FEATURE ARTICLE
Explosives Detection with Nuclear Quadrupole Resonance
An emerging technology will help to uncover land mines and terrorist bombs
Joel Miller, Geoffrey Barrall
Ground Truth
Although this technique shows a great deal of promise as a means for
finding hidden explosives, military tacticians consider NQR
detectors to be too slow to be useful in clearing roads of land
mines. In recognition of this concern, we are designing NQR
equipment to function as a "confirmation sensor." These
devices will supplement the conventional tools now being applied to
the task of finding land mines: metal detectors and
ground-penetrating radar, which can be quite sensitive but tend to
produce many false alarms. The idea is that an NQR sensor will be
used to test for the presence of explosives only at those spots
identified as suspicious by these other methods, reducing the number
of false alarms.
In an effort to gauge the effectiveness of NQR in this context, we
and some of our colleagues arranged in 2003 to test a prototype
confirmation sensor (built by Quantum Magnetics) under realistic
conditions. This equipment was designed to detect antitank and
antivehicle mines buried in roads. We performed these experiments at
two U.S. government test sites, one situated in the desert, the
other located in a temperate environment, so as to be able to gauge
whether damp soil, which can interfere with conventional detection
methods, posed special problems for NQR. (It didn't.)
We used a variety of mines for these trials: Some contained from 5
to 8 kilograms of TNT, whereas others used anywhere from 2 to 10
kilograms of an explosive called "Comp B," which is a
combination of TNT (40 percent) and the explosive compound
cyclotrimethylene trinitramine, better known as Royal Demolition
Explosive or RDX (60 percent). The mines were buried at realistic
depths, varying from 2.5 to 12.5 centimeters (as measured from the
surface of the ground to the top of the mine). These were blind
tests, in the sense that the people operating the NQR equipment did
not know ahead of time which of the hundreds of spots they examined
held mines.
We carried out the first set of trials at the desert site, both
during the day and at night. Why test day and night? Because we
anticipated that radio-frequency interference would pose a bigger
problem at night than during the day. (Recall how many more stations
your short-wave radio picks up after the sun goes down.)
Fortunately, the equipment dealt with this interference well, and
the results for day and night proved to be statistically identical:
The overall probability of detection was about 95 percent, and the
probability of false alarm was only between 4 and 7 percent.
In the second test at the temperate site, the TNT detection
probability was slightly reduced compared with what we had
determined under arid conditions. But we obtained similar results at
both sites for RDX. And again, the day and night tests gave nearly
identical outcomes: The overall probability of detection was once
more around 95 percent, and the probability of false alarms was
about 5 percent.
These tests clearly showed the feasibility of detecting antitank
land mines by NQR, but antipersonnel mines are a different matter.
Many antipersonnel land mines contain as little as 50 grams of
explosive, pushing current NQR detection sensitivity to its limits.
The 2003 tests made apparent some of the practical difficulties that
still limit NQR detection sensitivity. For the past few years, we
and our colleagues at the Naval Research Laboratory and at Quantum
Magnetics have worked (with support from the Army, the Marine Corps
and the Office of Naval Research) to overcome these problems with an
eye to developing rugged, portable hardware that can detect mines
swiftly and reliably under harsh field conditions. We've made
excellent progress in improving the sensitivity of our NQR
detectors, while at the same time making them more immune to
radio-frequency noise. These advances are bringing the NQR detection
of antipersonnel land mines into the realm of possibility.
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