Gamma-ray vision

A UK company has built a highly sensitive drive-through cargo scanner for ports and airports that can better distinguish between a terrorist dirty bomb and less harmful radioactive material.

The technology, developed by Symetrica, could increase the accuracy of gamma-ray radiation detectors planned for installation at UK ports to prevent terrorism and avoid time-wasting false alarms, the company claims.

Gamma-rays are emitted naturally by many materials and can therefore trigger conventional detectors, according to Dr. Brian Lever, Symetrica operations director. ‘Materials such as ceramic tiles, cat litter or bananas are radioactive to some extent,’ said Lever. ‘Our equipment immediately distinguishes between materials that do not pose a threat and those that do. Nobody has ever before created a large detector that gives a resolution beyond simply calling an alarm.’

An oversensitive alarm would prevent any dangerous radioactive material slipping through, but create more work for customs officials and congestion at ports and airports, said Lever.

The government pledged earlier this year to screen all cargo entering the UK to prevent terrorists acquiring the materials to build a dirty bomb. Customs officials are installing permanent large screening machines at ports and airports to detect radioactive material after a three-month trial at Dover, Felixstowe and Portsmouth.

However, the planned radiation detectors are first-generation technology, and were originally developed for the steel industry to sound an alarm if scrap metal inside lorries is radioactive, said Lever.

Conventionally a large plastic DVT sheet is doped with scintillating gamma-ray detector material. The company has re-designed the sheet to offer clearer fingerprint detection.

‘Our large plastic sheet detectors go up to a resolution of 10 per cent – that’s almost as good as sodium-iodide detectors.’ These are used for environmental surveys and as laboratory instruments.

The company has built a prototype, and after tests expects the detector to be available next year.

Symetrica was reluctant to reveal the technology behind the detector, but said that the improved performance comes from ‘better detector design, signal processing software and faster processing electronics’. One way of improving the processing of gamma fingerprints and improving resolution is to distinguish between the scattered rays bounced off other materials and the rays direct from the source.

Scattered photons have a slightly lower energy level after a collision compared with direct photons, so if a detector can distinguish between the energy levels more accurately, the resolution can be improved.

Symetrica’s gamma-ray technology for medical use and handheld scanners already offers a ten-fold increase in resolution and a four-fold increase in sensitivity, said Lever.

Symetrica was spun out from the University of Southampton in 2002 and received a grant from the National Endowment for Science, Technology and the Arts to develop the drive-through scanner.

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