New hope for landmine detection

The first steps in a new method of detecting landmines by determining the presence of tiny quantities of the explosive TNT (trinitrotoluene) are described in research published today in the Institute of Physics publication Journal of Physics D. Markus Nolte, Alexei Privalov and Franz Fujara of Darmstadt Technical University in Germany, together with Jurgen Altmann of Dortmund University and Vladimir Anferov from the Kalingrad State University in Russia, describe in the journal how they have devised a new way of detecting the nitrogen present in TNT so that small quantities of the explosive can be detected from a distance.

Armed conflicts have left some 60 – 100 million landmines spread through more than 60 countries. Around 20,000 people a year are maimed or killed as a result. In 1997, the Ottawa Convention set a goal of clearing all mines within 10 years, but this simply cannot be achieved if detection continues using only conventional methods — searching with an induction coil and then probing with a metal spike.

Because mine producers have reduced the metal content of their mines, search devices have had to become so sensitive that they can now detect less than 1 gram of metal from 20 centimetres away. The downside, though, is that they detect every rusty nail or piece of shrapnel, forcing the operator to spend maybe 10 or 20 minutes prodding unnecessarily. Telling the difference between mines with a low metal content and harmless pieces of metal could be done much more quickly if the explosive itself could be sensed. Various techniques for doing this are currently being investigated.

One such method involves inducing and then detecting a particular kind of magnetisation in nitrogen nuclei, which are present in most explosives, using a technique called nuclear quadrupole resonance (NQR). This works well for an explosive like hexogen (RDX) but not so well for trinitrotoluene (TNT), which is very widely used. Large quantities of TNT — such as the 5 kg used in an anti-tank mine — can certainly be detected from a few centimetres away using a coil moved over the surface, but most anti-personnel mines use only tens of grams of TNT and the NQR signal is too weak.

Rather than seeking to improve the signal detection techniques or data handling, Dr Privalov and his colleagues have gone back to basics and looked for ways of enhancing the nitrogen nuclear resonance signal itself.
In preliminary experiments in the laboratory, they have placed small TNT samples inside the coil of a home-built field-cycling NMR spectrometer. A magnetic field is then applied, but in such a way that rather than magnetizing the nitrogen nuclei directly, the protons (or hydrogen nuclei) present are polarized. By then reducing the magnetic field to a certain level, the proton magnetization can be transferred to the nitrogen nuclei. In this way, the researchers have produced a strong signal from which they can subtract the signal due to the protons alone — leaving the nitrogen information.

With this technique, it has proved possible to detect as little as 0.5 grams of TNT positioned inside the coil. Even better, the method is so sensitive that different TNT sources — ie different producers — can be identified by distinguishing minor variations in the signal caused by small structural or molecular changes in the explosive.

“This new technique is providing us with stunningly accurate results which we hope will one day save many lives”, says Dr Privalov. “But this is only the first step. We now need to construct a proper surface magnet minesweeping system and investigate whether we can transfer the technique to this kind of detection device.”