Decay Study of Hot Nuclei below the Multifragmentation Threshold with the FOBOS Detector at Dubna

Abstract
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 ⁷Li (43 AMeV) + ²³²Th, ¹⁴N (34 AMeV) + ¹⁹⁷Au, and ¹⁴N (53 AMeV) + ¹⁹⁷Au, ²³²Th. Two- and three-fragment events were analysed on the base of masses and velocity vectors measured independently for each fragment. The events were sorted into excitation energy bins according to the linear momentum transfer following the massive transfer approach. Binary events were treated as fission. Fragment mass distributions as well as total kinetic energies were studied for an excitation energy range of 100 - 500 MeV. A new TKE parametrisation is proposed extending the Viola systematics to large mass asymmetries. With rising excitation energy the mass dispersion develops unexpectedly. Two new effects have been fourid and are discussed as consquences of the cooling down during the slow fission process at moderate E*, and of a strong decrease of the fission time at large E*. Ternary events were analysed by comparing measured velocity correlations with results of Coulomb trajectory simulations. If one fragment has intermediate mass (A = 10...30), two components in the relative velocities and the Z/A ratios confirm a sequential and a neck mechanism. For events with three fragments of comperable size neither the assumption of two sequential independent fission acts nor a nlultifragnlentaion-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 280 fm/c, a very short time scale compared with usual saddle-to-scission intervals.