Neutron Capture Cross Section of Plutonium-240

The average ²⁴⁰Pu capture cross section was measured from 200 eV to 350 keV. The cross section was normalized at thermal-neutron energies (0.02 to 0.03 eV), and this normalization was confirmed at the 1.06-eV resonance by the black resonance technique. The source of pulsed neutrons was the Oak Ridge Electron Linear Accelerator. The capture gamma-ray detector used was the “total energy detector,” which is a modification of the Moxon-Rae detector. The shape of the neutron flux was measured relative to the ¹⁰B(n, α) cross section up to 2 keV and the ⁶Li(n, α) cross section at higher neutron energies. The results of the measurement define the average capture cross section of ²⁴⁰Pu over a wide neutron energy range to an accuracy of ∼8%, which is significantly better than previously known. The results indicate that the ENDF/B-IV evaluation is ∼25% low above 30-keV neutron energy. The cross section is important in fast plutonium-fueled reactors.