5He,7He,and8Li(E*=2.26 MeV)intermediate ternary particles in the spontaneous fission of252Cf

Abstract

The neutron-unstable odd-$N$ isotopes ${}^{5}\mathrm{He},$ ${}^{7}\mathrm{He},$ and ${}^{8}\mathrm{Li}$ (in its excited state of ${E}^{*}=2.26 \mathrm{MeV})$ were measured to show up as short-lived $(\ensuremath{\tau}\ensuremath{\simeq}{10}^{\ensuremath{-}21}\ensuremath{-}{10}^{\ensuremath{-}20} \mathrm{s})$ 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: $1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}21} \mathrm{s},$ and $4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}21} \mathrm{s},$ 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 ${E}^{*}=2.26 \mathrm{MeV}$ (lifetime: $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}20} \mathrm{s})$ 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 \ensuremath{\alpha} particles registered, and to 16.4(3) MeV for the true ternary \ensuremath{\alpha} 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.