On the spin of fission fragments, an orientation pumping mechanism

We discuss a new mechanism of high angular momentum states population in fission fragments, based on the quantum mechanical uncertainty relations between the orientation angles of the fragments and the angular-momentum content of their intrinsic states. The latter is evaluated assuming an important rigidity of the fissioning system with respect to “bending” distortions of the field and approximating the wave functions by deformed Slater determinants or BCS wavefunctions having well fixed orientations. The angular momentum is “pumped” into the fragments by the forces responsible for their mutual orientation. Such an “orientation pumping” mechanism is found to yield a predominant contribution to the angular momentum of fission fragments in a majority of spontaneous fission events. Only when at least one of the fragments has at scission a nearly spherical shape the other ways of angular momentum generation (as e.g. bending mode activation) may successfully compete with it. Angular distributions of emitted γ rays are discussed as providing a discriminating test between the bending mode approach and ours. Recent experimental data seems to be in favour of our spin generation scheme.