Constraints on the low-energy [ital E]1 cross section of [sup 12]C([alpha],[gamma])[sup 16]O from the [beta]-delayed [alpha] spectrum of [sup 16]N

The shape of the low-energy part of the [beta]-delayed [alpha]-particle spectrum of [sup 16]N is very sensitive to the [alpha]+[sup 12]C reduced width of the 7.117 MeV subthreshold state of [sup 16]O. This state, in turn, dominates the low-energy [ital p]-wave capture amplitude of the astrophysically important [sup 12]C([alpha],[gamma])[sup 16]O reaction. The [alpha] spectrum following the decay of [sup 16]N has been measured by producing a low-energy [sup 16]N[sup 14]N[sup +] beam with the TRIUMF isotope separator TISOL, stopping the molecular ions in a foil, and counting the [alpha] particles and [sup 12]C 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[sup 6] events, has been fitted by [ital R]- and [ital K]-matrix parametrizations which include the measured [sup 12]C([alpha],[gamma])[sup 16]O cross section and the measured [alpha]+[sup 12]C elastic scattering phase shifts. The model space appropriate for these parametrizations has been investigated. For [ital S][sub [ital E]1](300), the [ital E]1 part of the astrophysical [ital S] factor for the [sup 12]C([alpha],[gamma])[sup 16]O reaction at [ital E][sub c.m.]=300 keV, values of 79[plus minus]21 and 82[plus minus]26 keVmore » b have been derived from the [ital R]- and [ital K]-matrix fits, respectively.« less