Almost medium-free measurement of the Hoyle state direct-decay component with a TPC

Abstract

Background: The structure of the Hoyle state, a highly α-clustered state at 7.65 MeV in 12 C, has long been the subject of debate. Understanding if the system comprises of three weakly-interacting α-particles in the 0s orbital, known as an α-condensate state, is possible by studying the decay branches of the Hoyle state. Purpose: The direct decay of the Hoyle state into three α-particles, rather than through the 8 Be ground state, can be identified by studying the energy partition of the 3 α-particles arising from the decay. This paper provides details on the break-up mechanism of the Hoyle stating using a new experimental technique. Method: By using beta-delayed charged-particle spectroscopy of 12 N using the TexAT (Texas Active Target) TPC, a high-sensitivity measurement of the direct 3 α decay ratio can be performed without contributions from pileup events. Results: A Bayesian approach to understanding the contribution of the direct components via a likelihood function shows that the direct component is < 0.043% at the 95% confidence level (C.L.). This value is in agreement with several other studies and here we can demonstrate that a small non-sequential component with a decay fraction of about 10 −4 is most likely. Conclusion: The measurement of the non-sequential component of the Hoyle state decay is performed in an almost medium-free reaction for the first time. The derived upper-limit is in agreement with previous studies and demonstrates sensitivity to the absolute branching ratio. Further experimental studies would need to be combined with robust microscopic theoretical understanding of the decay dynamics to provide additional insight into the idea of the Hoyle state as an α-condensate.

Investigating the predicted breathing-mode excitation of the Hoyle state

“…Phenomenological Rmatrix models employed in astrophysics typically assume that only two-body effects are important [12,47]. Whilst the direct 3α decay branch has been shown to be small at the primary peak of the Hoyle state [5,6,7,9], at higher excitation energies, the direct branch may be non-negligible due to the enhanced penetrability through the Coulomb barrier (see Ref. [8]).…”

“…Of particular interest is 12 C, which exhibits shell-model and α-cluster structures. The Hoyle state in 12 C is the archetypal α-cluster state [3], remaining the focus of considerable study, including efforts to measure its direct [4,5,6,7,8,9], γ [10] and E0 decay branching ratios [11]. Such decay branches are astrophysically significant; the Hoyle state mediates the 3α reaction producing 12 C. The 3α reaction rate requires an accurate description of the Hoyle-state properties (e.g., resonance energy E r , partial and total widths, Γ i and Γ) and their evolution with excitation energy.…”

“…The ghost of the Hoyle state extends some considerable energy above the main peak of the Hoyle state, overlying the region in which the breathing-mode is predicted and resulting in interference effects with higher-lying monopole resonances [32]. The region above the Hoyle state was studied by Itoh et al through the 12 C(α, α ′ ) 12 C reaction and a peak-fitting analysis with Gaussian lineshapes required an additional peak at E x ≈ 9.04 (9) MeV with Γ = 1.45 (18) MeV [13]. However, the Gaussian lineshapes employed in Ref.…”

High precision proton angular distribution measurements of $^{12}$C(p,p') for determination of the $E0$ decay branching ratio of the Hoyle state

“…At this point, the "3α process" becomes significant, which allows carbon to be produced in stars that have an equilibrium concentration of 8 Be (t 1/2 = 10 −16 s) [1]. The small probability of a third α particle to fuse with the 8 Be before it decays enables the production of 12 C, (α + α → 8 Be) + α → 12 C * . The existence of the 0 + 2 state at 7.65 MeV above the ground state of 12 C is crucial for the 3α process.…”

“…The Hoyle state disintegrates back to 8 Be+α or 3α with probability > 99.94% [9,10]. Stable carbon is only produced when the Hoyle state instead electromagnetically decays directly to the ground state via an electric monopole (E0) transition, or via the 2 + 1 state by two electric quadrupole (E2) transitions.…”