Extrapolation of low-energy reaction cross sections

Many applications in astrophysical and controlled-fusion research require values of nuclear-reaction cross sections at interaction energies considerably below the range in which absolute measurements are practical. Measured cross sections thus must be extrapolated to the energies of interest. For exoergic charged-particle reactions, which are not dominated by resonances within the range of interest, it is shown that a physically meaningful extrapolation formula can be derived from the R-matrix theory. For energies such that only incident s-waves contribute to the reaction, the resultant formula contains at most three 'free' parameters. These results are used to extrapolate cross sections for various d + 6Li reactions, measured at energies above 100 keV, to energies in the range of 1 keV. (Auth.)