High-energy break-up of 6Li as a tool to study the Big-Bang nucleosynthesis reaction 2H(alpha,gamma)6Li

Abstract

The recently claimed observations of non-negligible amounts of 6 Li in old halo stars have renewed interest in the Big-Bang Nucleosynthesis (BBN) of 6 Li. One important ingredient in the predicted BBN abundance of 6 Li is the low-energy 2 H(α,γ) 6 Li cross section. Up to now, the only available experimental result for this cross section showed an almost constant astrophysical S-factor below 400 keV, contrary to theoretical expectations. We report on a new measurement of the 2 H(α,γ) 6 Li reaction using the break-up of 6 Li at 150 A MeV. Even though we cannot separate experimentally the Coulomb contribution from the nuclear one, we find clear evidence for Coulomb-nuclear interference by analyzing the scattering-angular distributions. This is in-line with our theoretical description which indicates a drop of the S24-factor at low energies as predicted also by most other models. Consequently, we find even lower upper limits for the calculated primordial 6 Li abundance than before.

Standard big bang nucleosynthesis and primordial CNO abundances after Planck

“…Very recently, a direct measurement has been performed at LUNA [1]. The results of this very difficult experiment agree well with those from the indirect method [32]. So, this confirmed low cross section comforts the prediction of the BBN calculations of a low primordial 6 Li value, 6 Li/H = 10 −14 .…”

“…The calculation of 6 Li/H depends directly on the D(α, γ) 6 Li reaction rate that was plagued with by large uncertainty [3], New measurement of the D(α, γ) 6 Li which is the main way to produce primordial 6 Li have been performed. In the absence of direct measurements at BBN energy, [32] used the Coulomb breakup technique to extract the cross section. Very recently, a direct measurement has been performed at LUNA [1].…”

“…in [17]) we generally adopted f =3, except when the uncertainty is not provided but is obviously smaller e.g. "Ham10" [32] or "Nag06" where we adopted a 40% uncertainty (f =1.4). This may look arbitrary, but one of the main goal of this work is to identify potentially influential reaction rates that may have to be improved in a subsequent stage.…”

Cosmic rays during BBN as origin of Lithium problem

“…Primordial 7 Li abundance from measurements of metal-poor halo stars is 7 Li/H= (1 ∼ 2) × 10 −10 [2]- [9], while prediction by the standard BBN (SBBN) model is three to five times higher: 7 Li/H= (5.24 +0.71 −0.67 ) × 10 −10 [10]. Meanwhile 6 Li abundance from observation is 6 Li/H≈ 6 × 10 −12 [3], a factor of about 1000 higher than the SBBN model prediction [11].…”