New portal to the ${}^{15}\mathbf{O}(\alpha ,\gamma ){}^{19}\mathbf{Ne}$ resonance triggering CNO-cycle breakout

The ${}^{15}\mathrm{O}(\alpha ,\gamma ){}^{19}\mathrm{Ne}$ reaction is expected to trigger the initial path for breakout from the CNO hydrogen-burning cycles to the rapid proton capture $\left(rp\right)$ process in type I x-ray bursts on accreting neutron stars. The thermonuclear reaction rate has a major impact on models of type I x-ray burst observables and it depends on the small $\alpha$-particle branching ratio, ${\mathrm{\Gamma }}_{\alpha }/\mathrm{\Gamma }$, of the 4.03 MeV state in ${}^{19}\mathrm{Ne}$. Attempts to measure ${\mathrm{\Gamma }}_{\alpha }/\mathrm{\Gamma }$ by populating the 4.03 MeV state using nuclear reactions have only led to strong upper limits. In the present work, we report the first experimental evidence that the 4.03 MeV ${}^{19}\mathrm{Ne}$ state is populated in ${}^{20}\mathrm{Mg} \beta$-delayed proton emission. This new channel has the potential to provide the necessary sensitivity to detect a finite value of ${\mathrm{\Gamma }}_{\alpha }/\mathrm{\Gamma }$.

Figure 1

Simplified ${}^{20}\mathrm{Mg}$ ($T_{1/2}=91.4$ ms [16], $Q_{\mathrm{EC}}=10627$ keV [3, 17, 18]) $\beta$ decay scheme focusing on the transitions relevant to the present work (blue online). Energies are shown in units of keV. The proton separation energy of ${}^{20}\mathrm{Na}$ is 2190 keV and the $\alpha$-particle separation energy of ${}^{19}\mathrm{Ne}$ is $3529\mathrm{keV}$ [3]. Branches are given as intensities in percent of ${}^{20}\mathrm{Mg} \beta$ decays. The values for the ${}^{20}\mathrm{Mg}\left({\beta }^{+}\nu \right){}^{20}\mathrm{Na}$ branches are adopted from Ref. [16] and the value for the ${}^{20}\mathrm{Na}^{*}\left(p\right){}^{19}\mathrm{Ne}_{4033}^{*}$ branch shown is from the present work.

Figure 2

Energy spectra of SeGA events. The upper spectrum shows all SeGA events in coincidence with events of any energy in the scintillator, including both ion implantations and $\beta$ decays. This spectrum includes prompt $\gamma$ rays from nuclear reactions and $\beta$-delayed $\gamma$ rays. The lower spectrum selects events in coincidence with events depositing less than 10 MeV in the scintillator. This spectrum includes $\beta$-delayed $\gamma$ rays and excludes prompt $\gamma$ rays. $\gamma$-ray photopeaks are labeled by the nuclide in which the $\gamma$-ray transition occurs. First and second 511-keV $\gamma$-ray escape peaks are labeled by one and two asterisks, respectively.

Figure 3

(b): the points show the energy spectrum of SeGA events in coincidence with events depositing less than 10 MeV in the scintillator. This spectrum includes $\beta$-delayed $\gamma$ rays and excludes prompt $\gamma$ rays. The spectrum is identical to the lower spectrum in Fig. 2, but the binning is different. The error bars associated with the data points are statistical. The smooth line is a functional fit to the data comprised of a Gaussian function added to a linear background. (a): the points show the difference between the data and the linear background component of the fit shown in (b). The smooth line is the Gaussian function derived from the fit shown in (b).