${}^5\mathrm{He},$ ${}^7\mathrm{He},$ and ${}^8\mathrm{Li}$ ${(E}^{*}=2.26\mathrm{}\hspace{0.5em}\mathrm{MeV})$ intermediate ternary particles in the spontaneous fission of ${}^{252}\mathrm{Cf}$

The neutron-unstable odd-$N$ isotopes ${}^5\mathrm{He},$ ${}^7\mathrm{He},$ and ${}^8\mathrm{Li}$ (in its excited state of $E^{*}=2.26\mathrm{}\hspace{0.5em}\mathrm{MeV})$ were measured to show up as short-lived $\left(\tau \simeq {10}^{-21}-{10}^{-20}\hspace{0.5em}\mathrm{s}\right)$ intermediate light charged particles (LCPs) in ternary fission of ${}^{252}\mathrm{Cf}.$ For the study a high-efficiency angular correlation measurement between neutrons, LCPs, and main fission fragments has been performed. The evidence for the ternary ${}^5\mathrm{He}$ and ${}^7\mathrm{He}$ particles (lifetimes: $1\times {10}^{-21}\hspace{0.5em}\mathrm{s},$ and $4\times {10}^{-21}\hspace{0.5em}\mathrm{s},$ respectively) was disclosed from the measured angular distributions of their decay neutrons focused by the emission in flight towards the direction of motion of ${}^4\mathrm{He}$ and ${}^6\mathrm{He}$ ternary particles. Similarly, neutrons observed to be peaked around Li-particle motion could be attributed to the decay of the second excited state at $E^{*}=2.26\mathrm{}\hspace{0.5em}\mathrm{MeV}$ (lifetime: $2\times {10}^{-20}\hspace{0.5em}\mathrm{s})$ of ${}^8\mathrm{Li}.$ The fractional yields of the intermediate ${}^5\mathrm{He}$ and ${}^7\mathrm{He}$ ternary fission modes relative to the “true” ternary ${}^4\mathrm{He}$ and ${}^6\mathrm{He}$ modes, respectively, were determined to be 0.21(5) for both cases. The mean energy of the ${}^4\mathrm{He}$ residues resulting from the ${}^5\mathrm{He}$ decay was determined to be 12.4(3) MeV, compared to 15.7(2) MeV for all ternary α particles registered, and to 16.4(3) MeV for the true ternary α particles. The mean energy of the ${}^6\mathrm{He}$ residues from the ${}^7\mathrm{He}$ decay is 11.0(15) MeV, compared to 12.3(5) MeV for all ternary ${}^6\mathrm{He}$ particles. The population of ${}^8{\mathrm{Li}}^{*}$ was deduced to be 0.06(2) relative to Li ternary fission, and 0.33(20) relative to the yield of particle stable ${}^8\mathrm{Li}.$ The perspective of using the observed intermediate LCPs for probing the ternary scission configuration in ${}^{252}\mathrm{Cf}$ fission with the aid of trajectory calculations is briefly discussed.