In one sense, detection of landmines
is easy. But resolving, false alarms, that is distinguishing
a landmine from, for example, a nail is really the tough
problem.
The main tool for the military
and for humanitarian deminers is a metal detector with technology
similar to that used during World War II. However, many mines,
especially the antipersonnel (AP) mines, now use a plastic case
and have very little (<1g) of metal in the firing mechanism.
While these low metal, mines can be detected by a skilled operator,
the sensitivity of the detector must be increased from that required
to detect, say, a metal-cased antitank (AT) mine. But this increase
in sensitivity will also pick up all the detritus of a battlefield
(shell casings, metal fragmentation) or of other human activity
(nails, wire). Since the metal detector cannot in general distinguish
between a firing pin in a landmine and an empty shell casing,
such an alarm, must be resolved. The next step is to probe mechanically
with a pointed stick. Such archaeology must be performed very
carefully since the mine may be sensitive to mechanical shock
or be rigged with anti-handling devices. If the gentle probing
does indicate the presence of a mine, then the mine is either
marked, removed, or destroyed depending on the application. But,
more frequently, a mine is not found, and it is these false alarms
that can dominate the pace of demining. Depending on the terrain,
the amount of clutter, and the details of mine emplacement, there
might be hundreds or thousands more false alarms than actual
mines.
Other more advanced methods of
mine detection are being explored, such as ground penetrating
radar (GPR), passive microwave, infrared techniques, and advanced
metal detection systems. But to a great degree these all suffer
from this problem of clutter: their detection methods are based
on non-unique properties of the mine, e.g. the presence of a
small quantity of metal, the discontinuity in the dielectric
constant, etc. It would be a revolutionary step to detect the
essence of the mine, that is the explosives itself.
Nuclear Quadrupole Resonance
(NQR) is a method that unambiguously detects the presence of
explosives. While
there are many technical hurdles remaining, NQR has the potential
to strike at the heart of the terrible problem of finding
landmines.
NQR is a radio frequency spectroscopy
similar to nuclear magnetic resonance (NMR) and to magnetic resonance
imaging (MRI). In contrast to NMR and MRI, no magnet is required
to align the nuclear spins. In NQR the valence electrons align
the nuclear spins, here 14N, along preferred directions.
The energy associated with this alignment is quite small: at
room temperature, almost equal numbers of spins are aligned parallel
and antiparallel to these directions, but a slight population
excess, about 1 in 107 nitrogen spins, is preferentially
aligned. Application of an RF pulse with a frequency corresponding
to this energy causes the populations depart from their thermal
equilibrium, and following such an RF pulse, an NQR signal can
be observed. For nitrogen, the resonance frequencies range from
0 - 6 MHz. TNT, the main explosive of interest, has 18 different
resonance lines, of which 12 are clustered from 700 - 900 kHz,
frequencies in the commercial am radio band.
The NQR frequencies for explosives
are quite specific, and are not shared by other nitrogenous materials.
The NQR explosives detection approach is conceptually simple:
one broadcasts an RF pulse at the particular frequency or frequencies
of interest and looks for a return signal with a sensitive radio
receiver. The intensity of the return signal is proportional
to the amount of explosive, and does not reflect the presence
of any other material except that explosive.
It is this specificity to explosives
that overcomes the fundamental problems of mine detection: all
other methods are susceptible to clutter,, those real signals
that arise from things other than explosives.
While the specificity of NQR
is a major advantage over other methods, the NQR signal is very
weak in comparison to the thermal noise in any conventional detection
system. A second problem is that significant RF power may be
required to interrogate large volumes. The Naval Research Laboratory
has made progress in both these problem areas: increasing the
signal-to-noise ratio by use of efficient pulse sequences and
by improved design of the detector coil; and pioneering the use
of low power RF for NQR.
The Naval Research Laboratory
and other researchers throughout the world are examining NQR
for this application. Indeed NQR for landmine detection was explored
in the 1970's by the US Army and in the 1980's by the Soviet
Army. Results at that time were limited to the military explosive
RDX that is present in some but not all mines. TNT is the most
common explosive in mines and is technically more difficult to
see by NQR. However, we at NRL have applied some of the advanced
methods of solid state nuclear magnetic resonance to NQR and
have succeeded in improving the detactability of these explosives,
especially TNT.
The NRL technology for detection
of landmines by NQR has been licensed to Quantum Magnetics (San
Diego, CA), a division of InVision Technologies. QM has pioneered
the use of NQR under field conditions, and has begun two programs
to develop a man-portable system for the US Marines and a vehicle
mounted system for the US Army. The mine detection work at NRL
and QM has been funded by the Defense Advanced Research Projects
Agency (US Department of Defense).
Dr. Allen Garroway of NRL will
present an invited talk on "Detection of Landmines by Nuclear
Quadrupole Resonance (NQR) on 22 February 2000 at the American
Association for the Advancement of Science (AAAS) meeting in
Washington DC. Dr. Garroway will discuss the general use of NQR
for explosives detection and then focus on three technical areas
for landmine detection:
-- efficient RF
pulse sequences that maximize the signal-to-noise ratio of the
NQR signal from explosives,
-- detection of TNT in one-shot,, obviating the need to wait
tens of seconds while the TNT signal recovers and thereby
increasing
the rate of advance of the detection system,
-- the use of a figure-eight, NQR detection coil to reduce external
RF interference.
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