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Figure 2: Dependence of relative amplitude of x-ray scatter (in this case, through 180°) on atomic number. Truck is being irradiated from the side, and backscattered radiation is imaged by an image receptor on the same side as the radiation source. Organic and other materials with a lower atomic number, such as drugs, explosives, plastic weapons, or people, scatter x-rays much more readily than do high-atomic-number items, such as trucks and guns. The amount of back-scatter from an object also depends, of course, on its physical density. (Image courtesy of American Science and Engineering, Billerica, Mass.)
