Background: The O15(α,γ)Ne19 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 Mg20(βpα)O15 decay sequence. The key O15(α,γ)Ne19 resonance at an excitation of 4.03 MeV is now known to be fed in Mg20(βpγ)Ne19; however, the energies of the protons feeding the 4.03 MeV state are unknown. Knowledge of the proton energies will facilitate future Mg20(βpα)O15 measurements. Purpose: To determine the energy of the proton transition feeding the 4.03 MeV state in Ne19. Method: A fast beam of Mg20 was implanted into a plastic scintillator, which was used to detect β particles. 16 high purity germanium detectors were used to detect γ rays emitted following βp decay. A Monte Carlo method was used to simulate the Doppler broadening of Ne19γ-ray lines and compare to the experimental data. Results: The center of mass energy between the proton and Ne19, feeding the 4.03 MeV state, is measured to be 1.21-0.22+0.25MeV, corresponding to a Na20 excitation energy of 7.44-0.22+0.25MeV. Absolute feeding intensities and γ-decay branching ratios of Ne19 states were determined including the 1615 keV state, which has not been observed before in this decay. A new γ decay branch from the 1536 keV state in Ne19 to the ground state is reported. The lifetime of the 1507 keV state in Ne19 is measured to be 4.3-1.1+1.3 ps resolving discrepancies in the literature. Conflicting Mg20(βp) decay schemes in published literature are clarified. Conclusions: The utility of this Doppler broadening technique to provide information on β-delayed nucleon emission and excited-state lifetimes has been further demonstrated. In particular, knowledge of the proton energies feeding the 4.03 MeV Ne19 state in Mg20β decay will facilitate future measurements of the α-particle branching ratio.
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