Measurement of the decay branching ratios of the α-unbound states in 19Ne and the 15O(α,γ) reaction rate

The breakout reaction O-15(alpha,gamma)Ne-19 from the hot CNO cycle is critical to understanding explosive astrophysical phenomena such as x-ray bursts. In spite of considerable past experimental effort via indirect methods, this reaction rate remained mostly uncertain until our recent measurement [Phys. Rev. Lett. 98, 242503 (2007)] of the eluding alpha-decay branching ratios of the near-threshold states in Ne-19, particularly the critical level at 4.03 MeV. In this paper, we present more details of and deeper insights into the measurement and the uncertainties of the experimental results. The alpha-unbound states in Ne-19 were populated via the reaction F-19(He-3,t), and the alpha-decay branch was observed from t-alpha coincidences using a low-energy particle detection array and the TWINSOL facility at the University of Notre Dame. In particular, the measured branching ratio of the 4.03-MeV state is 2.9 +/- 2.1x10(-4). In combination with previous measurements of the lifetimes of these states, a new experimental reaction rate of O-15(alpha,gamma)Ne-19 is proposed and discussed in the astrophysical scenario. Further experimental investigations are necessary to reduce the remaining uncertainties.