments based on these considerations. Our calculated results
confirm that isospin behaves as an approximately good quantum number in
neutron-rich systems, in this case, the fission fragments.
