The 7Be(n,alpha)4He 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 7Be(n,alpha)4He reaction cross section across a wide energy range to better understand its role in the Cosmological Lithium Problem, revealing that this reaction likely has a minor impact on Big Bang Nucleosynthesis.

Contribution

First measurement of the 7Be(n,alpha)4He cross section from 10 meV to 10 keV neutron energy using n_TOF at CERN, providing new data for cosmological models.

Findings

Cross section measurements are consistent with previous data at thermal energies.

Results suggest the reaction plays a minor role in Big Bang Nucleosynthesis.

Current cross section estimates are inadequate for BBN calculations.

Abstract

The energy-dependent cross section of the 7Be(n,alpha)4He reaction, of interest for the so-called Cosmological Lithium Problem in Big Bang Nucleosynthesis, has been measured for the first time from 10 meV to 10 keV neutron energy. The challenges posed by the short half-life of 7Be and by the low reaction cross section have been overcome at n_TOF thanks to an unprecedented combination of the extremely high luminosity and good resolution of the neutron beam in the new experimental area (EAR2) of the n_TOF facility at CERN, the availability of a sufficient amount of chemically pure 7Be, and a specifically designed experimental setup. Coincidences between the two alpha-particles have been recorded in two Si-7Be-Si arrays placed directly in the neutron beam. The present results are consistent, at thermal neutron energy, with the only previous measurement performed in the 60's at a nuclear reactor. The energy dependence here reported clearly indicates the inadequacy of the cross section estimates currently used in BBN calculations. Although new measurements at higher neutron energy may still be needed, the n_TOF results hint to a minor role of this reaction in BBN, leaving the long-standing Cosmological Lithium problem unsolved.