Abstract
Under astrophysical conditions of high temperature and density, such as for example found in X-ray bursts, breakout can occur from the hot CNO cycles into the rapid proton capture process. A key breakout route is via the sequence O-15(alpha. gamma)Ne-19(p. gamma)Na-20. The Ne-19(p, gamma)Na-20 reaction rate is expected to be dominated by a single resonance at 457(3) key. The identity of the resonance has been under discussion for a long time, with J(pi) = 1(+) and 3(+) assignments suggested. In this study of the beta-delayed proton decay of Mg-20 we report a new, significantly more stringent, upper limit on the beta-decay branch to this state of 0.02% with a confidence level of 90%. This makes a 1(+) assignment highly unlikely and favours a 3(+) assignment for which no branch is expected to be observed. The 3(+) state is predicted to have a significantly higher resonance strength, and to produce a proportionately higher Ne-19(p. gamma)Na-20 reaction rate in X-ray burst conditions. (C) 2012 Elsevier B.V. All rights reserved.