Constraints on the low-energy E1 cross section of 12C(α,γ)16O from the β-delayed α spectrum of 16N

Abstract

The shape of the low-energy part of the β-delayed α-particle spectrum of 16N is very sensitive to the α+12C reduced width of the 7.117 MeV subthreshold state of 16O. This state, in turn, dominates the low-energy p-wave capture amplitude of the astrophysically important 12C(α,γ)16O reaction. The α spectrum following the decay of 16N has been measured by producing a low-energy 16N14N+ beam with the TRIUMF isotope separator TISOL, stopping the molecular ions in a foil, and counting the α particles and 12C 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 106 events, has been fitted by R- and K-matrix parametrizations which include the measured 12C(α,γ)16O cross section and the measured α+12C elastic scattering phase shifts. The model space appropriate for these parametrizations has been investigated. For SE1(300), the E1 part of the astrophysical S factor for the 12C(α,γ)16O reaction at Ec.m.=300 keV, values of 79±21 and 82±26 keV b have been derived from the R- and K-matrix fits, respectively.

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