Prescission neutrons in the fission of U-235 and Cf-252 nuclei

Experimental data obtained previously for the energy-angular distribution o f neutrons originating from the fission of Cf-252 (spontaneous fission) and U-235 (thermal-neutron-induced fission) nuclei are analyzed, the angle bei ng measured with respect to the direction of fission-fragment motion. A reg ularity common to all independent experiments is revealed: at an angle of a bout 90 degrees, there exists an excess of neutrons (30% for Cf-252 and 60% for U-235) that does not admit explanation within the model of neutron emi ssion from fully accelerated fragments. Two possible explanations of this e xperimental fact-neutron emission during the acceleration process and the e xistence of an additional source of neutrons (predominantly, prescission ne utrons)-are considered. It is shown that the latter conjecture describes th e observed features for both nuclei more adequately. The total yield of pre scission neutrons and their energy and angular distributions are determined . (C) 2001 MAIK "Nauka/Interperiodica".