Detection of Landmines by NQR at the Naval Research Laboratory

2/22/2000 - 13-00r
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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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