Explosives Detection using Sound

The following article is reproduced with permission from "Find A Better Way" about research performed at the University of Sheffield Chemistry Department into research for the detection of explosives. For more information on the charity, visit their website at www.findabetterway.org.uk

A new Find A Better Way funded programme for detecting traces of vapour emitted from explosive material began at the University of Sheffield last month. Called ‘Mid-IR CERPAS for Humanitarian Demining,’ the technology being developed will act much like an artificial nose, taking in air samples and searching for trace amounts of explosive compounds.

Dr Hipplers new explosive detector projectThis searching is done by using a laser which can be tuned to a special part of the infrared frequency known as the ‘fingerprint region.’ By using frequencies that are absorbed by molecules associated with explosives, the Sheffield team can effectively ‘sniff out’ tiny trace amounts of explosive material from the air, possibly indicating if a landmine or unexploded ordinance is nearby.

The challenge for the Sheffield team is to make a device sensitive enough to detect absolutely tiny amounts of explosive material, possibly as little as one part in one billion in a given air sample.

To make their detector more sensitive the team are adapting a technology called cavity-enhanced resonant photoacoustic spectroscopy (CERPAS), previously developed by the Sheffield team’s leader Dr Michael Hippler. CERPAS traps the infrared laser beam between two mirrors, which forces the laser to bounce between them several hundred times. This makes it easier to detect a compound absorbing energy from the infrared laser, but even this process is not sensitive enough to detect ultra-low levels of interaction. For this (believe it or not) highly sensitive microphones are used to listen for the sound of molecules make when they absorb the laser’s energy – a process known as photoacoustics.

Dr Hippler is very enthusiastic about the project so far. ‘Although we’re only one month in, the project has started well,’ he explained. ‘We have procured some essential optical and electronic components and a suitable mid-infrared quantum cascade laser, plus we have recruited Dr Mark Stringer, an experienced research associate to join me.’

The two researchers have previously identified the technical issues that must be addressed, and have demonstrated their first prototype explosive detector. The prototype works well in laboratory conditions, so their task during the remaining two years of the project is to build a new version (smaller, stronger, and battery operated) that could conceivably be used by deminers in the field.

The team already know what is required for the next version. Dr Stringer explains, ‘Whenever you finish a prototype you immediately know what you should have done differently. If we implement what we already know from making the first version into the next device we will make good progress.’