A dual-energy x-ray source located a distance of one half of the maximum width of the subject from the subject emits a cone beam to a horizontal slit in an x-ray-blocking sheet, producing a fan beam that is chopped into a pencil beam by a rotating disk with radial slots. The pencil beam sweeps a subject, producing backscatter read by a plastic scintillator detector situated very close to and curved around the sides of the subject. The entire assembly translates vertically to produce a complete image of the subject. Pencil beam area is increased farther from the center by increasing the width of the slit toward both ends and increasing the width of the slots toward the outer end. High and low peak x-ray energies of 50 KeV or more and 30 KeV or less, respectively, enable differentiation between innocent and contraband materials that both contain low Z materials.