Carbon burning powers scenarios that influence the fate of stars, such as the late evolutionary stages of massive stars (exceeding eight solar masses) and superbursts from accreting neutron stars. It proceeds through the C +C fusion reactions that produce an alpha particle and neon-20 or a proton and sodium-23-that is, C(C, α)Ne and C(C, p)Na-at temperatures greater than 0.4 × 10 kelvin, corresponding to astrophysical energies exceeding a megaelectronvolt, at which such nuclear reactions are more likely to occur in stars. The cross-sections for those carbon fusion reactions (probabilities that are required to calculate the rate of the reactions) have hitherto not been measured at the Gamow peaks below 2 megaelectronvolts because of exponential suppression arising from the Coulomb barrier. The reference rate at temperatures below 1.2 × 10 kelvin relies on extrapolations that ignore the effects of possible low-lying resonances. Here we report the measurement of the C(C, α)Ne and C(C, p)Na reaction rates (where the subscripts 0 and 1 stand for the ground and first excited states of Ne andNa, respectively) at centre-of-mass energies from 2.7 to 0.8 megaelectronvolts using the Trojan Horse method and the deuteron in N. The cross-sections deduced exhibit several resonances that are responsible for very large increases of the reaction rate at relevant temperatures. In particular, around 5 × 10 kelvin, the reaction rate is boosted to more than 25 times larger than the reference value . This finding may have implications such as lowering the temperatures and densities required for the ignition of carbon burning in massive stars and decreasing the superburst ignition depth in accreting neutron stars to reconcile observations with theoretical models .

The 13 C(α, n) 16 O reaction is the neutron source for the main component of the s-process, responsible for the production of most of the nuclei in the mass range 90 A 208. This reaction takes place inside the heliumburning shell of asymptotic giant branch stars, at temperatures 10 8 K, corresponding to an energy interval where the 13 C(α, n) 16 O reaction is effective in the range of 140-230 keV. In this regime, the astrophysical S(E)-factor is dominated by the −3 keV sub-threshold resonance due to the 6.356 MeV level in 17 O, giving rise to a steep increase in the S-factor. Its contribution is still controversial as extrapolations, e.g., through the R-matrix and indirect techniques such as the asymptotic normalization coefficient (ANC), yield inconsistent results. The discrepancy amounts to a factor of three or more precisely at astrophysical energies. To provide a more accurate S-factor at these energies, we have applied the Trojan horse method (THM) to the 13 C( 6 Li, n 16 O)d quasi-free reaction. The ANC for the 6.356 MeV level has been deduced through the THM as well as the n-partial width, allowing us to attain unprecedented accuracy for the 13 C(α, n) 16 O astrophysical factor. A larger ANC for the 6.356 MeV level is measured with respect to the ones in the literature, (C 17 O(1/2 + ) α 13 C ) 2 = 7.7 ± 0.3 stat +1.6 −1.5 norm fm −1 , yet in agreement with the preliminary result given in our preceding letter, indicating an increase of the 13 C(α, n) 16 O reaction rate below about 8 × 10 7 K if compared with the recommended values. At ∼10 8 K, our reaction rate agrees with most of the results in the literature and the accuracy is greatly enhanced thanks to this innovative approach.

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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...

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A new measurement of the 11B(p,α0)8Be has been performed applying the Trojan horse method (THM) to the 2H(11B,α80Be)n quasi-free reaction induced at a laboratory energy of 27 MeV. The astrophysical S(E) factor has been extracted from ∼600 keV down to zero energy by means of an improved data analysis technique and it has been compared with direct data available in the literature. The range investigated here overlaps with the energy region of the light element LiBeB stellar burning and with that of future aneutronic fusion power plants using the 11B+p fuel cycle. The new investigation described here confirms the preliminary results obtained in the recent TH works. The origin of the discrepancy between the direct estimate of the 11B(p,α0)8Be S(E)-factor at zero energy and that from a previous THM investigation is quantitatively corroborated. The results obtained here support, within the experimental uncertainties, the low-energy S(E)-factor extrapolation and the value of the electron screening potential deduced from direct measurements.