NON-DESTRUCTIVE ASSAY OF ²⁴²PU BY RESONANCE NEUTRON CAPTURE

For the accurate assay of plutonium by neutron correlation measurements, especially for material derived from high-burnup reactor fuel, the content of 242PU in a sample must be determined. Since242PU has a long half-life (387,000 yr.) and decays to 238U by alpha particle emission with the accompanying emission of only weak, low-energy gamma rays, gamma-ray spectrometry methods which are ordinarily employed to determine the isotopic composition of a plutonium sample are not feasible for 242PU -The existence of a resonance in the neutron capture cross section of 242PU at 3H energy of 2.67 electron volts (eV) with a large (72,000 barn) cross section affords the possibility for the quantitative assay of this isotope by epithermal neutron capture. Essential for this purpose is an appropriately designed geometry of neutron moderators and absorbers which will provide maximum flux in the eV region while suppressing thermal neutron capture by the fissile plutonium isotopes. Signatures for neutron capture in 242PU include the decay of 242PU (4.9 hour), prompt capture gamma rays (total energy 5.034 MeV), and the decay of an isomeric state (330 nanosecond). Experiments to determine the feasibility of this approach are currently in progress.