Testing for shielding of special nuclear weapon materials

Nuclear-weapon-material detection via gamma-ray sensing is routinely applied, for example, in monitoring cross-border traffic. Natural or deliberate shielding both attenuates and distorts the shape of the gamma-ray spectra of specific radionuclides, thereby making such routine applications challenging. We develop a Lagrange multiplier (LM) test for shielding. A strong advantage of the LM test is that it only requires fitting a much simpler model that assumes no shielding. We show that, under the null hypothesis and some mild regularity conditions and as the detection time increases, LM test statistic for (composite) shielding is asymptotically Chi-square with the degree of freedom equal to the presumed number of shielding materials. We also derive the local power of the LM test. Extensive simulation studies suggest that the test is robust to the number and nature of the intervening materials, which owes to the fact that common intervening materials have broadly similar attenuation functions.