Decay study of hot heavy nuclei below the multifragmentation threshold with the FOBOS detector at Dubna

摘要

The first series of experiments at the FOBOS detector, using beams of the U- 400M cyclotron of the Flerov Laboratory of Nuclear Reactions, was devoted to few-fragment decays of equilibrated systems. To exclude deep inelastic collisions or quasifission as sources of massive fragments, hot heavy nuclei were produced in the very asymmetric reactions (sup 7)Li(43 AMeV)+ (sup 232)Th, (sup 14)N(34 AMeV)+(sup 197)Au, and (sup 14)N(53 AMeV)+(sup 197)Au, (sup 232) Th. Two- and three-fragment events were analysed on the base of masses and velocity vectors measured independently for each fragment. Fragment mass distributions as well as total kinetic energies were studied for an excitation energy range of 100-500 MeV. For events with three fragments of comperable size neither the assumption of two sequential independent fission acts nor a multifragmentation-like scenario can reproduce the data. Agreement is achieved if these three-fragment decays are characterized by a collinear intermediate state followed by two scissions separated by no more than 200 fm/c, a very short time scale compared with usual saddle-to-scission intervals. (orig.)