R-matrix extrapolations that take into account the effects of subthreshold levels are reported for the a+12,13C reaction cross sections. The relatively low extrapolated value SEI(300 keV) = 20 keV-b for a+12C capture seems compatible with measurements of the fl-delayed a-decay spectrum of I~N, while the extrapolated 13C(a, n)160 S-factor is found to be nearly two orders of magnitude larger than the Caughlan-Fowler value.
1. E1 CAPTURE FOR a+12C
The analysis of the 160 system upon which the extrapolation is based includes differential cross sections for c~+12C elastic scattering [1] and integrated cross sections for 12C(a,V)laO E1 capture [2-5]. R-matrix parameters are obtained from a direct fit to the measurements (rather than to derived quantities such as S-factors and phase shifts), thereby avoiding unnecessary correlations in the input data. It was not assumed ~ priori that the subthreshold J" = 1-state at E = -45 keV was necessary to fit the data. Therefore, a somewhat surprising first result, in strong contradiction to previous analyses, was that most of the data could be well represented in the region where they had been measured without including the subthreshold state at all. This parameter set includes only two J" = 1-levels, one for the positive-energy resonance at E = 2.37 MeV, and a distant background level.