Neutron Capture and Fission Cross Sections of Plutonium-241

Neutron capture and fission cross sections of ²⁴¹Pu have been measured from 0.01 eV to 30 keV, and their ratio has been measured up to 250 keV. The cross sections were normalized at thermal-neutron energies (0.02 to 0.03 eV) to the ENDF/B-IV evaluation. The source of pulsed neutrons was the Oak Ridge Electron Linear Accelerator. The gamma-ray detector used to detect capture and fission events was the “total energy detector,” which is a low-efficiency detector whose average efficiency is forced to be proportional to the energy of the interacting gamma rays by weighting these events according to their pulse height in the scintillator. Fast-neutron scintillation detectors with pulse-shape discrimination were used to detect fission events. The shape of the neutron flux was measured relative to the ¹⁰B(n, α) cross section. The measurements are unique for ²⁴¹Pu in that absorption and fission were determined directly and simultaneously over a wide neutron energy range rather than indirectly by inferring capture from separate fission and total cross-section measurements. The results indicate that the neutron resonance region of the ENDF/B-IV evaluation underestimates capture by a factor of ∼2. Above the resonance region (∼100 eV), there are no previous measurements of the differential capture cross section. These cross sections are important in plutonium-fueled reactors.