Constraints on the low-energyE1 cross section ofC12(α,γ)16O from the β-delayed α spectrum ofN16

The shape of the low-energy part of the β-delayed α-particle spectrum of ${}_{}{}^{16}{}_{}{}^{}\mathrm{N}$ is very sensitive to the α${+}^{12}$C reduced width of the 7.117 MeV subthreshold state of ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$. This state, in turn, dominates the low-energy p-wave capture amplitude of the astrophysically important ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$(α,γ${)}^{16}$O reaction. The α spectrum following the decay of ${}_{}{}^{16}{}_{}{}^{}\mathrm{N}$ has been measured by producing a low-energy ${}_{}{}^{16}{}_{}{}^{}\mathrm{N}_{}^{14}{}_{}{}^{}$ ${\mathrm{N}}^{+}$ beam with the TRIUMF isotope separator TISOL, stopping the molecular ions in a foil, and counting the α particles and ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ recoil nuclei in coincidence, in thin surface-barrier detectors. In addition to obtaining the α 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 ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$(α,γ${)}^{16}$O cross section and the measured α${+}^{12}$C elastic scattering phase shifts. The model space appropriate for these parametrizations has been investigated. For ${\mathit{S}}_{\mathit{E}1}$(300), the E1 part of the astrophysical S factor for the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$(α,γ${)}^{16}$O reaction at ${\mathit{E}}_{\mathrm{c}.\mathrm{m}.\mathrm{}}$=300 keV, values of 79±21 and 82±26 keV b have been derived from the R- and K-matrix fits, respectively.