The n_TOF collaboration published a study providing for the first time a precise measurement of
the 7Be(n,?)He4 reaction, one of the processes involved in the production of lithium during the Big Bang Nucleosynthesis.
The theory of the production of nuclei during the early phases of the universe – known as Big Bang Nucleosynthesis (BBN) – states that, right after the Big Bang, all the lightest and most abundant elements of the universe (hydrogen, helium and lithium) were formed. But, while observations and theory are perfectly aligned as regards hydrogen and helium, the amount of lithium that we actually observe is about three times smaller than that deduced by theoretical predictions. This discrepancy is known as the cosmological lithium problem.
One explanation could be linked to the transformation of an unstable isotope of beryllium – beryllium-7 – into lithium. The production and destruction of beryllium-7 essentially regulate the abundance of cosmological lithium: the more beryllium-7, the more lithium there is, but if beryllium somehow gets destroyed, the amount of lithium consequently decreases. Therefore, a possible explanation for the higher theoretical value could be an underestimation of the destruction of primordial beryllium-7, in particular in reactions with neutrons.
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 and the availability of a sufficient amount of chemically pure Be7.
The energy dependence 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 at a minor role (ten times smaller than that used in theoretical calculations) of this reaction in BBN, leaving the long-standing cosmological lithium problem unsolved
