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.
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