All militarily important explosives contain 
very large amounts and densities of either nitrogen or chlorine - notably, much larger 
densities than are found in natural materials.  Both nitrogen [shown below] and chlorine 
emit unusually energetic gamma rays when exposed to thermal neutrons.  And 
thermal neutrons and energetic gamma rays penetrate well thru steel, lead, 
concrete, and soil.  Thermal neutrons are safer than x-rays and much safer 
than energetic neutrons, and the 
returning gamma rays are very low intensity.  So using thermal neutrons and 
returning gamma rays to find explosives the clear choice for finding IEDs and WMDs 
safely and effectively.