The 19F(alpha, p)22Ne and 23Na(p,alpha)20Ne reactions in astrophysical environment with the Trojan horse Method

Abstract

Fluorine abundance in the universe is far from being reproduced by models. In particular, it is strongly underestimated with respect to what experimentally observed in AGB-stars, considered to be the main production sites for 19F, the only stable isotope of Fluorine. AGB-stars are composed by a degenerate CO core surrounded by a He and a H shell. Those shells are then divided by a thin layer (10^-2-10^-3 solar radii). If temperature is T =10^8K, the 14N produced in the CNO cycle can be processed into 19F by the chain of reactions 14N( alpha,gamma)18F(beta + )18N(p, alpha)19F. If the production pattern of 19F is quite clear, the same thing can not be said for its destruction pathways. In AGB-environment two reactions are considered to be the main destructions pattern for 19F: 19F(p, alpha)16O and 19F( alpha,p)22Ne. We focused on this second reaction: try to obtain informations about this reaction is, indeed, very di cult with direct nuclear physics experiments. That is due to the Coulomb barrier, that strongly reduces the cross-section (to some pico-nanobarn). The direct measurements of the 19F(alpha,p)22Ne reaction at lowest energy is at ECM=0.91 MeV. However, given the typical temperatures for a low-mass AGB-stars (2 ·10^8 K, 2- 4 solar masses ), the Gamow window for this reaction is between 390 and 800 keV. For cases like that, in the last 20 years indirect methods have been used. Among these, the Trojan Horse Method is one of the most important for astrophysical measurements. For these reasons we decided perform an experiment at Rudjer Boskovic Institut (Zagreb), that consisted in using a 6Li beam to investigate the two body reaction 19F( alpha,p)22Ne using the 6Li(19F,p 22Ne)2H three-body one. In this why we were able the extract a cross-section in absolute unit, and to calculate the S(E)-factor, for the reaction rate and for the astrophysical impact of this measure. In the end, some preliminary results of the study of the 23Na(p, alpha)20Ne via the 23Na(d,p22Ne)n using THM is also discussed: in particular we were able to extract the two-body cross-section in arbitrary units, and to guess the involved resonances. The reaction 23Na(p,alpha )20Ne reaction is of primary importance for sodium destruction inside the nucleosynthesis path in the A>20 mass region in intermediate mass (4- 8 solar masses ) AGB-stars. This reaction is also involved in the branching point of the Ne-Na cycle, responsible for hydrogen burning at high temperatures (T=20 -100 T_6).