A Photoneutron Antimony-124-Beryllium System for Fissile Materials Assay

An ¹²⁴Sb-Be(γ,n) assay system has been designed and fabricated for the measurement of fissile content in small samples, fuel rods, and solutions. The instrument uses multiple ⁴He gas tube detectors to count fast neutrons induced in the assay sample by the ¹²⁴Sb-Be source neutrons. The ⁴He detectors are biased to count only neutrons with energies above the 26- and 360-keV source neutrons. Computer calculations were used to optimize the design of the irradiation unit, giving a factor of ∼3 improvement in the induced fission rate. The assay unit has been applied to the measurement of small UO₂ samples, pressurized water and boiling water reactor fuel rods, and high temperature gas-cooled reactor fuel.