Generation of Fragment Angular Momentum in Fission

A recent analysis of experimental data published in Nature found that the angular momenta of nuclear fission fragments are uncorrelated.  Based on this finding, the authors concluded that the spins are therefore determined only {\em after} scission has occurred.  We show here that the nucleon-exchange mechanism, as implemented in the well-established event-by-event fission model FREYA,  while agitating collective rotational modes in which the two spins are highly correlated, nevertheless leads to fragment spins that are largely uncorrelated.  This counter example invalidates the conclusion in the Nature paper that uncorrelated spins must necessarily have been generated after scission (a potentious conclusion that would rule out all models that generate the fragment spins prior to scission).  Furthermore, they reported that the mass dependence of the average fragment spin has a sawtooth structure.  We demonstrate that such a behavior naturally emerges when shell and deformation effects are included in the moments of inertia of the fragments at scission.