Study of the 12C(p,g)13N at astrophysical energies at LUNA

abstract: The 12C/13C ratio is a sensitive indicator of the degree of stellar nucleosynthesis and thus it can be used as a tracer of galactic chemical evolution. Nevertheless, the C isotopic ratio variation after the dredge-up phenomenons occuring in Asymptotic Giant Branch (AGB) and Red Giant Branch (RGB) stars is highly dependant on the adopted rate for the proton capture reaction on 12 C. In a RGB star, the Gamow peak of the reaction lies between 20 and 70 keV. Given the exponential drop of the cross section, the reactions at such low energies are very difficult to measure because of the very low signal counting rate. Nevertheless, a precise measurement of the 12C(p, γ)13N reaction magnitude is necessary to make reliable predictions about the evolution of RGB stars. On the contrary, the Gamow peak for an AGB stars lies between 50 and 150 keV. This energy range can be measured through the detection of prompt γ-rays. Measurements of the 12C(p,γ)13N reaction cross section were performed at the Laboratory for Underground Nuclear Astrophysics (LUNA), located at the Labaratori Nazionali del Gran Sasso (LNGS) in Italy. Being located underground, the γ-ray background is suppressed by more than three orders of magnitude, thus providing a unique environment for low-energy measurements of reaction cross sections. Prompt γ-rays associated with the formation of 13 N nuclei were analysed to determine the non-resonant contribution to the reaction cross section. The total non-resonant S-factor was determined at energies between E cm ≈ 75 − 350 keV, obtained with great precision. Nevertheless, the preliminary results are discordant with the literature data. The reason of such a discrepancy is still unkown and has to be understood in future analysis.