Resonances in 19 Ne with relevance to the astrophysically important 18 F( p , α ) 15 O reaction

The most intense $\\ensuremath{\\gamma}$-ray line observable from novae is likely to be from positron annihilation associated with the decay of ${}^{18}$F. The uncertainty in the destruction rate of this nucleus through the ${}^{18}$F($p$,$\\ensuremath{\\alpha}$)${}^{15}$O reaction presents a limit to interpretation of any future observed $\\ensuremath{\\gamma}$-ray flux. Direct measurements of the cross section of both this reaction and the ${}^{18}$F($p,p$)${}^{18}$F reaction have been performed between center of mass energies of 0.5 and 1.9 MeV. Simultaneous fits to both data sets with the $R$-matrix formalism reveal several resonances, with the inferred parameters of populated states in ${}^{19}$Ne in general agreement with previous measurements. Of particular interest, extra strength has been observed above ${E}_{\\mathrm{c}.\\mathrm{m}.}\\ensuremath{\\sim}$1.3 MeV in the ${}^{18}$F($p,p$)${}^{18}$F reaction and between 1.3--1.7 MeV in the ${}^{18}$F($p$,$\\ensuremath{\\alpha}$)${}^{15}$O reaction. This is well described by a broad 1/2${}^{+}$ state, consistent with both a recent theoretical prediction and an inelastic scattering measurement. The astrophysical implications of a broad subthreshold partner to this state are discussed.