Constraints on the low-energy E1 cross section of 12C( alpha, gamma )16O from the beta -delayed alpha spectrum of 16N

First Accurate Normalization of the β-delayed α Decay of ^{16}N and Implications for the ^{12}C(α,γ)^{16}O Astrophysical Reaction Rate

The ^{12}C(α,γ)^{16}O reaction plays a central role in astrophysics, but its cross section at energies relevant for astrophysical applications is only poorly constrained by laboratory data. The reduced α width, γ_{11}, of the bound 1^{-} level in ^{16}O is particularly important to determine the cross section. The magnitude of γ_{11} is determined via sub-Coulomb α-transfer reactions or the β-delayed α decay of ^{16}N, but the latter approach is presently hampered by the lack of sufficiently precise data on the β-decay branching ratios. Here we report improved branching ratios for the bound 1^{-} level [b_{β,11}=(5.02±0.10)×10^{-2}] and for β-delayed α emission [b_{βα}=(1.59±0.06)×10^{-5}]. Our value for b_{βα} is 33% larger than previously held, leading to a substantial increase in γ_{11}. Our revised value for γ_{11} is in good agreement with the value obtained in α-transfer studies and the weighted average of the two gives a robust and precise determination of γ_{11}, which provides significantly improved constraints on the ^{12}C(α,γ) cross section in the energy range relevant to hydrostatic He burning.

E2 interference effects in the 12C(α,γ0)16O reaction

The E1-E2 interference sign between the E(c.m.)=2.68-MeV E2 resonance and an underlying E1 strength has been measured for the first time. An E1-E2 asymmetry parameter of a=0.07±0.05 was extracted from the thick-target γ-ray yields of the narrow resonance at angles of 45° and 135°. The positive sign of a corresponded to constructive interference at forward angles and, further, allowed the interference between the resonance and an E2 background to be identified as constructive below the resonance energy. The E2-E2 interference was then used to evaluate the global S(E2) data within the vicinity of the resonance 2.5≤E(c.m.)≤3.0  MeV. An analysis of the global S(E2) data that agreed with the interference scenario has determined the E2-E2 interference scheme of the 4.34-MeV resonance and background, resulting in a value of S(E2)(300)=62(-6)(+9)  keV b.

New determination of the astrophysical S factor SE1 of the 12C(alpha,gamma)16O reaction

A new measurement of the beta-delayed alpha decay of 16N has been performed using a set of high efficiency ionization chambers. Sources were made by implantation of a 16N beam, yielding very clean alpha spectra down to energies as low as 400 keV. Our data are in good agreement with earlier results. For the S factor S(E1), we obtain a value of 74 +/- 21 keV b. In spite of improvements in the measurement, the error in S(E1) remains relatively large because of the correlations among the fit parameters and the uncertainties inherent to the extrapolation.

Elastic alpha-12C scattering and the 12C(alpha,gamma)16O E2 S factor

Angular distributions of 12C(alpha,alpha)12C have been measured for E(alpha) = 2.6-8.2 MeV, at angles from 24 to 166, yielding 12 864 data points. R-matrix analysis of the ratios of elastic scattering yields a reduced width amplitude of gamma12 = 0.47 +/- 0.06 MeV(1/2) for the Ex = 6.917 MeV (2+) state in 16O(a = 5.5 fm). The dependence of the chi2 surface on the interaction radius a has been investigated and a deep minimum is found at a = 5.42(+0.16)(-0.27) fm. Using this value of gamma12, radiative alpha capture and 16N beta-delayed alpha-decay data, the S factor is calculated at E(c.m.) = 300 keV to be S(E2)(300) = 53(+13)(-18) keV b for destructive interference between the subthreshold resonance tail and the ground state E2 direct capture.

12C(alpha,gamma)16O: the key reaction in stellar nucleosynthesis

The angular distributions of gamma rays from the 12C(alpha,gamma)16O reaction have been measured at 20 energy points in the energy range E(cm) = 0.95 to 2.8 MeV. The sensitivity of the present experiment compared to previous direct investigations was raised by 1-2 orders of magnitude, by using an array of highly efficient ( 100%) Ge detectors shielded actively with BGOs, as well as high beam currents of up to 500 microA that were provided by the Stuttgart Dynamitron accelerator. The S(E1) and S(E2) factors deduced from the gamma angular distributions have been extrapolated to the range of helium burning temperatures applying the R-matrix method, which yielded S(300)(E1) = (76+/-20) keV b and S(300)(E2) = (85+/-30) keV b.