# The $^{7}$Be(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 measured 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 direct measurement of the $^{7}$Be(n,p)$^{7}$Li cross section from thermal to 325 keV at CERN, addressing previous data discrepancies and refining BBN models.

## Key findings

- Higher cross section at low energies compared to evaluations.
- Results align with the inverse $^{7}$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.