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Abstract

The neutron-unstable odd-N isotopes He-5, He-7, and Li-8 (in its excited state of E*=2.26 MeV) were measured to show up as short-lived (tausimilar or equal to10(-21)-10(-20) s) intermediate light charged particles (LCPs) in ternary fission of Cf-252. For the study a high-efficiency angular correlation measurement between neutrons, LCPs, and main fission fragments has been performed. The evidence for the ternary He-5 and He-7 particles (lifetimes: 1x10(-21) s, and 4x10(-21) 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 He-4 and He-6 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 MeV (lifetime: 2x10(-20) s) of Li-8. The fractional yields of the intermediate He-5 and He- 7 ternary fission modes relative to the "true" ternary He-4 and He-6 modes, respectively, were determined to be 0.21(5) for both cases. The mean energy of the He-4 residues resulting from the He-5 decay was determined to be 12.4(3) MeV, compared to 15.7(2) MeV for all ternary alpha particles registered, and to 16.4(3) MeV for the true ternary alpha particles. The mean energy of the He-6 residues from the He-7 decay is 11.0(15) MeV, compared to 12.3(5) MeV for all ternary He-6 particles. The population of Li-8* was deduced to be 0.06(2) relative to Li ternary fission, and 0.33(20) relative to the yield of particle stable Li-8. The perspective of using the observed intermediate LCPs for probing the ternary scission configuration in Cf-252 fission with the aid of trajectory calculations is briefly discussed.