β-delayed α emission of N18: Broad Jπ=1- states in the C14 +α system

The beta-delayed alpha-spectrum of N-18 has been measured at the TISOL facility at TRIUMF resolving the structure of a broad alpha-particle distribution at alpha-energies of 1.5-5.5 MeV into several states that show a distinct interference pattern. R-matrix fits assuming a solely J(pi)=1(-) partial wave confirm this finding. The extrapolated low energy part of this structure constitutes the natural parity background in a possible experiment to observe the parity forbidden alpha-decay of the E-x=6.880 MeV, J(pi)=0(-) state in O-18 that is estimated from the fit to be 2x10(-8) of the intensity of the E-alpha=1081 keV peak. In addition, the tails of the broad 1(-) states observed likely contribute to the astrophysical C-14 (alpha,gamma)O-18 reaction rate though the experimental information available is too incomplete to allow precise predictions. The beta-delayed alpha-spectrum of N-18 also excludes complex beta-feeding amplitudes at a significant level. The states seen in the beta-delayed alpha-decay produce a pattern of approximately equally spaced 1(-) states in O-18 consistent with J(pi)=1(-) states seen at the same and higher excitation energies of O-18.