On the occasion of the $75^{th}$ anniversary of the fission phenomenon, wepresent a surprisingly simple result which highlights the important role ofisospin and its conservation in neutron rich fission fragments. We haveanalysed the fission fragment mass distribution from two recent heavy-ionreactions $^{238}$U($^{18}$O,f) and $^{208}$Pb($^{18}$O,f) as well as a thermalneutron fission reaction $^{245}$Cm(n$^{th}$,f). We find that the conservationof the total isospin explains the overall trend in the observed relative yieldsof fragment masses in each fission pair partition. The isospin values involvedare very large making the effect dramatic. The findings open the way for moreprecise calculations of fission fragment distributions in heavy nuclei and mayhave far reaching consequences for the drip line nuclei, HI fusion reactions,and calculation of decay heat in the fission phenomenon.
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