JEFF4: a major step forward in nuclear data for science and applications

The Joint Evaluated Fission and Fusion (JEFF) Library combines the existing experimental and theoretical knowledge on nuclear reactions and nuclear decays and makes it available in standard formats to serve a wide user community. The library is the result of an international collaborative effort led by the Nuclear Energy Agency Data Bank. It ensures the scientific quality, transparency, and usability of JEFF data for a wide range of applications, including reactor design, safety assessments, fusion research, medical isotope production, space and earth exploration, and nuclear science.

The JEFF-4.0 library was released in June 2025. This latest release represents a significant achievement for the nuclear data user community. It contains an impressive amount of information collected over many years in different types of experiments and facilities. The available data include neutron, photon and charged particle interaction data as well as radioactive decay data and fission product yields produced when actinides undergo nuclear fission. A sub-library dedicated to thermal neutron scattering provides a comprehensive dataset for 84 materials including 24 in elemental form and 60 compounds.

The Institut Laue-Langevin has made a major contribution, over many years, to the success of this project. Benefiting from the quality of its wide range of instruments, the ILL has played a key role in enhancing the precision of some neutron-induced observables. High-intensity diffractometers (D20, D4, D1B) and time-of-flight spectrometers (IN4, IN5, IN6-SHARPER) are among the instruments utilized for improving the thermal scattering laws of water (H₂O), uranium oxide (UO₂), thorium dioxide (ThO₂) and zircaloy (Zy4) which describe how slow neutrons interact with materials as a function of temperature. Thanks to its high resolution, the ILL's LOHENGRIN recoil mass spectrometer (PN1) is a unique instrument that has allowed highly accurate measurements of the mass, kinetic energy and nuclear charge distributions of fission fragments from uranium (²³³U, ²³⁵U) and Plutonium (²³⁹Pu, ²⁴¹Pu). Examples of fission product mass yields measured for ²³⁵U(n_th,f) are compared to JEFF-4.0 in the figure below. In parallel, the neutron beam available in the PF1B experimental area at the ILL has been successfully employed to measure the delayed neutrons emitted after the fission of the same fissile nuclei. In total, these experimental programs represent months of neutron beam time.