X-ray backscatter imaging exploits the unique properties
of ionizing radiation to penetrate visual concealment and
detect hidden contraband. The physical process which
generates backscatter is Compton scattering, in which a
photon interacts with a loosely bound or free electron and
scatters in an unpredictable direction [7]. Other interactions,
such as the photoelectric effect, are possible, and
the fraction of photons that interact and which particular
effect occurs depends on each photon’s energy and the
atomic composition of the mass. For a single-element
material, the determining factor is its atomic number Z,
while a compound material can be modeled by producing
an “effective Z,” or Zeff [46].
Under constant-spectrum X-ray illumination, the
backscattered intensity of a given point is largely determined
by the atomic composition of matter at that
location, and to a lesser extent its density. Thus, organic
materials, like flesh, can be easily differentiated from
materials such as steel or aluminum that are made from
heavier elements.
The Secure 1000 harnesses these effects for contraband
screening by operating as a “reverse camera,” as illustrated
in Figure 2. X-ray output from a centrally-located
tube (operating at 50 kVp and 5 mA) passes through slits
in shielding material: a fixed horizontal slit directly in
front of a “chopper wheel,” a rapidly spinning disk with