Neutron emission before full acceleration and the spectra of prompt neutron fission

The hypothesis that some or all fission fragments emit neutrons during acceleration is verified. It is shown that the kinetic energy of the fragment at the neutron-emission time (KEFNE) can be determined from the analysis of neutron spectra in the laboratory system if their parameters in the c.m.s. are known. For 252Cf, these data were taken from the experimental studies; for other fissile nuclei, they can be calculated using the 252Cf data and the Fermi gas model. For 252Cf, the ratio of the KEFNE to the total kinetic energy equals 0.86. Although this value is close to unity, the time interval between the scission and the neutron emission decreases by a factor of over 10. 18 refs., 7 figs