Constraints On The Low-Energy E1 Cross-Section Of C-12(Alpha,Gamma-O-16 From The Beta-Delayed Alpha-Spectrum Of N-16

The shape of the low-energy part of the beta-delayed alpha-particle spectrum of N-16 is very sensitive to the alpha+C-12 reduced width of the 7.117 MeV subthreshold state of O-16. This state, in turn, dominates the low-energy p-wave capture amplitude of the astrophysically important C-12(alpha, gamma)O-16 reaction. The alpha spectrum following the decay of N-16 has been measured by producing a low-energy (NN+)-N-16-N-14 beam with the TRIUMF isotope separator TISOL, stopping the molecular ions in a foil, and counting the alpha particles and C-12 recoil nuclei in coincidence, in thin surface-barrier detectors. In addition to obtaining the alpha spectrum, this procedure determines the complete detector response including the low-energy tail. The spectrum, which contains more than 10(6) events, has been fitted by R- and K-matrix parametrizations which include the measured C-12(alpha, gamma)O-16 cross section and the measured alpha+C-12 elastic-scattering phase shifts. The model space appropriate for these parametrizations has been investigated. For S(E1)(300), the E1 part of the astrophysical S factor for the C-12(alpha, gamma)O-16 reaction at E(c.m.) = 300 keV, values of 79 +/- 21 and 82 +/- 26 keV b have been derived from the R- and K-matrix fits, respectively.