Binary fission and coplanar cluster decay of 60Zn compound nuclei at high angular momentum

Fission decays with two heavy fragments in coincidence have been measured from 60Ni compound nuclei, formed in the 36Ar + 24Mg reaction at Elab(Ar36)=195MeV (5.4 MeV/A). The experiment was performed with a unique kinematic coincidence set-up consisting of two large area position sensitive (x,y) gas detector telescopes with Bragg-ionisation chambers. Very narrow out-of-plane correlations are observed for two heavy fragments emitted in either purely binary events or in events with a missing mass consisting of 2, 3 and 4 α-particles. The broad out-of-plane distributions correspond to binary fission with the evaporation of α-particles or nucleons from excited fragments. The narrow correlations are interpreted as ternary coplanar cluster decay from compound nuclei at high angular momenta through elongated shapes with large moments of inertia, and the lighter mass remains with very low momentum in the centre of mass frame. It is shown in calculations of shapes, that the large moments of inertia of the hyper-deformed configurations expected in these nuclei, will lower the ternary fission barrier so as to make a competition with binary fission possible.