The $^{7}$Be($\boldsymbol{n,p}$)$^{7}$Li reaction and the Cosmological Lithium Problem: measurement of the cross section in a wide energy range at n_TOF (CERN)

TL;DR

This study measures the $^{7}$Be($n,p$)$^{7}$Li reaction cross section across a wide energy range to better understand its role in Big Bang Nucleosynthesis and the Cosmological Lithium Problem, providing new experimental data at CERN.

Contribution

First measurement of the $^{7}$Be($n,p$)$^{7}$Li cross section in the energy range relevant for BBN, resolving discrepancies between previous data and informing cosmological models.

Findings

Higher cross section at low energies compared to evaluations.

Results align with the $^{7}$Li($p,n$)$^{7}$Be reaction data.

Minor impact on resolving the Cosmological Lithium Problem.

Abstract

We report on the measurement of the $^{7}$Be($n, p$)$^{7}$Li cross section from thermal to approximately 325 keV neutron energy, performed in the high-flux experimental area (EAR2) of the n\_TOF facility at CERN. This reaction plays a key role in the lithium yield of the Big Bang Nucleosynthesis (BBN) for standard cosmology. The only two previous time-of-flight measurements performed on this reaction did not cover the energy window of interest for BBN, and showed a large discrepancy between each other. The measurement was performed with a Si-telescope, and a high-purity sample produced by implantation of a $^{7}$Be ion beam at the ISOLDE facility at CERN. While a significantly higher cross section is found at low-energy, relative to current evaluations, in the region of BBN interest the present results are consistent with the values inferred from the time-reversal $^{7}$Li($p, n$)$^{7}$Be reaction, thus yielding only a relatively minor improvement on the so-called Cosmological Lithium Problem (CLiP). The relevance of these results on the near-threshold neutron production in the p+$^{7}$Li reaction is also discussed.