ABRUPT CHANGES OF THE CHARACTERISTICS OF THE PROTON-INDUCED FISSION OF U-238 AROUND 14-MEV EXCITATION

An experiment combining the conventional radiochemical stacked target technique and rapid measurements using a gas-jet transport system were carried out with a purpose of precise determination of charge dispers ion and distribution for the proton-induced fission of U-238 at two in cident energies of 9.4 and 12.4 MeV. We determined the charge dispersi on width as 1.01+/-0.05 charge unit at 17.9-MeV excitation and 0.70+/- 0.04 charge unit except for the symmetric mass region at 14.8-MeV exci tation. Fundamental differences were found also in the mass distributi on characteristics between 17.9-MeV and 14.8-MeV excitations. This abr upt change of the fission characteristics strongly supports the existe nce of different fission mechanisms below and above around 14-MeV exci tation.