Experimental Determination of α Widths of

The efficiency of the weak s process in low-metallicity rotating massive stars depends strongly on the rates of the competing ^{17}O(α,n)^{20}Ne and ^{17}O(α,γ)^{21}Ne reactions that determine the potency of the ^{16}O neutron poison. Their reaction rates are poorly known in the astrophysical energy range of interest for core helium burning in massive stars because of the lack of spectroscopic information (partial widths, spin parities) for the relevant states in the compound nucleus ^{21}Ne. In this Letter, we report on the first experimental determination of the α-particle spectroscopic factors and partial widths of these states using the ^{17}O(^{7}Li,t)^{21}Ne α-transfer reaction. With these the ^{17}O(α,n)^{20}Ne and ^{17}O(α,γ)^{21}Ne reaction rates were evaluated with uncertainties reduced by a factor more than 3 with respect to previous evaluations and the present ^{17}O(α,n)^{20}Ne reaction rate is more than 20 times larger. The present (α,n)/(α,γ) rate ratio favors neutron recycling and suggests an enhancement of the weak s process in the Zr-Nd region by more than 1.5 dex in metal-poor rotating massive stars.

An increase in the 12C + 12C fusion rate from resonances at astrophysical energies.

Carbon burning powers scenarios that influence the fate of stars, such as the late evolutionary stages of massive stars 1 (exceeding eight solar masses) and superbursts from accreting neutron stars2,3. It proceeds through the 12C + 12C fusion reactions that produce an alpha particle and neon-20 or a proton and sodium-23-that is, 12C(12C, α)20Ne and 12C(12C, p)23Na-at temperatures greater than 0.4 × 109 kelvin, corresponding to astrophysical energies exceeding a megaelectronvolt, at which such nuclear reactions are more likely to occur in stars. The cross-sections 4 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 4 below 2 megaelectronvolts because of exponential suppression arising from the Coulomb barrier. The reference rate 5 at temperatures below 1.2 × 109 kelvin relies on extrapolations that ignore the effects of possible low-lying resonances. Here we report the measurement of the 12C(12C, α0,1)20Ne and 12C(12C, p0,1)23Na reaction rates (where the subscripts 0 and 1 stand for the ground and first excited states of 20Ne and 23Na, respectively) at centre-of-mass energies from 2.7 to 0.8 megaelectronvolts using the Trojan Horse method6,7 and the deuteron in 14N. 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 × 108 kelvin, the reaction rate is boosted to more than 25 times larger than the reference value 5 . This finding may have implications such as lowering the temperatures and densities 8 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 3 .

Publisher Correction: An increase in the 12C + 12C fusion rate from resonances at astrophysical energies.

In equation (1) of this Letter, the closing bracket was missing; in Extended Data Fig. 1 and the accompanying legend, 'Φ(pd)' should have been 'Φ2(pd)', and in the Methods the text "Odd J assignments are uncertain by ±1." has been added. These errors have all been corrected online.

Advancement of Photospheric Radius Expansion and Clocked Type-I X-Ray Burst Models with the New

We report the first (in)elastic scattering measurement of ^{25}Al+p with the capability to select and measure in a broad energy range the proton resonances in ^{26}Si contributing to the ^{22}Mg(α,p) reaction at type I x-ray burst energies. We measured spin-parities of four resonances above the α threshold of ^{26}Si that are found to strongly impact the ^{22}Mg(α,p) rate. The new rate advances a state-of-the-art model to remarkably reproduce light curves of the GS 1826-24 clocked burster with mean deviation <9% and permits us to discover a strong correlation between the He abundance in the accreting envelope of the photospheric radius expansion burster and the dominance of ^{22}Mg(α,p) branch.

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

The decay path of the Hoyle state in ^{12}C (E_{x}=7.654 MeV) has been studied with the ^{14}N(d,α_{2})^{12}C(7.654) reaction induced at 10.5 MeV. High resolution invariant mass spectroscopy techniques have allowed us 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 3α cluster model calculations, where higher precision limits are needed.

Measurement of the 20 and 90 keV resonances in the 18O(p,alpha)15N reaction via the Trojan horse method.

The 18O(p,alpha)15N reaction is of primary importance in several astrophysical scenarios, including fluorine nucleosynthesis inside asymptotic giant branch stars as well as oxygen and nitrogen isotopic ratios in meteorite grains. Thus the indirect measurement of the low energy region of the 18O(p,alpha)15N reaction has been performed to reduce the nuclear uncertainty on theoretical predictions. In particular the strength of the 20 and 90 keV resonances has been deduced and the change in the reaction rate evaluated.

Suppression of the Coulomb interaction in the off-energy-shell p - p scattering from the p + d --> p + p + n reaction.

Off-energy-shell effects in p - p scattering have been investigated at p - p relative energies from 600 down to 80 keV applying the Trojan horse method (THM) to the p + d --> p + p + n reaction at 5 MeV. In contrast with the on-energy-shell case, no Coulomb-nuclear interference minimum has been found in the extracted THM p - p cross section, due to the suppression of the Coulomb amplitude as predicted by the half-off-energy shell calculations. This hypothesis is strengthened by the agreement between THM p - p data and calculated on-energy-shell n + n, n + p and nuclear p + p cross sections.