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Nuclear-weapon-material detection via gamma-ray sensing is routinely applied,for example, in monitoring cross-border traffic. Natural or deliberateshielding both attenuates and distorts the shape of the gamma-ray spectra ofspecific radionuclides, thereby making such routine applications challenging.We develop a Lagrange multiplier (LM) test for shielding. A strong advantage ofthe LM test is that it only requires fitting a much simpler model that assumesno shielding. We show that, under the null hypothesis and some mild regularityconditions and as the detection time increases, LM test statistic for(composite) shielding is asymptotically Chi-square with the degree of freedomequal to the presumed number of shielding materials. We also derive the localpower of the LM test. Extensive simulation studies suggest that the test isrobust to the number and nature of the intervening materials, which owes to thefact that common intervening materials have broadly similar attenuationfunctions.

Testing for shielding of special nuclear weapon materials