Speaker
Description
The triple $\alpha$ reaction is one of the most important reactions for
the nucleosynthesis in the universe because it is a doorway reaction
to synthesize heavier elements. An $\alpha$ particle is
captured by $^8$Be, which is a two $\alpha$ resonant state, to form a
triple $\alpha$ resonant state. Most of such triple $\alpha$ resonant
states decay back to 3$\alpha$ particles, but a tiny fraction of those
states decay to the ground state in $^{12}$C by emitting $\gamma$
rays. The branching ratio between the $\gamma$ and $\alpha$ decays of
the triple $\alpha$ resonant states is a key parameter to decide the
triple $\alpha$ reaction rate.
The triple $\alpha$ reaction proceeds via the Hoyle state at normal
stellar temperature, but the high-energy triple $\alpha$ resonant states
such as the $3^-_1$ and $2^+_2$ states in $^{12}$C play a very
important role at higher temperature $T_9 > 1$ like supernovae, first
stars, and so on. Nevertheless, the $\gamma$-decay probability of the
$3^-_1$ state is still unknown.
Recently, we measured the inelastic proton scattering off $^{12}$C
under the inverse kinematic condition in order to determine the $\gamma$
decay width of the $3^-_1$ state in $^{12}$C. The $\gamma$-decay
probability of the $3^-_1$ state is quite as small as $10^{-7}$,
therefore we introduced a thin solid hydrogen target and the recoil
proton counter ``Gion'' to realize the low background
measurement. We successfully identified the $\gamma$-decay events by
measuring the recoil proton and $^{12}$C in coincidence instead of
detecting the $\gamma$ ray.
With the careful data analysis, we finally determined the $\gamma$
decay width of the $3^-_1$ state for the first time.
