FEATURE ARTICLE
Explosives Detection with Nuclear Quadrupole Resonance
An emerging technology will help to uncover land mines and terrorist bombs
Joel Miller, Geoffrey Barrall
Explosive Mix
All of these efforts (going back as far as Pound's first tests) were
predicated on the realization that, because their chemical bonds are
somewhat unstable, nitrogen compounds are employed in virtually all
explosives. That use has a long history. Gunpowder, for example, was
first concocted some seven centuries ago from a mix of charcoal,
sulfur and potassium nitrate. The 19th century saw the introduction
of TNT—again another nitrogen compound: trinitrotoluene. And
such modern horrors as the truck bomb Timothy McVeigh used to blow
up the Alfred P. Murrah Federal Building in Oklahoma City contained
the fertilizer compound ammonium nitrate.
Thankfully (for our purposes), the nucleus of the common isotope of
nitrogen, 14N, is not spherical. It thus possesses an
appreciable electric quadrupole moment and can be detected using
NQR. Better yet, because the frequencies at which an NQR signal is
obtained reflect the chemical environment of the nitrogen nuclei,
one can distinguish dangerous explosive compounds from innocuous
materials that also happen to contain nitrogen.
The basic scheme for detecting hidden explosives is fundamentally
simple: One positions a loop antenna around a suspect suitcase or
over a patch of mine-infested ground and applies a short pulse of
radio-frequency magnetic field near the NQR frequency of interest,
which is usually something less than a few megahertz. The loop
antenna then serves to detect a faint return signal at the NQR
frequency if the material of interest is present in the vicinity.
One complication is that the strong outgoing pulse tends to set up
electrical reverberations in the antenna, just as banging on a bell
with a hammer sets up mechanical vibrations that can last a long
time. Although it might take only a few milliseconds for the
oscillations in a typical antenna to decay to negligible levels, the
return signal from some kinds of explosives lasts only a short time
too. The signal one gets back from TNT, for example, has a
characteristic decay time of less than one millisecond. So something
must be done to ensure that the left-over oscillations from the
transmitted pulse do not interfere with detection of the signal. A
similar concern arises in radar equipment, where the same antenna is
used to transmit powerful bursts of electromagnetic energy and to
receive weak echoes from distant objects. One solution (for both
radar and NQR) is to use special circuitry to dissipate the energy
left in the antenna right after the transmitted pulse is finished.
Another option for NQR is to use outgoing pulses that generate what
are called spin echoes.
Spin echoes are a phenomenon unique to nuclear resonance. Their
effect is to produce a measurable return from the nuclei under study
after the signal has nominally died out. How in the world can that
happen? The key is to understand that the reason the signal
disappears in the first place is not that the individual nuclei have
expended all the energy they have to give up. Rather, the overall
signal is lost because the separate emanations from individual
nuclei get out of synchrony. Spin echoes are induced using a
specially designed sequence of pulses, ones that coax the resonating
nuclei to come back into step at some later time.
A simple way to get the general idea is to imagine several runners
lined up at the beginning of a road race. When the gun goes off,
they all speed away from the starting line. Initially, it appears as
though the runners are advancing in unison. But because some go
slightly faster than others, after a short while they get out of
alignment. This is analogous to what happens in nuclear-resonance
experiments: The nuclei resonate at slightly different frequencies,
which causes their oscillations to drift out of phase, producing
little overall signal.
Now consider what would happen if the race officials instructed the
runners suddenly to turn around 180 degrees and head back to where
they started. At that moment, they would be at different places, but
(assuming that they all kept to their established paces) eventually
they would all arrive back at the starting line at the same time,
the slower ones having less far to go. In nuclear resonance, a
second pulse is used in essence to turn all the resonating nuclei
around so that at some later moment they all get back into phase and
produce a return signal that is well separated in time from the
outgoing pulse.
This tactic then helps to solve the ringing-bell problem. But there
is another fundamental concern in NQR: The signals are generally
quite weak. Indeed they are usually comparable in magnitude to the
noise that results from thermal agitation alone. So a considerable
effort has to be made to extract a reliable signal from background noise.
Because thermal noise arises in a completely random fashion, one can
boost an NQR signal simply by averaging the results over time or,
rather, over many repeated spin echoes. The NQR signal will increase
in approximate proportion to the number of spin echoes, whereas the
noise will rise only with the square root of that number. More
difficult is the problem presented by other forms of radio-frequency
interference, which could come from, say, distant AM radio stations
or from electronic equipment in the vicinity. In a controlled
environment (such as within a device for inspecting baggage), one
can employ suitable shielding, typically a grounded metal cage.
Dealing with such radio-frequency noise is, however, a greater
challenge for land-mine detection, where the space to be examined
cannot be enclosed. The solution adopted at Quantum Magnetics has
been to employ not one but several antennae. The additional
antennae, positioned remotely from the first, are used to record the
radio-frequency background at the moment the NQR measurements are
taken. This noise is then digitally subtracted from the signal
obtained from the main antenna.
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