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

The direct 3α decay branch from the 0+2 state at Ex = 7.65 MeV in 12C, 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 3α decay from this state using the 12C(12C, 3α)12C reaction at E12C = 110 MeV. The branching ratio of the direct 3α 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 8Be, the direct decay with equal energies of three a particles, and the direct decay to the phase space uniformly, we have obtained the upper limit of 0.2% on the direct 3α decay. [ABSTRACT FROM AUTHOR]