A PGNAA method provides the nondestructive assay of a homogeneous or a non-homogeneous contaminant (50), such as a hazardous RCRA metal, in a container (36), such as a sealed 55 gallon waste drum (36). The method includes employing an interrogation chamber (40) having a wall (42) made of a neutron-reflecting material; and disposing a platform (30) within the interrogation chamber (40). At least one of rotating (31) the platform (30), and elevating (32) the platform (30) is employed. The method also includes disposing the container (36) on the platform (30) and within the interrogation chamber (40); and employing at least one neutron source (46) between the wall (42) of the interrogation chamber (40) and the container (36), with thermal neutrons (52) reacting with a container (36) and thereby effecting an emission of thermal-neutron induced gamma radiation (54) from the container (36). The container (36) is irradiated with a fast-neutron burst (44) from the neutron source (46) or as reflected by the wall (42). The fast-neutron burst (44) is characterized by a duty cycle and is thermalized and/or moderated by the container (36). Gamma radiation data is acquired (10, 11) indicative of the number or intensity of gamma rays and energies of the gamma rays during a predetermined time interval. The acquiring step includes a separate identification of thermal neutron-induced gamma rays, and the gamma radiation data is acquired over only a fraction of the duty cycle. The gamma radiation is analyzed (14, 15, 16) to detect the presence and determine the amount of the non-homogeneous contaminant (50) in the container (36).
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