Experimental study of fast fission and quasifission in the 40Ca + 208Pb reaction leading to the formation of the transfermium nucleus 248No

Background: The stability of the transfermium nucleus against fission is mainly determined by the shell correction depending on its angular momentum and excitation energy. Purpose: The study of the fast fission process of the transfermium nucleus No-248 and its dependence on the interaction energy and introduced angular momentum. Methods: Mass-energy distributions of the No-248 fission fragments formed in the Ca-40 + Pb-208 reaction at energies above the Coulomb barrier have been measured using the double-arm time-of-flight spectrometer CORSET at the Ca-40-beam energies of 223, 250, and 284 MeV. Results: The contribution of the fast fission process is determined from the calculations of the driving potential, taking into account shell effects and rotational energy and amounts to 39% and 61% at 250 and 284 MeV, respectively. The mass-energy distributions of the quasifission and fast fission fragments have been extracted by subtracting the mass-energy matrices associated with compound nucleus fission from those of all measured fissionlike events. The asymmetric fragments with masses 97 and 151 u were found to be the most probable in the fast fission of No-248. With increasing Ca-40 energy from 250 to 284 MeV the mass distributions of the fast fission fragments change slightly. Conclusions: Contrary to quasifission in which the fragments are focused mainly around the closed neutron or proton shells, the influence of known proton or neutron shells on the asymmetric mass distribution in the fast fission process was not observed.