Spin-parities of subthreshold resonances in the ${}^{18}\mathrm{F}(p,\alpha ){}^{15}\mathrm{O}$ reaction

The ${}^{18}\mathrm{F}(p,\alpha ){}^{15}\mathrm{O}$ reaction is key to determining the ${}^{18}\mathrm{F}$ abundance in classical novae. However, the cross section for this reaction has large uncertainties at low energies largely caused by interference effects. Here, we resolve a longstanding issue with unknown spin-parities of subthreshold states in ${}^{19}\mathrm{Ne}$ that reduces these uncertainties. The ${}^{20}\mathrm{Ne}({}^3\mathrm{He},{}^4\mathrm{He}){}^{19}\mathrm{Ne}$ neutron pick-up reaction was used to populate ${}^{19}\mathrm{Ne}$ excited states, focusing on the energy region of astrophysical interest $(\approx 6–7 \mathrm{MeV})$. The experiment was performed at the Triangle Universities Nuclear Laboratory using the high resolution Enge split-pole magnetic spectrograph. Spins and parities were found for states in the astrophysical energy range. In particular, the state at 6.133 MeV $(E_r^{\text{c.m.}}=-277 \mathrm{keV})$ was found to have spin and parity of $3/2^{+}$ and we confirm the existence of an unresolved doublet close to 6.288 MeV $(E_r^{\text{c.m.}}=-120 \mathrm{keV})$ with $J^{\pi }=1/2^{+}$ and a high-spin state. Using these results, we demonstrate a significant factor of two decrease in the reaction rate uncertainties at nova temperatures.

Figure 1

RBS spectrum. The red and blue lines are the experimental data and SIMNRA [24] simulation, respectively. The analysis done shows that, apart from the natural carbon substrate, the target contains a $3.6\pm 0.4$ $\mu \mathrm{g}$/${\mathrm{cm}}^2$ thick ${}^{20}\mathrm{Ne}$ layer and an ${}^{16}\mathrm{O}$ contamination layer $0.22\pm 0.02$ $\mu \mathrm{g}/{\mathrm{cm}}^2$ thick located at the front.

Figure 2

Two-dimensional energy loss vs front position spectrum collected at ${\theta }_{\mathrm{lab}}={20}^{\circ }$ from signals coming from these sections of the focal plane detector. Bands of events coming from protons, deuterons, ${}^3\mathrm{He}$, and ${}^4\mathrm{He}$ are indicated, with the software gate used for $\alpha$ particles shown as a black polygon.

Figure 3

Energy calibrated $\alpha$-particle gated spectrum at ${\theta }_{\text{lab}}={20}^{\circ }$. The peaks corresponding to ${}^{19}\mathrm{Ne}$ excited states are labeled with the energies (in MeV) in column 2 of Table 2. The peaks with a red label correspond to excited states coming from reactions involving the substrate or contaminants. The inset figure highlights the region around the subthreshold resonances, which strongly influence the interference effects at high energies. The vertical dashed blue line denotes the proton separation energy.

Figure 4

Angular distributions obtained from the ${}^{20}\mathrm{Ne}({}^3\mathrm{He},{}^4\mathrm{He}){}^{19}\mathrm{Ne}$ experiment for ${}^{19}\mathrm{Ne}$ excited states with energies of astrophysical interest (6–8 MeV). The curves were calculated with fresco. Each panel shows the ${}^{19}\mathrm{Ne}$ level energy together with the angular momentum transfers that were found to best describe the data. The downward-facing arrow for the state at 5.351 MeV indicates an upper limit.

Figure 5

Astrophysical $S$ factor for this work (black lines), Kahl et al. 2021 [21] set VIII (purple dot-dashed lines) and set XVI (solid green lines), and Hall et al. 2020 [19] (orange dashed lines). To calculate interference, relative signs for each resonance are entered into azure2. The signs used to obtain the maximum and minimum rates correspond to the $1/2^{+}$ resonances ($-120$, 1380 keV) and the $3/2^{+}$ resonances ($-277$, 31, 665 keV). For all rates, the maximum is obtained using relative signs ($+,-$) for the $1/2^{+}$ resonances and ($-,-,+$) for the $3/2^{+}$ resonances. The minimum “destructive” rate is obtained by assigning the same sign to all resonances: ($+,+$) and ($+,+,+$).

Figure 6

${}^{18}\mathrm{F}(p,\alpha ){}^{15}\mathrm{O}$ reaction rates ratios between constructive interference and this work's destructive interference for Kahl et al. [21], set XVI (green shade) and set VIII (purple shade), Hall et al. 2020 [19] (orange shade), and this work (grey hatched region). The solid black line at unity represents the case were both interference possibilities are the same. The figure shows that unknown interference causes a factor of about 2 uncertainty with our present rates at 150 MK in contrast to factors of 3 and 4 in the Hall et al. 2019 and Kahl et al. 2020 set XVI rates, respectively.