COMPARISON OF THE AVERAGE KINETIC ENERGIES OF THE FRAGMENTS FROM THE FISSION OF URANIUM-235 DUE TO THERMAL NEUTRONS AND TO 15-Mev AVERAGE ENERGY NEUTRONS

It is at present assumed that the average kinetic energy Ekof the fission fragments is determined primarily by the Coulomb repulsion forces at the time of the fission, being also weakly influenced by the excitation energy E/sub x/ of the fissioning nucleus. It was shown earlier (Atomnaya Energiya, 12: 461 (1962) No. 6) that the E/sub k/ remains the same in the case of fission of U/sup 235/ by thermal and 5-Mev neutrons. This work was extended to neutrons with higher energy; an ionization chamber containing A + 3% CO/sub 2/ and a 128channel analyzer were used for the measurements. The fast neutrons were obtained by bombarding a heavy tritium target with 2.2-Mev deuterons. It was found that the E/sub k/ of the fragments resulting from fission due to the 15-Mev neutrons was by 1.5 plus or minus 0.3 Mev lower than that of the fragments originating from thermal neutron induced fission, thus disagreeing with the data reported by P. Stevenson et al. (Phys. Rev., 117: 186 (1960)). These results are difficult to interpret because in case of bombardment with 15-Mev neutrons (n,n'f) and (n,2n'f) reactions take place together with fission; they might be due to the thermal expansion ofmore » the nucleus and the increase of the portion of symmetric fission, during which a substantial reduction of the kinetic energy is noted. (TTT)« less