TIME-OF-FLIGHT FISSION STUDIES ON U233, U235 AND Pu239

Abstract

The prompt mass and kinetic energy distributions resulting from the thermal neutron fission of U²³³, U²³⁵, and Pu²³⁹ have been reinvestigated using time-of-flight methods to measure simultaneously the velocities of the fragment pairs. A new feature shown by the present work is the existence of fine structure in the prompt mass yields. This fine structure is most pronounced at high total kinetic energies where the fragments have little excitation energy and may be associated with irregularities in the energy release as a function of mass. The fine structure is most noticeable in U²³⁵ and least in Pu²³⁹; the fragments of U²³⁵ have the lowest average excitation and those of Pu²³⁹, the highest. Another feature, which is confirmed by this work, is the large drop in total kinetic energy when the fragments are near symmetry. This decrease is about 35 Mev and is consistent either with a picture in which the nucleus with 50 protons is especially preferred or with one in which fragments at symmetric fission have an abnormally high excitation energy and a consequent large number of neutrons. The mean kinetic energies for thermal neutron fission of U²³³, U²³⁵, and Pu²³⁹ were found to be 167.6, 168.3, and 175.0 Mev with an error of ± 1.7 Mev.