1. A method of assaying a waste container comprising the steps of: forming a plurality of slices through a volume of the waste container; performing a relatively fast passive prescan to measure a level of gamma-ray radioactivity of each of the plurality of slices; identifying at least some slices of the plurality of slices having a relatively high level of gamma-ray radioactivity; determining a passive assay time for the identified slices based upon the measured radioactivity; and performing a relatively slow passive scan of the identified slices based upon the determined passive assay time to quantify a source of the measured radioactivity by weight.