Radiative decay branching ratio of Hoyle state in ¹²C

The triple-alpha process is one of the most important reactions in nuclear astrophysics. It is a sequence of two reactions a) α+α →⁸Be(g.s.) and b) ⁸Be+α ⇔ γ+¹²C leading to the production of carbon. The second reaction proceeds through a special excited 0⁺ state at 7.65 MeV excitation energy in ¹²C, the so-called Hoyle state. The reaction rate of the triple-alpha process is proportional to Γ_αΓ_rad/(Γ_α+Γ_rad) ≈ Γ_rad since Γ_α≫Γ_rad. One way to establish the Γ_rad is to measure the branching ratio for electromagnetic decay and utilize the known partial width Γ_π (E0) for the electron-positron pair production. Recent measurement of branching ratio for electromagnetic decay is more than 3σ away from the adopted value. Our work is to verify the result by measuring the total radiative decay branching ratio of Hoyle state in ¹²C.

The Holye state in ¹²C was populated through ¹²C (α, α’) ¹²C’ inelastic scattering. The scattered α-particles were detected by a ΔE-E telescope, and the heavy ¹²C recoils were detected by the newly developed MDM-TexPPAC system. TOF technology and track reconstruction are used to identify the decay products of ¹²C (7.65). The data analysis is ongoing and the result is intended to be finalized by summer 2022.