A high energy resolution gas target setup to study selected NeNa cycle reactions

The neon-sodium cycle of hydrogen burning occurs in several astrophysical sites, such as asymptotic giant branch stars and novae, affecting the production of neon and sodium isotopes. To enhance the accuracy of predicted nucleosynthesis yields, there is a pressing need for new experimental investigations of the cross sections of the reactions involved in this cycle at energies relevant to astrophysics. The 400 kV accelerator at the Laboratory for Underground Nuclear Astrophysics (LUNA) provides a unique advantage relative to above-ground laboratories thanks to its deep underground location within the Gran Sasso National Laboratory (INFN-LNGS) in Italy. We performed experiments at LUNA on two of the reactions of the NeNa cycle: 20Ne(p,γ )21Na and 21Ne(p,γ )22Na using a high-purity gas target system for isotopically enriched gases coupled with two high-resolution germanium detectors, surrounded by copper and lead shielding to further reduce the natural background at LUNA. We describe the detailed characterization of the experimental setup performed through Monte Carlo simulations, and the method for the precise determination of resonance energies, giving improved values of Er = 127.3 ±0.5 keV, 271.4 ± 0.4 keV, 272.3 ± 0.4 keV, 291.5 ± 0.5 keV and 352.6 ± 0.4 keV. Additionally, decay branching ratios for the Ex = 7016.4 keV excited state in 22Na, and three new transitions (R → 4770 keV, R → 3059.4 keV and R → 583.05 keV) in the Er = 291.5 keV resonance, are also reported.