Measurement of Radiative Proton Capture on ${}^{18}\mathbf{F}$ and Implications for Oxygen-Neon Novae

The rate of the ${}^{18}\mathrm{F}(p,\gamma ){}^{19}\mathrm{Ne}$ reaction affects the final abundance of the $\gamma$-ray observable radioisotope ${}^{18}\mathrm{F}$, produced in novae. However, no successful measurement of this reaction exists and the rate used is calculated from incomplete information on the contributing resonances. Of the two resonances thought to play a significant role, one has a radiative width estimated from the assumed analogue state in the mirror nucleus, ${}^{19}\mathrm{F}$. The second does not have an analogue state assignment at all, resulting in an arbitrary radiative width being assumed. Here, we report the first successful direct measurement of the ${}^{18}\mathrm{F}(p,\gamma ){}^{19}\mathrm{Ne}$ reaction. The strength of the 665 keV resonance ($E_x=7.076\mathrm{MeV}$) is found to be over an order of magnitude weaker than currently assumed in nova models. Reaction rate calculations show that this resonance therefore plays no significant role in the destruction of ${}^{18}\mathrm{F}$ at any astrophysical energy.

Figure 1

Energy loss vs energy loss plot obtained from the first two anodes in the IC. The attenuated beam run is shown in black and two well-separated loci are clearly visible signifying the presence of both ${}^{18}\mathrm{F}$ and ${}^{18}\mathrm{O}$. Circles (triangles), both red online, correspond to observed ${}^{19}\mathrm{Ne}$ (${}^{18}\mathrm{F}$) events when the separator was tuned to recoils. Squares (blue online) correspond to ${}^{19}\mathrm{F}$ recoils during the separate ${}^{18}\mathrm{O}$ beam run.

Figure 2

MCP time of flight spectrum showing where ${}^{18}\mathrm{F}$ was observed during the attenuated beam run (blue) and events when tuned to ${}^{19}\mathrm{Ne}$ recoils (red). Leaky beam during the recoil run is clearly identified as it appears in the ${}^{18}\mathrm{F}$ peak.

Figure 3

Energy loss plot from the IC’s first anode showing the position of ${}^{18}\mathrm{O}(p,\gamma ){}^{19}\mathrm{F}$ (solid, blue) and ${}^{19}\mathrm{Ne}$ events (shaded, red).

Figure 4

Fractional resonant and direct capture contributions to the total ${}^{18}\mathrm{F}(p,\gamma ){}^{19}\mathrm{Ne}$ reaction rate at ONe nova peak temperatures. The upper graph shows the 665 keV rate (solid line, red) using the experimental upper limit taken from Rehm et al. [14] while the lower graph has the rate using the current work. The direct capture contribution (dashed line, green) and the 330 keV resonance (fine dashed line, blue) contribution was computed using data from 7, 10, 17, 18, 28, respectively. Note that at lower temperatures the resonance at 38 keV together with a subthreshold state at $-122\mathrm{keV}$ account for the remaining contribution to the total rate [29].