Year
1975
Abstract
Small samples are assayed for fissile content by cylically bombarding them with 300-500-keV neutron pulses and counting between pulses delayed neutrons from fission. Neutron absorption in the sample,with resultant response nonlinearity,is mimimized by shielding the sample from thermal and resonance- region neutrons with cadmium and B^C. A production small-sample assay system has been built using the above principle. Biases of the order of 1-2% can occur in the assay of samples in this system, due to gross differences in makeup between assay samples and calibration standards. These biases are minimized or removed by matching standards and samples and by measurement o£ the effect of sample parameters on the assay response. Typical parameters studied and their effects are: fissile quantity (+0.077%/g fissile), inert dilution (up to 2% in a 5-cm^ vial), sample height (1% over a range of 25 mm), hydrogen content (+O.ll/mg of H), and thorium content (+0.12%/g of Th). Assay precision as good as 0.34% (1 a) has been achieved.