The emission of ${}_{}{}^5{}_{}{}^{}\mathrm{He}$ ($T_{\frac{1}{2}}=8\mathrm{}\times {10}^{-22\mathrm{}}$ sec for decay into $\alpha +n$) in spontaneous fission of ${}_{}{}^{252}{}_{}{}^{}\mathrm{Cf}$ is established from a correlation measurement involving the direction and the kinetic energies of the neutrons and the long-range $\alpha$ particles. Approximately 11% of these $\alpha$'s are products of ${}_{}{}^5{}_{}{}^{}\mathrm{He}$ breakup. The initial energy of the fragments of the ${}_{}{}^5{}_{}{}^{}\mathrm{He}$ at scission are calculated from the properties of the ${}_{}{}^5{}_{}{}^{}\mathrm{He}$ decay. They are 31 ± 11 and 3.2 ± 0.9 MeV, respectively.

• Received 26 June 1972

DOI:https://doi.org/10.1103/PhysRevLett.29.805