Energy dependence of prompt fissions neutron multiplicity in the 239Pu(n,f) reaction

Accurate multiplicities of prompt fission neutrons emitted in neutron-induced on a large energy range are essential for fundamental and applied physics. Measuring them to high precision for radioactive fissioning remains, however, an experimental challenge. In this work, the average-neutron multiplicity emitted in the 239Pu(n,f) reaction was extracted as function of the incident-neutron energy, over the range 1-700~MeV, with a technique, which allowed to minimize and correct for the main sources of and thus achieve unprecedented precision. At low energies, our data validate for the first time the ENDF/B-VIII.0 data evaluation with an independent measurement and reduce the uncertainty by up to $60\%$. This work opens up the possibility of measuring prompt fission neutron multiplicities on highly radioactive relevant for an essential component of energy production world-wide.