Measurement of the ²⁴¹Am Neutron Fission Cross Section

The fission cross section of ²⁴¹Am has been measured from 0.02 eV to 20 MeV using time-of-flight techniques at the Oak Ridge Electron Linear Accelerator. A “honeycomb” fission ionization chamber that contained six deposits totaling 14.3 mg of ²⁴¹Am, six deposits totaling 116 mg of ²³⁵U, and a single deposit of ²⁵²Cf, which served as a monitor for the chamber performance, was used. The ²³⁵U fission served as the cross-section standard for energies above 101 keV while ⁶Li(n, α), normalized to ²³⁵U fission in the 7.8- to 11.0-eV interval, served as a shape standard below 101 keV. Approximately 700 h of data were obtained at a flight path distance of 9.1 m, primarily with 40-ns bursts. Because the fission cross section of ²⁴¹Am is very small in the midrange of neutron energies, particular attention was paid to correction of backgrounds, particularly inscattered neutron-induced events. The fission resonance integral was found to be 14.1 ± 0.9 b.