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The Mystery of Cosmological Lithium Remains Unsolved

​The deficit in lithium observed in the early universe is not explained by the poor understanding of a nuclear reaction involving beryllium-7. In an experiment carried out at CERN, a collaboration involving scientists from IRFU has characterized, with unparalleled precision, neutron capture by 7Be, a reaction with implications in primordial nucleosynthesis as much as in nuclear technology.

Published on 18 January 2017

​The chemical composition of certain stars is very close to the early universe. While their content in hydrogen and helium is in perfect concordance with theoretical predictions, their content in lithium (7Li, the most common stable isotope) is approximately three times smaller than expected. A team of researchers has been seeking to determine if this anomaly could be explained by a deficit in the precursor of this nucleus, a radioactive isotope of beryllium (7Be). To do so, they studied the capture of a neutron by 7Be, producing helium-4 and an α particle.

At a new experimental area of CERN, the neutron Time-Of-Flight (n_TOF) collaboration bombarded a sample of 7Be with neutrons and measured the neutron capture rate by 7Be as a function of the energy of incident neutrons. Their results are consistent with the theory. The cosmological lithium anomaly therefore remains unsolved.

This work was achieved using a new neutron source, more intense than the preceding one, in partnership with INFN, Italy, and the Institute for Nuclear Physics at Université Paris Saclay (IPN), in particular. The beryllium-7 nuclei were produced by Institut Paul Scherrer, Switzerland.

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