Proton branching ratios of ${}^{23}\mathrm{Mg}$ levels

Background: The anomalous ${}^{22}\mathrm{Ne}$ abundance measured in certain presolar graphite grains is thought to arise from the decay of ${}^{22}\mathrm{Na}$ that was synthesized at high temperatures in core-collapse supernovas. To better interpret this abundance anomaly, the primary destruction mechanism of ${}^{22}\mathrm{Na}$, the ${}^{22}\mathrm{Na}(p,\gamma ){}^{23}\mathrm{Mg}$ reaction, must be better understood.

Purpose: Determine proton branching ratios of several ${}^{23}\mathrm{Mg}$ excited states that play a role in the high-temperature ${}^{22}\mathrm{Na}(p,\gamma ){}^{23}\mathrm{Mg}$ reaction rate.

Methods: Particle decays of ${}^{23}\mathrm{Mg}$ excited states populated with the previously reported ${}^{24}\mathrm{Mg}(p,d){}^{23}\mathrm{Mg}$ transfer reaction measurement [Kwag et al., Eur. Phys. J. A 56, 108 (2020)] were analyzed to extract proton branching ratios. The reaction was studied using a 31-MeV proton beam from the Holifield Radioactive Ion Beam Facility of Oak Ridge National Laboratory and ${}^{24}\mathrm{Mg}$ solid targets.

Results: Proton branching ratios of several ${}^{23}\mathrm{Mg}$ excited states in the energy range $E_x=8.044\text{−}9.642$ MeV were experimentally determined for the first time for the $p0$ and $p{1}^{\prime }$ ($p1+p2+p3$) decay channels.

Conclusions: These new branching ratios for ${}^{23}\mathrm{Mg}$ levels can provide an experimental foundation for an improved high-temperature rate of the ${}^{22}\mathrm{Na}(p,\gamma ){}^{23}\mathrm{Mg}$ reaction needed to understand production of anomalously high ${}^{22}\mathrm{Ne}$ abundance in core-collapse supernovas.

Figure 1

Decay particle energy versus coincident reaction deuteron energy plot for all identified events. Four major diagonal bands labeled as $p0, p{1}^{\prime }, p4$, and $p{5}^{\prime }$ are evident. Each band represents a proton decay channel. Only the events falling in the red and purple gates were used to determine branching ratios, and are associated with the ground state ($p0$) and three closely located levels of ${}^{22}\mathrm{Na}$ at $E_x=582.8$, 657.0, and 890.9 keV ($p{1}^{\prime }$), respectively. The weak band in the black dotted gate shows the decay events originated from the impurity of the target.

Figure 2

Reconstructed ${}^{22}\mathrm{Na}$ excitation energy spectrum from the proton-deuteron coincidences in Fig. 1. The peaklike structures labeled as $p{1}^{\prime }$ and $p{5}^{\prime }$ are associated with three closely spaced levels each that could not be fully resolved in the present work.

Figure 3

(a) Deuteron energy spectrum obtained from the ${}^{24}\mathrm{Mg}(p,d){}^{23}\mathrm{Mg}$ reaction measurement [20]. Identified ${}^{23}\mathrm{Mg}$ levels are labeled with their excitation energies in MeV. (b) Gated spectra of the coincident protons for the $p0$ (blue solid line) and $p{1}^{\prime }$ (red dotted line) channels.

Figure 4

A diagram of the involved energy levels of ${}^{22}\mathrm{Na}$ and ${}^{23}\mathrm{Mg}$ along with decay channels. Energies are in units of MeV.

Figure 5

Proton energy spectrum for the coincident events falling in the $p0$ gate (black solid line). Identified ${}^{23}\mathrm{Mg}$ energy levels are labeled with their excitation energies in MeV. The expected proton energy spectrum (displayed as the red dotted line) well reproduces the experimental result.

Figure 6

Similar to Fig. 5, the empirical proton energy spectrum for the $p{1}^{\prime }$ channel (black solid line). (a) The expected proton energy spectra obtained by assuming equal contributions from three final levels are also shown. (b) The expected spectra after the fit are shown (see text).

Figure 7

Counts of decay protons from the 8.924-MeV (9.642-MeV) level as a function of relative angle for the $p0$ ($p{1}^{\prime }$) channel shown in the top (bottom) panel as a blue solid line. The expected distributions assuming isotropic and anisotropic decay of protons are shown as a black solid and a red dotted line, respectively.