The beta -delayed alpha spectrum of 16N and the low-energy extrapolation of the 12C( alpha , gamma )16O cross section

The low-energy part of the beta alpha -decay spectrum of 16N permits the determination of the reduced alpha -width theta alpha 2 of the 7.12 MeV subthreshold state of 16O, which dominates the low-energy p-wave capture amplitude of the astrophysically important reaction 12C( alpha , gamma )16O. In a first run, a total of 7.1*103 alpha particles from the decay of 16N nuclei, selected by the TRIUMF isotope separator. TISOL, have been observed in a 10.6 mu m detector in coincidence with the recoiling 12C nuclei in a second detector, thus removing most sources of low-energy alpha background. These new data show a narrower width for the main alpha peak than previously determined. In addition, there is an indication of the low-energy interference anomaly predicted by several authors. The spectrum obtained can be parameterized in K- and R-matrix calculations, which also include the previously-measured 12C( alpha , gamma )16O cross sections and elastic scattering phase shifts. Results of a data collection run in June with much higher alpha yields will be presented.