Direct measurement of the break-out ¹⁹F(p, γ)²⁰Ne reaction in the China Jinping underground laboratory (CJPL)

Calcium production and the stellar evolution of first-generation stars remain fascinating mysteries in astrophysics. As one possible nucleosynthesis scenario, break-out from the hot carbon–nitrogen–oxygen (HCNO) cycle was thought to be the source of the calcium observed in these oldest stars. However, according to the stellar modeling, a nearly tenfold increase in the thermonuclear rate ratio of the break-out \(^{19}\) $_{}^{19}$ F(p,  \(\gamma\) $\gamma$ ) \(^{20}\) $_{}^{20}$ Ne reaction with respect to the competing \(^{19}\) $_{}^{19}$ F(p, \(\alpha\) $\alpha$ ) \(^{16}\) $_{}^{16}$ O back-processing reaction is required to reproduce the observed calcium abundance. We performed a direct measurement of this break-out reaction at the China Jinping underground laboratory. The measurement was performed down to the low-energy limit of \(E_\mathrm {c.m.}\) $E_{\mathrm{c}.\mathrm{m}.}$ = 186 keV in the center-of-mass frame. The key resonance was observed at 225.2 keV for the first time. At a temperature of approximately 0.1 GK, this new resonance enhanced the thermonuclear \(^{19}\) $_{}^{19}$ F(p,  \(\gamma\) $\gamma$ ) \(^{20}\) $_{}^{20}$ Ne rate by up to a factor of \(\approx\) $\approx$  7.4, compared with the previously recommended NACRE rate. This is of particular interest to the study of the evolution of the first stars and implies a stronger breakdown in their “warm” CNO cycle through the \(^{19}\) $_{}^{19}$ F(p,  \(\gamma\) $\gamma$ ) \(^{20}\) $_{}^{20}$ Ne reaction than previously envisioned. This break-out resulted in the production of the calcium observed in the oldest stars, enhancing our understanding of the evolution of the first stars.