PROMPT NEUTRON EMISSION-SPECTRA AND MULTIPLICITIES IN THE THERMAL-NEUTRON INDUCED FISSION OF U-235

The emission spectra of prompt fission neutrons from mass and kinetic energy selected fission fragments have been measured in U-235(n(th),f) . Neutron energies were determined from the measurement of the neutron time of flight using a NE213 scintillation detector. The fragment ene rgies were measured by a pair of surface barrier detectors in one set of measurements and by a back-to-back gridded ionization chamber in th e second set of measurements. The data were analysed event by event to deduce neutron energy in the rest frame of the emitting fragment for the determination of neutron emission spectra and multiplicities as a function of the fragment mass and total kinetic energy. The results ar e compared with statistical model calculations using shell and excitat ion energy dependent level density formulations to deduce the level de nsity parameters of the neutron rich fragment nuclei over a large rang e of fragment masses.