?"Little information exists on the performance of x-ray backscatter machines now being deployed through UK, US and other airports. We implement a Monte Carlo simulation using as input what is known about the x-ray spectra used for imaging, device specifications and available images to estimate penetration and exposure to the body from the x-ray beam, and sensitivity to dangerous contraband materials. We show that the body is exposed throughout to the incident x-rays, and that although images can be made at the exposure levels claimed (under 100 nanoGrey per view), detection of contraband can be foiled in these systems. Because front and back views are obtained, low Z materials can only be reliable detected if they are packed outside the sides of the body or with hard edges, while high Z materials are well seen when placed in front or back of the body, but not to the sides. Even if exposure were to be increased significantly, normal anatomy would make a dangerous amount of plastic explosive with tapered edges difficult if not impossible to detect.
It is very likely that a large (15-20 cm in diameter), irregularly-shaped, cm-thickpancake with beveled edges, taped to the abdomen, would be invisible to thistechnology, ironically, because of its large volume, since it is easily confused withnormal anatomy. Thus, a third of a kilo of PETN, easily picked up in a competent patdown, would be missed by backscatter "high technology". Forty grams of PETN, apurportedly dangerous amount, would fit in a 1.25 mm-thick pancake of thedimensions simulated here and be virtually invisible. Packed in a compact mode,say, a 1 cm×4 cm×5 cm brick, it would be detected.
The images are very sensitive to the presence of large pieces of high Z material, e.g., iron, but unless the spatial resolution is good, thin wires will be missed becauseof partial volume effects. It is also easy to see that an object such as a wire or a boxcutter blade, taped to the side of the body, or even a small gun in the same location,will be invisible. While there are technical means to mildly increase the conspicuityof a thick object in air, they are ineffective for thin objects such as blades when theyare aligned close to the beam direction."
Best Regards Tom.