ASTROPHYSICAL NUCLEAR REACTIONS ON NEON ISOTOPES AT LUNA

This thesis reports the direct measurements of the 22Ne(α,γ)26Mg and 20Ne(p,γ)21Na reactions at astrophysical energies of interest. The 22Ne(α,γ)26Mg reaction competes with the 22Ne(α,n)25Mg reaction which is the main source of neutrons for the s-process in low-mass Asymptotic Giant Branch and massive stars. At temperatures T < 300 MK where the (α,γ) channel becomes dominant, the rate of the 22Ne(α,γ)26Mg reaction is influenced by several resonances studied only indirectly. The first part of this thesis concerns the direct measurement of one of these resonances, Er = 334 keV, which so far was studied only indirectly leading to six orders of magnitude range of possible values for its resonance strength. The experiment has been performed at LUNA (Laboratory for Underground Nuclear Astrophysics) using the intense alpha beam of the LUNA 400 kV accelerator and a windowless gas target combined with a high-efficiency BGO detector. In the present study, an upper limit of 4.0·10−11 eV has been determined for the resonance strength. Taking into account these results, an up-dated 22Ne(α,γ)26Mg thermonuclear reaction rate was obtained and its role on the predicted 25Mg/26Mg ratio of a 5M⊙ AGBs was investigated. The data show a decrease by a factor of 15 of the intershell 25Mg/26Mg ratio. The 20Ne(p,γ)21Na is the slowest reaction of the NeNa cycle. It determines the velocity of the cycle and defines the final abundances of the isotopes synthesized in this cycle. The uncertainties on the NeNa cycle are affected by the 20Ne(p,γ)21Na reaction rate. The main goal of the second part of this thesis was the direct measurement of the Ecm = 366 keV resonance which dominates the total rate in the temperature range between 0.2 GK and 1 GK. The measurement has been performed at LUNA using the windowless gas target and two high-purity germanium detectors placed at different positions. This measurement allowed to reduce the uncertainty on the strengths of the 366 keV resonance from 18% to 7%. These results were used to update the 20Ne(p,γ)21Na reaction rate.