Pre- and postsaddle fission dynamics using lead on proton reactions in complete kinematic measurements

Abstract

Fission is a clear example of the evolution from a metastable to a quasi-equilibrium regime. The dynamics of this process can be described in terms of the potential-energy surface, friction, diffusion and inertia. The investigation of different experimental observables has shown evidences that the nuclear friction parameter or viscosity of the medium changes with the deformation, but also with the nuclear temperature. However, these ideas are still under debate because these conclusions could be biased by the experimental conditions. Therefore, we propose to investigate these effects with complete kinematic measurements of the fission products at high excitation energy, low angular momentum and small compound nucleus deformation, where dissipative effects should manifest in a clear way. In this work, we will report recent results obtained at GSI (Darmstadt) for the reactions 208Pb(370A, 500A, and 650A MeV) + p and 208Pb(500A MeV) + 27Al. These reactions fulfill the optimum conditions for the investigation of dissipative effects in fission. Moreover, the use of the SOFIA (Studies On FIssion with Aladin) setup together with the inverse kinematics technique allowed us to construct new observables that provide information on the fissioning compound nucleus, and its saddle and scission configurations. In particular, the unambiguous identification in mass and atomic number of both fission fragments, obtained for the first time in this experiment, was a key achievement. Total and partial fission cross sections, light-charged particle multiplicities and charge distributions of the fission fragments are used to characterize the fission dynamics at small deformation. We also present the results concerning to the neutron excess and isotopic widths of the fission fragments, their kinematic properties and the average pre- and postscission neutron multiplicities, which should help us to investigate the postsaddle dynamics.