Number of neutrons emitted by individual fission fragments of U235

A large detector filled with a liquid organic scintillator containing cadmium has been used to measure the number of neutrons emitted by individual fragments in U235 fission by thermal neutrons. Using 4 π geometry the dependence of the number of neutrons emitted by fragment pairs on the mass ratio has been measured. The excitation energy used in neutron evaporation is determined on the basis of the semiempirical Weizsacker formula. A sharp asymmetry in the distribution of excitation energy between the heavy and light fragments is noted. The data which are obtained are found to be in disagreement with the statistical theory of fission proposed by Fong.