An experiment at CERN’s neutron time-of-flight (n_TOF) facility has filled in a missing piece of the cosmological-lithium problem puzzle, according to a report published in Physical Review Letters. Along with a few other light elements such as hydrogen and helium, much of the lithium in the universe is thought to have been produced in the very early universe during a process called Big-Bang nucleosynthesis (BBN). For hydrogen and helium, BBN theory is in excellent agreement with observations. But the amount of lithium (7Li) observed is about three times smaller than predicted – a discrepancy known as the cosmological-lithium problem.

The n_TOF collaboration has now made a precise measurement of one of the key processes involved – 7Be(n,α)4He – in an attempt to solve the mystery. The production and destruction of the unstable 7Be isotope regulates the abundance of cosmological lithium, but estimates of the probability of 7Be destruction via this channel have relied on a single measurement made in 1963 of thermal energies at the Ispra reactor in Italy. Therefore, a possible explanation for the higher theoretical value could be an underestimation of the destruction of primordial 7Be, in particular in reactions with neutrons.

Now, n_TOF has measured the cross-section of the 7Be(n,α)4He reaction over a wide range of neutron energies with a high level of accuracy. This was possible thanks to the extremely high luminosity of the neutron beam in the recently constructed experimental area (EAR2) at the n_TOF facility.

The results indicate that, at energies relevant for BBN, the probability for this reaction is 10 times smaller than that used in theoretical calculations. The destruction rate of 7Be is therefore even smaller than previously supposed, ruling out this channel as the source of the missing lithium and deepening the mystery of the cosmological-lithium problem