Prompt Neutron Emission From Spontaneous Fission Of Cf-252 At Long Times

A search for neutron emission from spontaneous fission of $^{252}\mathrm{Cf}$ in the time range ${10}^{\ensuremath{-}15}$-${10}^{\ensuremath{-}8}$ 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 ${10}^{\ensuremath{-}13}$ s, but possibly there is a sizable fraction with a mean lifetime in the ${10}^{\ensuremath{-}14}$ s range. A competition between neutron and $\ensuremath{\gamma}$-ray emission can therefore possibly take place in the time region ${10}^{\ensuremath{-}14}$-${10}^{\ensuremath{-}13}$ s after fission, as can be expected at excitation energies near the yrast levels.RADIOACTIVITY $^{252}\mathrm{Cf}$ (sf); measured upper limit of ${I}_{n}$ ($t={10}^{\ensuremath{-}15}\ensuremath{-}{10}^{\ensuremath{-}8}$ s).