Doppler broadening in Mg-20(beta p gamma)(19) Ne decay

Background: The O-15(alpha, gamma)Ne-19 bottleneck reaction in Type I x-ray bursts is the most important thermonuclear reaction rate to constrain experimentally, to improve the accuracy of burst light-curve simulations. A proposed technique to determine the thermonuclear rate of this reaction employs the Mg-20(beta p alpha)O-15 decay sequence. The key O-15(alpha, gamma)(19) Ne resonance at an excitation of 4.03 MeV is now known to be fed in 20 Mg(beta p gamma)Ne-19; however, the energies of the protons feeding the 4.03 MeV state are unknown. Knowledge of the proton energies will facilitate future Mg-20(beta p alpha)O-15 measurements. Purpose: To determine the energy of the proton transition feeding the 4.03 MeV state in Ne-19. Method: A fast beam of Mg-20 was implanted into a plastic scintillator, which was used to detect beta particles. 16 high purity germanium detectors were used to detect gamma rays emitted following beta p decay. A Monte Carlo method was used to simulate the Doppler broadening of Ne-19 gamma-ray lines and compare to the experimental data. Results: The center of mass energy between the proton and Ne-19, feeding the 4.03 MeV state, is measured to be 1.21(-0.22)(+0.25) MeV, corresponding to a Na-20 excitation energy of 7.44(-0.22)(+0.25) MeV. Absolute feeding intensities and gamma-decay branching ratios of Ne-19 states were determined including the 1615 keV state, which has not been observed before in this decay. A new gamma decay branch from the 1536 keV state in Ne-19 to the ground state is reported. The lifetime of the 1507 keV state in Ne-19 is measured to be 4.3(-1.1)(+1.3) ps resolving discrepancies in the literature. Conflicting Mg-20(beta p) decay schemes in published literature are clarified. Conclusions: The utility of this Doppler broadening technique to provide information on -delayed nucleon emission and excited-state lifetimes has been further demonstrated. In particular, knowledge of the proton energies feeding the 4.03 MeV Ne-19 state in Mg-20 beta decay will facilitate future measurements of the alpha-particle branching ratio.