Particular features of ternary fission induced by polarized neutrons in the major actinides $^{233,235}$U and $^{239,241}$Pu

International audienceTernary fission in (n,f) reactions was studied with polarized neutrons for the isotopes $^{233,235}$U and $^{239,241}$Pu. A cold longitudinally polarized neutron beam was available at the High Flux Reactor of the Institut Laue-Langevin in Grenoble, France. The beam was hitting the fissile targets mounted at the center of a reaction chamber. Detectors for fission fragments and ternary particles were installed in a plane perpendicular to the beam. In earlier work it was discovered that the angular correlations between neutron spin and the momenta of fragments and ternary particles were very different for $^{233}$U or $^{235}$U. These correlations could now be shown to be simultaneously present in all of the above major actinides though with different weights. For one of the correlations it was observed that up to scission the compound nucleus is rotating with the axis of rotation parallel to the neutron beam polarization. Entrained by the fragments also the trajectories of ternary particles are turned away albeit by a smaller angle. The difference in turning angles becomes observable upon reversing the sense of rotation by flipping neutron spin. All turning angles are smaller than 1∘. The phenomenon was called the ROT effect. As a distinct second phenomenon it was found that for fission induced by polarized neutrons an asymmetry in the emission probability of ternary particles relative to a plane formed by fragment momentum and neutron spin appears. The asymmetry is attributed to the Coriolis force present in the nucleus while it is rotating up to scission. The size of the asymmetry is typically 10$^{−3}$. This asymmetry was termed the TRI effect. The interpretation of both effects is based on the transition state model. Both effects are shown to be steered by the properties of the collective (J,K) transition states which are specific for any of the reactions studied. The study of asymmetries of ternary particle emission in fission induced by slow polarized neutrons provides a new method for the spectroscopy of transition states (J,K) near the fission barrier. Implications of collective rotation on fragment angular momenta are discussed