Direct Measurement of the Astrophysical ${}^{19}\mathrm{F}(p,\alpha \gamma ){}^{16}\mathrm{O}$ Reaction in the Deepest Operational Underground Laboratory

Fluorine is one of the most interesting elements in nuclear astrophysics, where the ${}^{19}\mathrm{F}(p,\alpha ){}^{16}\mathrm{O}$ reaction is of crucial importance for Galactic ${}^{19}\mathrm{F}$ abundances and CNO cycle loss in first generation Population III stars. As a day-one campaign at the Jinping Underground Nuclear Astrophysics experimental facility, we report direct measurements of the essential ${}^{19}\mathrm{F}(p,\alpha \gamma ){}^{16}\mathrm{O}$ reaction channel. The $\gamma$-ray yields were measured over $E_{\mathrm{c}.\mathrm{m}.}=72.4–344\mathrm{keV}$, covering the Gamow window; our energy of 72.4 keV is unprecedentedly low, reported here for the first time. The experiment was performed under the extremely low cosmic-ray-induced background environment of the China JinPing Underground Laboratory, one of the deepest underground laboratories in the world. The present low-energy $S$ factors deviate significantly from previous theoretical predictions, and the uncertainties are significantly reduced. The thermonuclear ${}^{19}\mathrm{F}(p,\alpha \gamma ){}^{16}\mathrm{O}$ reaction rate has been determined directly at the relevant astrophysical energies.

Figure 1

$\gamma$-ray spectra of the ${}^{19}\mathrm{F}+p$ experiments measured by a $4\pi$ BGO array at a proton energy of $E_p=130\mathrm{keV}$. The aboveground (at CIAE) and underground (at JUNA) spectra are shown for comparison. Two background lines at 1460.8 keV (from ${}^{40}\mathrm{Ar}$) and 2614.5 keV (from ${}^{208}\mathrm{Tl}$) are used for energy calibration. The $\gamma$ rays induced by the ${}^{12}\mathrm{C}$, ${}^{13}\mathrm{C}$, and ${}^{11}\mathrm{B}$ contaminants are also indicated.

Figure 2

Typical $\gamma$-ray spectra of the ${}^{19}\mathrm{F}(p,\alpha \gamma ){}^{16}\mathrm{O}$ reaction measured at JUNA with a $4\pi$ BGO array: (a) at $E_p=310$, (b) at $E_p=200$, (c) at $E_p=88\mathrm{keV}$, respectively. For (a), the inset shows the reaction scheme [18]; for (c), the inset shows the rebinned net spectrum after subtracting the normalized background contribution (blue shaded), which was evaluated by a pure Fe target (covered also with a 50-nm Cr layer) run. Here, the $\gamma$-peak positions for the possible contaminant reactions from ${}^2\mathrm{H}$ and ${}^{13}\mathrm{C}$ are indicated by the arrows and integral region for the 6130-keV $\gamma$ ray by the curly bracket.

Figure 3

Experimental yields for the ${}^{19}\mathrm{F}(p,\alpha \gamma ){}^{16}\mathrm{O}$ reaction measured by the JUNA experiment (with statistical uncertainties only). The results for two implanted targets are shown separately, where the target 2 data are scaled down by a factor of 1000. The geant4 simulated curve is indicated by the solid line for each target, by using the $R$-matrix calculated $S$ factor as shown in “fit1” in Fig. 4.

Figure 4

Astrophysical $S$ factors of the ${}^{19}\mathrm{F}(p,\alpha \gamma ){}^{16}\mathrm{O}$ reaction derived from the JUNA experiment (with statistical uncertainties only). The previous experimental data (‘SP00 Expt’) [23] and theoretical predictions (‘SP00 Calc’) [22, 23] are shown for comparison. Here we show the exact $S(E)$ without taking into account the individual experimental target thicknesses. Three most probable $R$-matrix fits are shown. See text for details.

Figure 5

Ratio of present (labeled as JUNA) relative to Spyrou et al.’s rate (labeled as SP00 [23]). The corresponding ratios for deBoer et al.’s rate [22] and Zhang et al.’s rate [18] are also shown for comparison. The associated uncertainties are shown as the colored bands. The inset shows the ratio between the present and deBoer et al.’s rates. For more information, refer to Supplemental Material [32].