β-Delayed proton-decay study of 20Mg and its implications for the breakout reaction in X-ray bursts

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 . The reaction rate is expected to be dominated by a single resonance at 457(3) keV. The identity of the resonance has been under discussion for a long time, with Jπ=1+ and 3+ assignments suggested. In this study of the β-delayed proton decay of 20Mg we report a new, significantly more stringent, upper limit on the β-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 reaction rate in X-ray burst conditions.