  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.
