An increased urgency has been associated with the development of new and improved means for the non-destructive characterization of nuclear waste or weapon grade materials that might be hidden in large storage blocks or cargo containers. Passive detection methods based on measurements of neutrons and/or photons are either inapplicable or impractical in many such cases. Active interrogation with neutrons or photons in a variety of forms currently depends upon the observation of β-delayed neutrons following induced fission to provide a unique signature for fissile material. However, the shielding provided by a thick waste container envelope, typically made of concrete of high density as well as with high water content, could be so large that this method will fail or will have very low detection sensitivity. Equally for identical reasons only high-energy neutrons or high-energy gammas have high enough range to activate the enclosed actinides.
In this paper we provide quantitative estimates on the detection limits of fissile material in the case of typical nuclear waste containers surrounded by massive concrete layers with variable contents of hydrogen. Signals of prompt photo-fission neutrons, β-delayed gammas and β-delayed neutrons are examined for direct comparison. Advantages and disadvantages detecting all these observables are discussed. We show that β-delayed gamma rays can offer an increase in sensitivity for the detection of fissile materials by as much as 2-3 orders of magnitude (depending on the hydrogen content in the waste envelope) when compared to the detection of β-delayed neutrons. Due to the lack of experimental data on decay photons from photo-fission fragments an experimental program is proposed to confirm these investigations. |
