Sub-threshold states in Ne 19 relevant to F 18 (p,α) O 15

Background: Classical novae result from thermonuclear explosions producing several γ-ray emitters which are prime targets for satellites observing in the MeV range. The early ≤511keV γ-ray emission depends critically on the F18(p,α)O15 reaction rate which, despite many experimental and theoretical efforts, still remains uncertain. Purpose: One of the main uncertainties in the F18(p,α)O15 reaction rate is the contribution in the Gamow window of interference between sub-threshold Ne19 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. Methods: States in the Ne19 compound nucleus were studied at the Tandem-ALTO facility using the F19(He3,t)Ne19 charge-exchange reaction. Tritons were detected with an Enge Split-pole spectrometer while decaying protons or α particles from unbound Ne19 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. Results: The coincidence yield at Ex=6.29MeV 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. Conclusions: The levels schemes of Ne19 and F19 are still not sufficiently well known and further studies of the analog assignments are needed. The tentative broad state at 6 MeV may only play a role if the reduced proton width is large.N.d.S. and A.M.L. acknowledge very fruitful discussions with Pierre Descouvemont about the properties of sub-threshold 19Ne states, and with Michael Bentley on mirror energy differences. A.M.L. acknowledges the support of the Science and Technology Facilities Council and the Royal Society. This article is based upon work from the “ChETEC” COST Action (CA16117), supported by COST (European Cooperation in Science and Technology)