Abstract
Background: The 15O(α,γ)19Ne 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 20Mg(βpα)15O decay sequence. The key 15O(α,γ)19Ne resonance at an excitation of 4.03 MeV is now known to be fed in 20Mg(βpγ)19Ne; however, the energies of the protons feeding the 4.03 MeV state are unknown. Knowledge of the proton energies will facilitate future 20Mg(βpα)15O measurements.
Purpose: To determine the energy of the proton transition feeding the 4.03 MeV state in 19Ne.
Method: A fast beam of 20Mg 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 19Neγ-ray lines and compare to the experimental data.
Results: The center of mass energy between the proton and 19Ne, feeding the 4.03 MeV state, is measured to be 1.21+0.25−0.22MeV, corresponding to a 20Na excitation energy of 7.44+0.25−0.22MeV. Absolute feeding intensities and γ-decay branching ratios of 19Ne 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 19Ne to the ground state is reported. The lifetime of the 1507 keV state in 19Ne is measured to be 4.3+1.3−1.1 ps resolving discrepancies in the literature. Conflicting 20Mg(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 19Ne state in 20Mgβ decay will facilitate future measurements of the α-particle branching ratio.
