An innovative system for detecting plastic explosives in airline luggage promises to improve significantly the effectiveness of existing systems.
Its inventors say it should reduce the number of false positives (indicating explosive present when none is, typically 25% in current systems) with zero false negatives (failing to detect explosive when it is present).
The system uses a combination of neutron scattering and gamma-ray detection to build up a three-dimensional image of an item of airline baggage using a technique similar to computer-aided tomography scanning in medicine.
Dr John Hassard of London’s Imperial College says that a plastic explosive detection system has to cope with a wide range of possible explosives, which can take many forms (a strip, a sheet or a sausage shape, for example) and can be concealed in an infinite number of ways.
The new detector system, developed by Imperial with Eppra of France and with support from the Ministry of Defence and Department of Transport, bases its effectiveness on maximising the information available and the way in which it is used.
The package being scanned is bombarded with neutrons. ‘Neutrons can penetrate most conceivable packaging, or if they can’t, that is suspicious in itself,’ says Hassard.
The bombardment by neutrons excites the nuclei of material within it in such a way that characteristic gamma-ray photons are emitted, while the neutron beam is scattered.
The gamma-ray data is combined with data from the neutron scattering to produce a tomographic reconstruction of the objects in the bag. By combining the data in an optimal way, an accurate identification of the nuclei causing the scattering is obtained, and false positives are reduced, says Hassard.
Though identifying materials from gamma-ray emissions is an established technique, the innovation is the detection methods used, which remain secret.
Eppra provided a plasma focus source of neutrons, cheaper than using a radioactive source and with the advantage of being able to be turned off when not in use.
The partners are now working with industrial partners to produce a marketable machine. Hassard believes a mobile machine is only four or five months away.