Stringent upper limit on the direct 3α decay of the Hoyle state in C12

Abstract

We investigate an implication of the most recent observation of a second J π = 2 + state in 12 C, which was measured using the 12 C(γ,α) 8 Be (g.s.) reaction. In addition to the dissociation of 12 C to an α-particle and 8 Be in its ground state, a small fraction of events (2%) were identified as direct decays and decays to excited states in 8 Be. This allowed a limit on the direct 3α partial decay width to be determined as Γ3α < 32(4) keV. Since this 2 + state is predicted by all theoretical models to be a collective excitation of the Hoyle state, the 3α partial width of the Hoyle state is calculable from the ratio of 3α decay penetrabilities of the Hoyle and 2 + states. This was calculated using the semi-classical WKB approach and we deduce a stringent upper limit for the direct decay branching ratio of the Hoyle state of Γ 3α Γ < 5.7 × 10 −6 , over an order of magnitude lower than previously reported. This result places the direct measurement of this rare decay mode beyond current experimental capabilities.

Wigner SU(4) symmetry, clustering, and the spectrum of $$^{12}$$C

“…[9] and recent related experimental findings are given in Refs. [18,19]. The ground state of 8 Be is a quasi-bound 2α state formed by a short-range repulsion and a medium-range attraction of the effective α-α interaction, which has been experimentally determined from the αα scattering phase shifts.…”

Expanding Nuclear Physics Horizons with the Gamma Factory

“…The study with gamma rays of high energy and therefore appropriate wavelength, a distant relative of X‐ray diffraction from crystals, can shed light on this structure that has no analogs in other nuclear phenomena. The $$(\gamma ,\alpha )$$ and possibly also $$(\gamma , 2\alpha )$$ reactions, [ 225 ] studied as a function of photon energy and polarization can provide unique information concerning specific features of alpha‐clusterization, especially in even–even $$N=Z$$ nuclei. [ 226 ]…”

Radiative Width of the Hoyle State from γ -Ray Spectroscopy

“…The entry state of the triple-alpha process, the second excited state in 12 C, is a 0 þ state at 7.65 MeV. It has attracted significant attention [1][2][3] since it was first proposed in 1953 by Hoyle [4]. The existence of the state was confirmed in the same year from the analysis of the α spectrum from the 14 Nðd; αÞ 12 C reaction [5].…”

“…Because of its unusual structure, the Hoyle state has attracted continuous attention; see the recent review of Freer and Fynbo [1] and other recent works [2,6,7]. The discussion includes nuclear clustering, a spacial arrangement of the three α particle clusters of which the state is believed to be composed, and discussion on a new form of nuclear matter, in analogy with the Bose-Einstein condensates.…”