Experimental investigation of the 7Li Cosmological Problem by means of the Trojan Horse Method applied to the 7Be(n,a)4He reaction.

With the term “7Li Cosmological Problem” we refer to the discrepancy between the primordial abundance of 7Li, observed in metal-poor halo stars, and the abundance predicted by the Big Bang Nucleo-synthesis (BBN). 7Li is mainly produced from 7Be that undergoes the electron capture process. We thus expect the primordial 7Li abundance to be essentially determined by the 7Be production and destruction rates. In order to find a solution of this problem, in the present work, the case of the 7Be destruction induced by the (n,a) reaction is investigated at the energies typical of the primordial nucleosynthesis by means of the Trojan Horse Method applied to the 2H(7Be,aa)p QF reaction. In the last 5 years many works have been published regarding the 7Be destruction channels involving neutrons, i.e., the 7Be(n,p)7Li, and 7Be(n,α)4He reactions. In particular, the (n,a) reaction channel has been the subject of recent studies. In this work I am going to present a new cross section measurement for the 7Be(n,a)4He performed via the Trojan Horse Method (THM) applied to the Quasi-Free (QF) 2H +7Be reaction. The 7Be(n,a)He cross-section has been measured in a single experiment from 2 MeV down to cosmological energies. The present study represents an important step in the application of THM to reactions between RIBs and neutrons and opens up new windows on many possible future applications. Our result is in remarkable agreement with direct measurements currently available in literature and with the more recent 7Be(n,a)4He measurements discussed in Hayakawa et al. (2020). The values of the 7Be(n,a)4He cross section are in good agreement with those obtained by Lamia et al. (2019) analyzing a complementary set of data collected during the same experiment. Combining both data sets will help in the next future to reduce the uncertainties in the evaluation of the reaction rates in the energy range of interest for the BBN, thus allowing for an overall better characterization of this reaction.