Measurement of the γ-Decay Probability of the Hoyle State

The triple alpha (3α) reaction is one of the most important processes in the nucleosynthesis. In this reaction, α particle is captured by the 2α resonance of ⁸Be, and form a 3α cluster state. Most of the 3α resonance states decay to three α particles, but a tiny fraction of them decays to the ground state in ¹²C via radiative processes of γ decay or e⁺e^--pair emission. Therefore, the γ-decay probability is an important parameter that directly determines the amount of ¹²C produced in the nucleosynthesis. Many γ-decay probability measurements were performed by 1976 and radiative-decay probability γ_rad/γ = 4.16(10) × 10^-4 [1] from the Hoyle state has been widely accepted.

Recently, a striking result of the γ-decay probability of the Hoyle state was reported from a measurement of two γ rays from the cascade decay of the Hoyle state. The new value of γ_rad/γ = 6.2(6) × 10^-4 [2] is 50% higher than the recommended value in Ref.[1]. The authors of Ref.[2] claimed that the discrepancy between the new and old results should be due to the different experimental methods. In order to solve this puzzle, it is necessary to measure surviving ¹²C nuclei and γ rays at the same time.

In this study, the experiment was performed at the tandem accelerator facility of IFIN-HH in Romania. We populated Hoyle state in ¹²C by the α+¹²C scattering using a α particle beam at E_beam = 25 MeV, and emitted charged particles are detected by a DSSD and γ rays by the ROSPHERE LaBr₃ detector array [3].

In this talk, we will report the experimental details and results of the γ-decay probability measurements.