Sub-threshold states in \(^{19}\)Ne relevant to \(^{18}\)F(p,\(\alpha\))\(^{15}\)O

Classical novae result from thermonuclear explosions producing several \(\gamma\)-ray emitters which are prime targets for satellites observing in the MeV range. The early 511 keV gamma-ray emission depends critically on the \(^{18}\)F(p,\(\alpha\))\(^{15}\)O reaction rate which, despite many experimental and theoretical efforts, still remains uncertain. One of the main uncertainties in the \(^{18}\)F(p,\(\alpha\))\(^{15}\)O reaction rate is the contribution in the Gamow window of interference between sub-threshold \(^{19}\)Ne states and known broad states at higher energies. Therefore the goal of this work is to clarify the existence and the nature of these sub-threshold states. States in the \(^{19}\)Ne compound nucleus were studied at the Tandem-ALTO facility using the \(^{19}\)F(\(^3\)He,t)\(^{19}\)Ne charge exchange reaction. Tritons were detected with an Enge Split-pole spectrometer while decaying protons or \(\alpha\)-particles from unbound \(^{19}\)Ne states were collected, in coincidence, with a double-sided silicon strip detector array. Angular correlations were extracted and constraints on the spin and parity of decaying states established. The coincidence yield at \(E_x\) = 6.29 MeV was observed to be high spin, supporting the conclusion that it is indeed a doublet consisting of high spin and low spin components. Evidence for a broad, low spin state was observed around 6 MeV. Branching ratios were extracted for several states above the proton threshold and were found to be consistent with the literature. R-matrix calculations show the relative contribution of sub-threshold states to the astrophysically important energy region above the proton threshold. The levels schemes of \(^{19}\)Ne and \(^{19}\)F are still not sufficiently well known and further studies of the analogue assignments are needed. The tentative broad state at 6 MeV may only play a role if the reduced proton width is large.