Year
1983
Abstract
Fast coincidence techniques based on detection of fissions are oft3n used to passively or actively assay SU'1. Severn! commercial instruments based on these techniques have been available since the early Seventies: ISAS, I5AF, and Random Driver. These techniques offer high sensitivities to ST1. They are applicable to small, as well as large size containers, e.g., 55 gallon drums. Like other fID/V techniques, geometrical and matrix effects can have large contributions to the measurement error. However, algebraic combinations of coincidence multiplicities can be formed which are relatively independent of detection efficiency, yet proportional to the amount of nuclear material being assayed. Considering these combinations, in addition to the coincidence rates alone as fission signatures, has the demonstrable advantage that the assay results are comparatively independent of sample geometry or even matrix. Systematic measurements to assess the degree of reduction of matrix and geometrical effects and its limitation were recently completed. The measurement results show that coinci lence ratios, and most specifically, twoto thres-fold ratio, reduce matrix and geometrical effects. The effect of polyethylene and sand on the standard signal was reduced by a factor of two to three. Spatial effects were reduced by 50-30*. fIon-linearity, i.e., variation in signal per gram 'J, for a soft neutron spectrum was reduced by 20-30*, with small loss of sensitivity.