Binary fragmentations of excited nuclear systems in the 372 MeV Fe-56+Th-232 reaction

The binary fragmentation of the excited superheavy system of Z=116 formed in the reaction 372 MeV Fe-56+Th-232 has been investigated. The fragment masses and kinetic energies were determined through measurement of the fragment velocities by the time-of-flight method. Neutron spectra were measured in coincidence with the fragments at several angles with respect to the fragment direction, which were analyzed to deduce the total as well as the pre-scission neutron multiplicities. We analyzed the correlations between fragment mass and kinetic energy, gated and nongated by the coincidence with neutrons, to learn about the dynamics of the reaction with respect to the two-body exit channels. The events in the near-mass-symmetric valley region appear to receive a significant contribution from the asymmetric mode of fission of the superheavy compound nucleus Z=116, which can be due to the influence of the closed proton and neutron shells of Z=50 and N=82 in the light fragments. From the observed number of pre-scission neutrons, it is inferred that the time scales of the fissionlike reactions leading to near-mass-symmetric splits are rather large, of the order of several times 10(-20) s. The average number of prompt neutrons emitted in the spontaneous fission of such a superheavy nucleus is nu=(12 +/- 1), as deduced from the neutron measurements.