First measurement of the low-energy direct capture in Ne 20 (p,γ) Na 21 and improved energy and strength of the Ec.m.=368 keV resonance

The Ne20(p,γ)21Na reaction is the slowest in the NeNa cycle and directly affects the abundances of the Ne and Na isotopes in a variety of astrophysical sites. Here we report the measurement of its direct capture contribution, for the first time below Ec.m.=352 keV, and of the contribution from the Ec.m.=368 keV resonance, which dominates the reaction rate at T=0.03-1.00 GK. The experiment was performed deep underground at the Laboratory for Underground Nuclear Astrophysics, using a high-intensity proton beam and a windowless neon gas target. Prompt γ rays from the reaction were detected with two high-purity germanium detectors. We obtain a resonance strength ωγ=(0.112±0.002stat±0.005sys)meV, with an uncertainty a factor of 3 smaller than previous values. Our revised reaction rate is 20% lower than previously adopted at T<0.1 GK and agrees with previous estimates at temperatures T≥0.1 GK. Initial astrophysical implications are presented.