?"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."

http://springerlink.com/content/g6620thk08679160/

Seems we are spending a lot of $$$$$ on equiptment that is fairly
easy to defeat by low tech terrorists. Feel any safer?

Best Regards
Tom.