Doppler Broadening in ${}^{20}$Mg(${\hat{I}}^{2}p{\hat{I}}^3$)${}^{19}$Ne Decay

Abstract

Background: The ${}^{15}$O($\hat{I}\pm ,{\hat{I}}^3$)${}^{19}$Ne bottleneck reaction in Type I x-ray bursts is the most important thermonuclear reaction rate to constrain experimentally, in order to improve the accuracy of burst light-curve simulations. A proposed technique to determine the thermonuclear rate of this reaction employs the ${}^{20}$Mg(${\hat{I}}^{2}p\hat{I}\pm$)${}^{15}$O decay sequence. The key ${}^{15}$O($\hat{I}\pm ,{\hat{I}}^3$)${}^{19}$Ne resonance at an excitation of 4.03 MeV is now known to be fed in ${}^{20}$Mg(${\hat{I}}^{2}p{\hat{I}}^3$)${}^{19}$Ne; however, the energies of the protons feeding the 4.03 MeV state are unknown. Knowledge of the proton energies will facilitate future ${}^{20}$Mg(${\hat{I}}^{2}p\hat{I}\pm$)${}^{15}$O measurements. Purpose: To determine the energy of the proton transition feeding the 4.03 MeV state in ${}^{19}$Ne. Method: A fast beam of ${}^{20}$Mg was implanted into a plastic scintillator, which was used to detect ${\hat{I}}^2$ particles. 16 high purity germanium detectors were used to detect ${\hat{I}}^3$ rays emitted following ${\hat{I}}^{2}p$ decay. A Monte Carlo method was used to simulate the Doppler broadening of ${}^{19}$Ne ${\hat{I}}^3$ rays and compare to the experimental data. Results: The center of mass energy between the proton and ${}^{19}$Ne, feeding the 4.03 MeV state, is measured to be 1.21${}_{-0.22}^{+0.25}$ MeV, corresponding to a ${}^{20}$Na excitation energy of 7.44${}_{-0.22}^{+0.25}$ MeV. Absolute feeding intensities and ${\hat{I}}^3$-decay branching ratios of ${}^{19}$Ne states were determined including the 1615 keV state. A new ${\hat{I}}^3$ decay branch from the 1536 keV state in ${}^{19}$Ne to the ground state is reported. The lifetime of the 1507 keV state in ${}^{19}$Ne is measured to be 4.3${}_{-1.1}^{+1.3}$ ps resolving discrepancies in the literature. Conflicting ${}^{20}$Mg(${\hat{I}}^{2}p$) decay schemes in published literature are clarified.

11 pages, 13 figures