AN ACCURATE COMPARISON OF THE AVERAGE KINETIC ENERGY OF THE FISSION PRODUCTS RESULTING FROM THE FISSIONING OF U$sup 235$ BY THERMAL AND BY 5-Mev AVERAGE ENERGY NEUTRONS

Exact knowledge of the dependence of the kinetic energy E/sub K/ on the excitation energy E*. of the fissioning nucleus, especially the experimental measurement of the possible change of the value of E/sub k/ in the range above the fission barrier is of great importance for the study of the mechanism of the fission process. Fowler's theory (reported by R. Leachman, Phys. Dev., 101: 1005(1956)) which postulates that the kinetic energy of the fragrnents is independent from the excitation energy, i.e., from the energy of the neutrons inducing the fission process, was therefore investigated by comparing the average kinetic energies of the fis sion fragments resulting from fissions induced by fast and thermal neutrons. It was attempted to detect a change of less than 0.1% in the value of E/sub k/ at an excitation energy difference of 1 Mev. A system incorporating a grid-type ionization chamber was used for measuring the average kinetic energy, making use of collimators for separating fragments fissioning under a body angle of about 1.7 pi , together with a 128-channel analyzer for recording the amplitude spectrum. Re sults showed that within an uncertainty of about O.1% the value of E/sub k/ does not change asmore » the excitation energy E* is increased up to 5 Mev. The results seem to indicate that the excess energy of the neutron causing the fission is used up to heat the fission fragments. (TTT)« less