FEATURE ARTICLE
Explosives Detection with Nuclear Quadrupole Resonance
An emerging technology will help to uncover land mines and terrorist bombs
Joel Miller, Geoffrey Barrall
Bad Bags
Of course, hidden explosives come in forms others than land mines.
Fortunately, one can design NQR detector coils to search for these
threats using fundamentally the same technology used to reveal land
mines. Instead of passing a small coil over a large patch of ground,
one typically moves a small (or perhaps not-so-small) object through
a large coil. Indeed, the coil can be quite sizable. Members of the
Defense Science and Technology Laboratory of the United Kingdom
recently constructed an NQR system for detecting ammonium
nitrate-based explosives hidden in the trunks of cars or the backs
of vans. They showed that a reasonable amount of the radio-frequency
field penetrates the vehicle, which is not too surprising when one
remembers that portable AM radios work just fine inside most cars.
They then demonstrated how a suspect vehicle could be driven into a
huge detector coil and rapidly scanned for the explosives.
Our initial efforts in NQR for the detection of explosives were
accelerated by the downing of Pan Am Flight 103 over Lockerbie,
Scotland, in 1988. Soon afterward, Miller and his colleagues at the
Naval Research Laboratory, with support from the FAA, began work on
an NQR system capable of scanning carry-on-sized baggage for the
presence of RDX-based explosives. This work showed that relatively
small quantities of this explosive (but enough to be a threat to
aircraft) could be detected in a reasonable time. Subsequently, the
Naval Research Laboratory licensed this technology to Quantum
Magnetics, which then built various prototype systems to scan larger
baggage for a range of explosive materials.
One component of NQR research at Quantum Magnetics has had the goal
of not only detecting the presence of an explosive substance but
also pinpointing its position within the suspect bag. It turns out
that localization, at least in one dimension, is easy enough to
accomplish. It requires only that many measurements be taken as the
bag passes along a conveyor belt through the NQR scanner. With these
observations, and knowing the physical characteristics of the coil
antenna used, one can readily calculate the position of an explosive
object (or objects) within the piece of luggage. Finding the
position of a problematic mass in two dimensions is not difficult
either: One needs only to rotate the bag 90 degrees and scan it
again. Indeed, a complete three-dimensional mapping can be
accomplished by rotating the bag a third time and scanning it once
more. (Actually, the results can be made more accurate by using a
larger number of scans, each one obtained after rotating the suspect
item into a different orientation.)
Of course, running a piece of luggage through a scanner many times
is bound to be tedious and time-consuming, particularly because the
operator would have to take care to adjust the orientation properly
during each pass. But the solution is straightforward: Run the bag
though once using multiple coil antennae of different orientations.
Quantum Magnetics has recently designed a system that employs two
perpendicular coils oriented at 45 degrees to the direction of the
conveyor belt. Although this arrangement does not allow the geometry
of an explosive to be mapped in any great detail, it does provide
two-dimensional localization in a single pass.
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