Nuclear Physics Division

Supported by CEA's "digital simulation" cross-disciplinary program, Irfu, the Laboratoire National Henri Becquerel of DRT and the Service d'Étude des Réacteurs et de Mathématiques Appliquées of DES teamed up to carry out a thorough review of calculations of antineutrino spectra from nuclear reactors. A complete revision of the summation method lays a new and solid foundations for these calculations, and was featured as the Physical Review C  journal editor’s suggestion [1] on November 27, 2023. This revision incorporates numerous improvements in the beta decay modeling of the thousands of branches making up a reactor antineutrino spectrum, and in the use of nuclear evaluated data. It also quantifies all the systematic effects known to influence the calculations, providing for the first time a complete uncertainty model. This major advance now makes the summation model, long criticized for being approximate and incomplete, a robust tool for predicting reactor antineutrino spectra and for interpreting current and future experimental measurements. This work will likely stimulate targeted research to check and improve the experimental inputs, with potentially wide-ranging impact, from weak-interaction physics to many aspects of nuclear reactor science and technology. It also sheds interesting light on the origin of reactor antineutrino anomalies [2,3].

For the first time, an experiment has provided key observations on the spectroscopy of the neutron-rich unbound oxygen nuclei (proton number Z = 8), oxygen 28 (N = 20) and its neighboring isotope at N = 19, oxygen 27.  They were produced in high-energy reactions and observed by direct detection of their decay products, ²⁴O and three or four neutrons. The study shows that it is possible to constrain the parameters of ab initio interactions from the energy differences of the observed states with respect to the last bound isotope - ²⁴O (N = 16). These groundbreaking results were published in the journal Nature [Nat23].

Given the complexity of studying unbound nuclei, an exceptional detection system was implemented at the world's most powerful radioactive ion beam facility: RIBF in Japan. The data were obtained by an international collaboration (Samurai21) of around a hundred physicists (from 36 laboratories), including a team* of physicists from Irfu who were responsible for operating a key detector for the measurements, Minos. The experiment, carried out on the Samurai area of the RIBF (Radioactive Ion Beam Factory) facility at RIKEN in Japan, was piloted by groups of physicists from Titech (Tokyo Institute of Technology) and by the RIKEN-RIBF teams.