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Surprisingly, for one of the best investigated nuclear reactions a new phenomenon was discovered. In an experiment performed at the High Flux Reactor of the Institut Laue Langevin in Grenoble, France, the reaction 235U( n , f ) was studied. Fission was induced by cold polarised neutrons. Besides the two main fragments also ternary light charged particles were measured. The centres or the detector assemblies for fragments and light particles were positioned at right angles relative to each other in a plane perpendicular to the neutron beam. It is well known that the majority of ternary particles are emitted closely perpendicular to the fission axis. With the neutron spin pointing parallel or anti-parallel to the neutron beam it was observed that, upon flipping periodically the neutron spin, the distributions of angles between fragments and light particles are wobbling back and forth. The phenomenon is traced to the rotation of the scissioning nucleus while the light particles are ejected. This interpretation is corroborated by trajectory calculations for ternary α-particles being accelerated in a rotating Coulomb field provided by the two main fragments. The angle through which the fission axis and the trajectories of α-particles rotate is very small and barely exceeds 0.2°. This so far unreported feature of nuclear fission has been called the “ROT-effect”.

Inducing fission by polarized neutrons allows studying subtle effects of the dynamics of the process. In the present experiments ternary fission of 235U and 239Pu was investigated with cold neutrons in the (n,f) reaction at the Institut Laue‐Langevin, Grenoble. Asymmetries in the emission of ternary particles were discovered by making use of the neutron spin flipping. It was found that two effects are interfering. There is first an asymmetry in the total yields of ternary particles having been called the TRI‐effect. Second, it was observed that the angular distributions of ternary particles are shifted back and forth when flipping the neutron spin. This shift was named ROT effect. Guided by trajectory calculations of the three‐body decay, the signs and sizes of the ROT effect are interpreted in terms of the K‐numbers of the transition states at the saddle point of fission.

Angular distributions of light charged particles relative to the momentum of the light fission fragment were studied in ternary fission induced by cold polarized neutrons. It was discovered that the angular distribution is asymmetric with respect to a plane determined by the direction of incident neutron spin and the light fragment momentum. The violation of the mirrorlike symmetry was established at the level of ∼10−2–10−4 for three reactions studied: 233U(n,f), 235U(n,f) and 239Pu(n,f). In spite of the smallness of the effects the neutron spin flip method allowed to make reliable measurements. The current status of the experimental studies of the effect and its interpretation are presented. Perspectives for further studies of the new phenomenon are discussed.