Energy Dependence of the Relative Yield and Period of Individual Groups of Delayed Neutrons in 239Pu Fission in Primary-Neutron Energy Range 0.37–4.97 MeV

The KG-2.5 cascade generator at the State Science Center of the Russian Federation – Physics and Power Engineering Institute is used as the basis to measure the relative yield and period of groups of delayed neutrons in fissioning of 239Pu in the incident-neutron energy range 0.37–4.97 MeV. A comparative analysis of the data obtained in the present work and in works by other authors is made in terms of the average half-life of the delayed-neutron precursor nuclei. This approach made it possible to show for the first time the character and scale of variation in the values of the group parameters of delayed neutrons as a function of the excitation energy of the fissioning compound nucleus. A substantial energy dependence of group parameters in fissioning of 239Pu by neutrons is found. It is expressed as a decrease in the average half-life of precursor nuclei by 10% in the primary-neutron energy range 2.85 eV–5 MeV. It is established that the relative change Δρ/ρ in the reactivity is approximately equal to the relative change in the average half-life of the delayed-neutron precursor nuclei / (provided that in both cases the same sets of group parameters are used).