Spin-parities of subthreshold resonances in the 18F(p, ?)15O reaction

The 18F(p, alpha)15O reaction is key to determining the 18F abundance in classical novae. However, the cross section for this reaction has large uncertainties at low energies largely caused by interference effects. Here, we resolve a longstanding issue with unknown spin-parities of subthreshold states in 19Ne that reduces these uncertainties. The 20Ne(3He, 4He)19Ne neutron pick-up reaction was used to populate 19Ne excited states, focusing on the energy region of astrophysical interest (approximate to 6-7 MeV). The experiment was performed at the Triangle Universities Nuclear Laboratory using the high resolution Enge split-pole magnetic spectrograph. Spins and parities were found for states in the astrophysical energy range. In particular, the state at 6.133 MeV (Ec.m. r = -277 keV) was found to have spin and parity of 3/2+ and we confirm the existence of an unresolved doublet close to 6.288 MeV (Erc.m.= -120 keV) with Jn = 1/2+ and a high-spin state. Using these results, we demonstrate a significant factor of two decrease in the reaction rate uncertainties at nova temperatures.