Spectroscopy of ${}^{19}\text{Ne}$ for the thermonuclear ${}^{15}\text{O}(\alpha ,\gamma ){}^{19}\text{Ne}$ and ${}^{18}\text{F}(p,\alpha ){}^{15}\text{O}$ reaction rates

Uncertainties in the thermonuclear rates of the ${}^{15}\text{O}(\alpha ,\gamma ){}^{19}\text{Ne}$ and ${}^{18}\text{F}(p,\alpha ){}^{15}\text{O}$ reactions affect model predictions of light curves from type I x-ray bursts and the amount of the observable radioisotope ${}^{18}\text{F}$ produced in classical novae, respectively. To address these uncertainties, we have studied the nuclear structure of ${}^{19}\text{Ne}$ over $E_x=4.0-5.1$ and 6.1–7.3 MeV using the ${}^{19}\text{F}({}^3\text{He},t){}^{19}\text{Ne}$ reaction. We find the $J^{\pi }$ values of the 4.14- and 4.20-MeV levels to be consistent with $9/2^{-}$ and $7/2^{-}$, respectively, in contrast to previous assumptions. We confirm the recently observed triplet of states around 6.4 MeV and find evidence that the state at 6.29 MeV, just below the proton threshold, is either broad or a doublet. Our data also suggest that predicted but yet unobserved levels may exist near the 6.86-MeV state. Higher resolution experiments are urgently needed to further clarify the structure of ${}^{19}\text{Ne}$ around the proton threshold before a reliable ${}^{18}\text{F}(p,\alpha ){}^{15}\text{O}$ rate for nova models can be determined.

Figure 1

Focal-plane position spectra of tritons from the ${}^{19}\text{F}({}^3\text{He},t){}^{19}\text{Ne}$ reaction at 25 MeV. The spectra were measured at ${\theta }_{\mathrm{lab}}={10}^{\circ }$ using the $50\mu \mathrm{g}/{\mathrm{cm}}^2{\mathrm{CaF}}_2$ target (a) and at ${\theta }_{\mathrm{lab}}={15}^{\circ }$ using the $10\mu \mathrm{g}/{\mathrm{cm}}^2{\mathrm{MgF}}_2$ target (b). Peaks are labeled by the corresponding ${}^{19}\text{Ne}$ excitation energies in keV.

Figure 2

Expanded views of Fig. 1. Overall best fits (solid red curve), constituent peaks (blue curve), and alternative, poorer fits (dashed red curves) are shown for levels within $E_x\left({}^{19}\text{Ne}\right)=6.2-6.9\mathrm{MeV}$. See text for details.

Figure 3

Triton angular distributions measured with the ${}^{19}\text{F}({}^3\text{He},t){}^{19}\text{Ne}$ reaction at 25 MeV. Theoretical curves calculated with fresco [21] have been fit to the data. Best-fit curves are plotted here, along with curves corresponding to previous, tentative assignments. Excitation energies (in keV) corresponding to the data and $J^{\pi }$ values corresponding to each curve are indicated. See text for full details.