Fingering explosives

An emerging technology has helped researchers from the

to develop sensors they say can quickly and effectively detect chemical, biological, nuclear and explosive material.

A spectroscopic technique utilising mm/THz frequencies — between microwave and infrared on the electromagnetic spectrum — has enabled the team to detect spectral 'fingerprints' that identify explosives and chemicals.

The technology detects the energy levels of a molecule as it vibrates, and the frequency distribution of this energy provides a unique and reproducible spectral pattern — a 'fingerprint' that identifies the material.

Supported by the US Department of Defence and NASA, the research achieved three important goals.

The detection and measurement of poison gas 60m away to an accuracy of 10 parts/million using active sensing — where a wave is sent out and identification is carried out on its return.

Identifying chemicals relating to defence applications such as nuclear weapons from 600m using passive sensing — where the wave remains static, so the device can be left unattended anywhere.

And recognising the various spectral fingerprints of trace levels of explosives such as DNT, TNT, semtex and C-4.

However, for the sensors to operate efficiently and be commercially attractive, a database of fingerprints is required to match and identify explosive or chemical signatures. project section manager Paul Raptis explained: 'In most cases if you want to detect something, you have got to know what you are looking for. In the case of the millimetre wave unless you have done the necessary work to build up the database you would be wasting your time.'

While previous research has developed a library for passive test sites, current research being carried out in collaboration with Sandia National Laboratory, Dartmouth College and the Sarnoff Corporation is looking to develop a database of trace explosive 'fingerprints'. Raptis said a database could be beneficial at airports, for example, to screen cargo for trace explosives. This would be far more accurate and eliminate lengthy delays due to false alarms.

He added the technology can also be used in an imaging capacity which would reveal weapons hidden under clothing, or even diversified into medical applications to locate tumours. According to Raptis, remote detection is also becoming possible. For example, a suicide bomber can be detected through imaging or trace explosives emitted from a package that leave fingerprints

The technology is believed to have advantages over conventional sensing techniques, such as optical or laser, which can be disrupted by atmospheric conditions, or x-rays, which can cause damage by ionisation.

Operating between 0.1 and 10 THz, the sensitivity of the new technique is four to five orders of magnitude higher with imaging resolution 100 to 300 times than those at microwave frequencies. 'Its selectivity is outstanding, it is like a fingerprint,' said Raptis. 'we can get sensitivity in parts per billion.'

To counter other security threats, researchers are testing millimetre-wave radars and developing models to detect radiation from nuclear accidents or reactor operations. In a test collaboration between Argonne and AOZT Finn-Trade of St Petersburg using instruments 9km from a nuclear power plant, results were promising — a clear difference being shown between when the plant was operational and when it was dormant.

It is also hoped this technology will be able to map plumes from nuclear radiation releases and be adapted for remote and rapid gas leaks from natural gas pipelines by detecting the fluctuations in the index-of-refraction caused by leaking gas into the surrounding air.

Together with ongoing sensor development for biological hazards, environmental monitoring and health, Raptis is hoping to expand the technology into the commercial sector within the next year or so.

'The next stage is do some more tests and increase the size of library of what we can detect. I think the passive technology can be commercialised with a year's work and could be put into an airport.'