Further Improvement of the Upper Limit on the Direct 3α Decay from the Hoyle State in 12C

The direct 3 alpha decay branch from the 0(2)(+) state at E-x = 7.65 MeV in C-12, which is known as the Hoyle state, is considered to affect the triple-a reaction rate strongly and to give crucial information on its structure. We have performed a high-precision measurement of the 3a decay from this state using the C-12(C-12, 3 alpha) C-12 reaction at E-12C = 110 MeV. The branching ratio of the direct 3 alpha decay was under the detection limit in the present experiment. By comparing with Monte Carlo simulations for three decay mechanisms as the sequential decay through the ground state of Be-8, the direct decay with equal energies of three alpha particles, and the direct decay to the phase space uniformly, we have obtained the upper limit of 0.2% on the direct 3 alpha decay.