Prompt neutron emission from spontaneous fission ofCf252at long times

A search for neutron emission from spontaneous fission of ${}_{}{}^{252}{}_{}{}^{}\mathrm{Cf}$ in the time range ${10}^{-15\mathrm{}}$-${10}^{-8\mathrm{}}$ s has been made by a method based upon the solid angle aberration. No positive evidence for neutron emission in this time range has been found, but an upper limit for the relative intensity of a single long-lived component as a function of its mean lifetime has been set. It is concluded that very few if any neutrons are emitted with mean lifetimes longer than ${10}^{-13\mathrm{}}$ s, but possibly there is a sizable fraction with a mean lifetime in the ${10}^{-14\mathrm{}}$ s range. A competition between neutron and $\gamma$-ray emission can therefore possibly take place in the time region ${10}^{-14\mathrm{}}$-${10}^{-13\mathrm{}}$ s after fission, as can be expected at excitation energies near the yrast levels.