In this paper we show how an understanding of the performance parameters of the
backscatter units can be obtained from what has been disclosed together with an
analysis of the published images.
Another factor to be considered is sensitivity to contraband. Two views of the subject
are presently obtained, one from the front and another from the back. The available
images show hard-edged low atomic number Z objects such as water-filled bottles or
“bricks” of explosive, and iron in the form of knives or guns. The low Z objects are
sometimes peripheral to the body, where the distortion of normal anatomy is clear, and
the high Z materials are always presented against the body as a background.
An important factor is safety, what the exposure levels mean in terms of hazards
to exposed humans. This is a complex matter, since it needs to take into account
dose to each organ, the skin, thyroid, fetus, etc. each responding differently, and the
predisposition, genetic or disease-induced, of the exposed individual. The authors
have no expertise in the field of the biologic effects of radiation, and we do not enter
into the debate as to whether the levels calculated are “safe” or not, or on risk/benefit
considerations. As a consequence, we present exposure levels in terms of energy
absorbed by a gram of tissue, using the definition of the gray as 10,000 erg/gram for
radiation with a quality factor of 1 (which is the case for x-rays and electrons).
Exposure levels are given as a function of depth in the tissue, this not being directly
comparable to “effective dose” in Sv presented in the published documents.