Is $\gamma$-Ray Emission from Novae Affected by Interference Effects in the ${}^{18}\mathbf{F}(p,\alpha ){}^{15}\mathbf{O}$ Reaction?

The ${}^{18}\mathrm{F}(p,\alpha ){}^{15}\mathrm{O}$ reaction rate is crucial for constraining model predictions of the $\gamma$-ray observable radioisotope ${}^{18}\mathrm{F}$ produced in novae. The determination of this rate is challenging due to particular features of the level scheme of the compound nucleus, ${}^{19}\mathrm{Ne}$, which result in interference effects potentially playing a significant role. The dominant uncertainty in this rate arises from interference between $J^{\pi }=3/2^{+}$ states near the proton threshold ($S_p=6.411\mathrm{MeV}$) and a broad $J^{\pi }=3/2^{+}$ state at 665 keV above threshold. This unknown interference term results in up to a factor of 40 uncertainty in the astrophysical $S$-factor at nova temperatures. Here we report a new measurement of states in this energy region using the ${}^{19}\mathrm{F}({}^3\mathrm{He},t){}^{19}\mathrm{Ne}$ reaction. In stark contrast to previous assumptions we find at least 3 resonances between the proton threshold and $E_{cm}=50\mathrm{keV}$, all with different angular distributions. None of these are consistent with $J^{\pi }=3/2^{+}$ angular distributions. We find that the main uncertainty now arises from the unknown proton width of the 48 keV resonance, not from possible interference effects. Hydrodynamic nova model calculations performed indicate that this unknown width affects ${}^{18}\mathrm{F}$ production by at least a factor of two in the model considered.

Figure 1

Raw focal-plane triton spectra at ${\theta }_{\mathrm{lab}}=10°$ (a) and 20° (b). Excitation energies are labeled in keV.

Figure 2

Partial raw focal-plane triton spectrum at ${\theta }_{\mathrm{lab}}=15°$ (a), 20° (b), and 30° (c). At 15°, the overall best fit (red) and three constituent Gaussian peaks (blue) are shown for the states within $E_x=6.4–6.5\mathrm{MeV}$.

Figure 3

Triton angular distributions measured with the ${}^{19}\mathrm{F}({}^3\mathrm{He},t){}^{19}\mathrm{Ne}$ reaction. Curves calculated with fresco have been fit to the data. Each panel (a–j) is labeled with the excitation energy (in keV) of the associated state in ${}^{19}\mathrm{Ne}$ and the $J^{\pi }$ values of the curves that best fit the data.

Figure 4

Triton angular distributions measured with the ${}^{19}\mathrm{F}({}^3\mathrm{He},t){}^{19}\mathrm{Ne}$ reaction at 25 MeV. Curves calculated as in Fig. 3. Fits shown are for previously accepted $J^{\pi }$ assignments.