New $\gamma$-ray transitions observed in ${}^{19}\mathrm{Ne}$ with implications for the ${}^{15}\mathrm{O}(\alpha ,\gamma ){}^{19}\mathrm{Ne}$ reaction rate

The ${}^{15}\mathrm{O}(\alpha ,\gamma ){}^{19}\mathrm{Ne}$ reaction is responsible for breakout from the hot CNO cycle in type I x-ray bursts. Understanding the properties of resonances between $E_x=4$ and 5 MeV in ${}^{19}\mathrm{Ne}$ is crucial in the calculation of this reaction rate. The spins and parities of these states are well known, with the exception of the 4.14- and 4.20-MeV states, which have adopted spin-parities of $9/2^{-}$ and $7/2^{-}$, respectively. $\gamma$-ray transitions from these states were studied using triton-$\gamma -\gamma$ coincidences from the ${}^{19}\mathrm{F}({}^3\mathrm{He},t\gamma ){}^{19}\mathrm{Ne}$ reaction measured with the GODDESS (Gammasphere ORRUBA Dual Detectors for Experimental Structure Studies) at Argonne National Laboratory. The observed transitions from the 4.14- and 4.20-MeV states provide strong evidence that the $J^{\pi }$ values are actually $7/2^{-}$ and $9/2^{-}$, respectively. These assignments are consistent with the values in the ${}^{19}\mathrm{F}$ mirror nucleus and in contrast to previously accepted assignments.

Figure 1

Partial level schemes of ${}^{19}\mathrm{F}$ and ${}^{19}\mathrm{Ne}$ highlighting two states near 4 MeV and showing mirror connections between levels (dashed lines); the mirror connections updated in this work are shown in blue. The ${}^{19}\mathrm{F}$ transitions, branching ratios (%), and energies (keV) are from Ref. [11]. The ${}^{19}\mathrm{Ne}$ 4.14- and 4.20-MeV state $\gamma$-ray transitions, branching ratios, and energies were determined in this work. The two red transitions were first observed in this measurement.

Figure 2

Rendering of the GODDESS setup showing the beam direction, target location, and ORRUBA in position inside Gammasphere [17].

Figure 3

Triton spectrum populating excitations in ${}^{19}\mathrm{Ne}$ at ${\theta }_{\mathrm{lab}}={20}^{\circ }$. The shaded region corresponds to ${}^{19}\mathrm{Ne}$ excitation energies between 3.8 and 4.4 MeV, which were gated on to produce the results shown.

Figure 4

Random-subtracted Gammasphere spectra generated by gating on tritons corresponding to ${}^{19}\mathrm{Ne}$ excitation energies between 3.8 and 4.4 MeV and $\gamma -\gamma$ coincidences. (a) Gated on the 1340-, 1377- and 1616-keV transitions depopulating the 1616-keV 3/2 state. The 2527.2-keV transition from the 4141.8-keV state is observed for the first time. (b) Gated on the 1232- and 1269-keV transitions depopulating the 1507-keV $5/2^{-}$ state. (c) Gated on the 238-keV $5/2^{+}$ to ground-state transition. The 3897.5-keV transition from the 4141.8-keV state is observed for the first time. The transitions labeled 4140 and 4364 keV are previously observed transitions from the 4378- and 4602-keV levels. The binning of the histograms is 20, 8, and 16 keV/bin, respectively.

Figure 5

Fractional contributions to the ${}^{15}\mathrm{O}(\alpha ,\gamma ){}^{19}\mathrm{Ne}$ reaction rate for the 4.14- and 4.20-MeV states assuming the two sets of spin-parity assignments and branching ratios of $B\left(\alpha \right)=1.2\times {10}^{-3}$ [7].