Year
2006
Abstract
The accuracy in determining the fissile content of waste drums and packages by gamma spectrometry is affected by several factors such as: matrix composition; matrix density; matrix homogeneity; the gamma source spatial distribution; the relative isotopic composition, and, emitter lump size. The effect of some of these factors may be calculated from the results of ancillary measurements such as gamma transmission for matrix density/composition, differential absorption for lump correction and low energy spectral analysis for isotopics. Factors affecting the result that remain unknown need to be taken into account in the total measurement uncertainty (TMU). A similar problem is encountered in the determination of 'less than' (LT) values to be quoted where the material is not detected but the effect of these factors should be incorporated. A method has been devised to calculate uncertainties and LT values for unknown waste streams for a gamma assay system. Matrix correction is applied with a transmission or weight-by-volume measurement to provide a result corrected assuming a volume weighted average (VWA) distribution of activity. The TMU contribution from a non-uniform source distribution is calculated from the far-field correction factor using computer modelled spatial response results. Where a lump correction has been applied, such as by differential absorption, the measurement uncertainties are propagated through the correction algorithms, otherwise a TMU contribution from the lump effect is calculated. The method will incorporate MGA/U isotopic results where available and propagate the associated uncertainties. For a multiple detector system the methods of combining results from individual detectors is described. This might include cases where some detectors provide a measured value and others a LT value, in which case an uncertainty contribution from the lessthan results is included in the TMU. This work represents an important step in refining the definition and statement of TMUs.