High-Precision Probe of the Fully Sequential Decay Width of the Hoyle State in C12

Abstract

The decay path of the Hoyle state in 12 C (Ex = 7.654MeV) has been studied with the 14 N(d, α2) 12 C(7.654) reaction induced at 10.5MeV. High resolution invariant mass spectroscopy techniques have allowed to unambiguously disentangle direct and sequential decays of the state passing through the ground state of 8 Be. Thanks to the almost total absence of background and the attained resolution, a fully sequential decay contribution to the width of the state has been observed. The direct decay width is negligible, with an upper limit of 0.043% (95% C.L.). The precision of this result is about a factor 5 higher than previous studies. This has significant implications on nuclear structure, as it provides constraints to 3-α cluster model calculations, where higher precision limits are needed.Exploring the structure of 12 C is extremely fascinating, since it is strongly linked to the existence of α clusters in atomic nuclei and to the interplay between nuclear structure and astrophysics. Furthermore, 12 C is one of the major constituents of living beings and ourselves. Our present knowledge traces the origin of 12 C to the so called 3α process in stellar nucleosynthesis environments. The 3α process, which occours in the Heburning stage of stellar nucleosynthesis, proceeds via the initial fusion of two α particles followed by the fusion with a third one [1, 2] and the subsequent radiative deexcitation of the so formed excited carbon-12 nucleus, 12 C * . The short lifetime of the 8 Be unbound nucleus (of the order of 10 −16 s), formed in the intermediate stage, acts as a bottle-neck for the whole process. Consequently, the observed abundance of carbon in the universe cannot be explained by considering a non-resonant two-step process. This fact led Fred Hoyle, in 1953, to the formulation of his hypothesis [3,4]: the second step of the 3α process, α+ 8 Be→ 12 C+γ, has to proceed through a resonant J π = 0 + state in 12 C, close to the α+ 8 Be emission threshold. The existence of such a state was then soon confirmed [5] at an excitation energy of 7.654MeV. This state was then named as the Hoyle state of 12 C [6]. The decay properties of this state strongly affect the creation of carbon and heavier elements in helium burning [7], as well as the evolution itself of stars [8,9]. At typical stellar temperatures of T ≈ 10 8 − 10 9 K, this reaction proceeds exclusively via sequential process consisting of the α + α s-wave fusion to the ground state * dellaquila@na.infn.it † ivlombardo@na.infn.it of 8 Be, followed by the s-wave radiative capture of a third α to the Hoyle state. However, in astrophysical scenarios that burn helium at lower temperatures, like for instance helium-accreting white dwarfs or neutron stars with small accretion rate, another decay mode of the Hoyle state completely dominates the reaction rate: the non-resonant, or direct, α decay [10][11][12], where the two αs bypass the formation of 8 Be via the 92keV resonance. Recent theoretical calculations show that, at temperatures below 0.07GK, the reaction rate...

Cluster structures in C12 from global energy density functionals

“…We note that the binding energy of the symmetry-restored oblate minimum is rather close to the experimental value: E B = −92. 16 MeV. This is consistent with the fact that the collective 0 + 1 state is expected to correspond to the band-head of an oblate-deformed rotational band.…”

“…First experimental evidence of triangular D 3h symmetry in 12 C was reported by Marín-Lambárri et al, with K = 0 and K = 3 bands reportedly merging to form a single rotational band built on triaxial ground state [15]. Exceptional results on the structure of the well known 0 + 2 (Hoyle) state have been simultaneously reported by two independent groups [16,17], indicating that this state predominantly decays by intermediate emission of an α-particle. In parallel with experimental advances, a number of theoretical methods have been employed to study the variety of shapes in 12 C, including the antisymmetrized molecular dynamics (AMD) model [6], the fermionic molecular dynamics (FMD) model [7,8], the Tohsaki-Horiuchi-Schuck-Röpke (THSR) wave function model [9,10], configuration-mixing Skyrme calculations [11], and the cranking relativistic mean-field theory [12].…”

Recent developments in radioactive charged-particle emissions and related phenomena

“…In particular, there have been tremendous efforts trying to detect the possible 12 C emission from 114 Ba [272,273] and even 112 Ba but so far there has been no success. That decay process from 114 Ba was calculated in Ref. [274] with different models.…”

“…The direct 3α decay or breakup from 12 C has been measured recently in Refs. [114,115], which provides constraints for the many theoretical models, both microscopic and empirical, that have been applied to study the clustering property of the nucleus.…”

Triple α -particle resonances in the decay of hot nuclear systems *

“…We plan to discuss this in future papers. More conventional analysis based on Dalitz plots [31][32][33][34][35][36][37][38] cannot be easily generalized when the α-multiplicity is larger than 3.…”