Rotation of the compound nucleus 236U* in the fission reaction 235U(n, f) induced by cold polarised neutrons

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 Lane Langevin in Grenoble, France, the reaction U-235(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 a-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 alpha-particles rotate is very small and barely exceeds 0.2 degrees. This so far unreported feature of nuclear fission has been called the ROT-effect. (c) 2007 Elsevier B.V. All rights reserved.