Neutron-Induced Fission Cross Section of Plutonium-240 in the Energy Range from 10 keV to 10 MeV

The neutron-induced fission cross section of ²⁴⁰Pu was measured in the neutron energy range from 10 keV to 10 MeV using the 7-MV Van de Graaff and the electron linear accelerator of the Central Bureau for Nuclear Measurements as pulsed neutron sources, which delivered monoenergetic and continuous neutron spectra, respectively. The neutron-induced fission events were detected with a parallel plate ionization chamber that provided a fast and narrow output signal allowing nanosecond timing, but where the time integral of the pulse contained, at the same time, the energy information of the ionizing particle. This detector permitted a high discrimination between alpha particles and fission fragments at an alpha emission rate of some 10⁷ s⁻¹. The fission cross-section data below 400 keV are especially remarkable since they were taken with an energy resolution almost one order of magnitude better than any other published data set. In this region, large structures in the fission cross section due to Class II states in the second well of the double-humped fission barrier were found. The spontaneous fission half-life of ²⁴⁰Pu was measured to be (1.15 ± 0.03)·10¹¹ yr.