Study of alpha-particle decay from certain exited levels in C^(12), O^(16), and Ne^(20) following population of the levels by beta-decay

Abstract

Beta-decay to, followed by alpha-particle deexcitation of, the following levels has been investigated: the 7.656-Mev and 10.1-Mev levels in c^(12), the 8.88-Mev, 9.59-Mev, and 9.85-Mev levels in O^(16), and the 5.63-Mev and 5.80-Mev levels in Ne^(20). Groups of delayed alpha particles were observed only from the first, second and fourth of these levels. The beta-unstable parent nuclei were formed through (d,p) reactions on B^(11), N^(15), and F^(19), after which the delayed alpha particles were observed for a period of time comparable to the half-life of the beta-decay process. The beta-decay branching ratio to the 9.59-Mev level in O^(16) was measured and is equal to (1.4 ± 0.2) • 10^(-5) corresponding to a log ft (9.59) = 6.5 ± 0.2. The beta-decay branching ratio to the 9.85-Mev level was found to be less than 2.7 • 10^(-7) which corresponds to a log ft (9. 85) ≥ 7.2 ± 0.2. The ratio of the decay probability for alpha-particle emission to the total decay probability for the 8.88-Mev level was found to be ^(Г_α)/_(Г_Τ) ≤ 6.6 • 10^(-5). This results in a lower limit for the amplitude of the parity nonconserving term in the nuclear potential of F ≤ 1. 5 • 10^(-5) where the following quantities were used: θ^2_(α2) = 0.007 and Г_y = 10^(-3) ev. Alpha-particle emission was not observed from the 5.63-Mev level or the 5.80-Mev level in Ne^(20). Limits on the log ft value to these levels were obtained: log ft (5.63) ≥ 8.0 ± 0.5; log ft (5.80) ≥ 8.0 ± 0.3. These large values for the log it are consistent with the assigned spin and parity of the levels involved in the beta-decay process.