Doppler broadening in Mg20(βpγ)Ne19 decay

Background: The $^{15}\mathrm{O}(\ensuremath{\alpha},\ensuremath{\gamma})^{19}\mathrm{Ne}$ 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 $^{20}\mathrm{Mg}(\ensuremath{\beta}p\ensuremath{\alpha})^{15}\mathrm{O}$ decay sequence. The key $^{15}\mathrm{O}(\ensuremath{\alpha},\ensuremath{\gamma})^{19}\mathrm{Ne}$ resonance at an excitation of 4.03 MeV is now known to be fed in $^{20}\mathrm{Mg}(\ensuremath{\beta}p\ensuremath{\gamma})^{19}\mathrm{Ne}$; however, the energies of the protons feeding the 4.03 MeV state are unknown. Knowledge of the proton energies will facilitate future $^{20}\mathrm{Mg}(\ensuremath{\beta}p\ensuremath{\alpha})^{15}\mathrm{O}$ measurements.Purpose: To determine the energy of the proton transition feeding the 4.03 MeV state in $^{19}\mathrm{Ne}$.Method: A fast beam of $^{20}\mathrm{Mg}$ was implanted into a plastic scintillator, which was used to detect $\ensuremath{\beta}$ particles. 16 high purity germanium detectors were used to detect $\ensuremath{\gamma}$ rays emitted following $\ensuremath{\beta}p$ decay. A Monte Carlo method was used to simulate the Doppler broadening of $^{19}\mathrm{Ne}\phantom{\rule{4pt}{0ex}}\ensuremath{\gamma}$-ray lines and compare to the experimental data.Results: The center of mass energy between the proton and $^{19}\mathrm{Ne}$, feeding the 4.03 MeV state, is measured to be $1.21{}_{\ensuremath{-}0.22}^{+0.25}\phantom{\rule{0.16em}{0ex}}\mathrm{MeV}$, corresponding to a $^{20}\mathrm{Na}$ excitation energy of $7.44{}_{\ensuremath{-}0.22}^{+0.25}\phantom{\rule{0.16em}{0ex}}\mathrm{MeV}$. Absolute feeding intensities and $\ensuremath{\gamma}$-decay branching ratios of $^{19}\mathrm{Ne}$ states were determined including the 1615 keV state, which has not been observed before in this decay. A new $\ensuremath{\gamma}$ decay branch from the 1536 keV state in $^{19}\mathrm{Ne}$ to the ground state is reported. The lifetime of the 1507 keV state in $^{19}\mathrm{Ne}$ is measured to be $4.3{}_{\ensuremath{-}1.1}^{+1.3}$ ps resolving discrepancies in the literature. Conflicting $^{20}\mathrm{Mg}(\ensuremath{\beta}p$) decay schemes in published literature are clarified.Conclusions: The utility of this Doppler broadening technique to provide information on $\ensuremath{\beta}$-delayed nucleon emission and excited-state lifetimes has been further demonstrated. In particular, knowledge of the proton energies feeding the 4.03 MeV $^{19}\mathrm{Ne}$ state in $^{20}\mathrm{Mg}\phantom{\rule{4pt}{0ex}}\ensuremath{\beta}$ decay will facilitate future measurements of the $\ensuremath{\alpha}$-particle branching ratio.