All militarily important explosives contain
very large amounts and concentrations of either nitrogen or chlorine - much larger
than anything found naturally. 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 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.