New portal to the O 15 ( α , γ ) Ne 19 resonance triggering CNO-cycle breakout

The O-15(alpha,gamma)Ne-19 reaction is expected to trigger the initial path for breakout from the CNO hydrogen-burning cycles to the rapid proton capture (rp) process in type I x-ray bursts on accreting neutron stars. The thermonuclear reaction rate has a major impact on models of type I x-ray burst observables and it depends on the small a-particle branching ratio, Gamma(alpha)/Gamma, of the 4.03 MeV state in Ne-19. Attempts to measure Gamma(alpha)/Gamma by populating the 4.03 MeV state using nuclear reactions have only led to strong upper limits. In the present work, we report the first experimental evidence that the 4.03 MeV Ne-19 state is populated in Mg-20 beta-delayed proton emission. This new channel has the potential to provide the necessary sensitivity to detect a finite value of Gamma(alpha)/Gamma.