Piece work

A gamma-ray baggage scanner spots explosives by identifying their chemical components

A team at Liverpool University is building a gamma-ray scanner for airports that creates clear images of hidden drugs and explosives by piecing together their components like a jigsaw.

The researchers will work with BAE Systems, Manchester Airport and the Police Scientific Development Branch to build a prototype of the gamma-ray scanner.

Explosives or drugs universally comprise oxygen, nitrogen, hydrogen and carbon in different quantities, and have a unique fingerprint based on the ratios of these four elements.

The Liverpool researchers’ device will bombard the bag or suitcase with neutrons, and any hidden explosives or drugs will release gamma rays from each of their four components at different energy levels. A detector measures the specific energy of the gamma rays from each substance to identify it, and the device’s fast electronics calculate the amount to yield the unique fingerprint and an image.

The detector itself is made from germanium, a semiconductor used in the nuclear industry to warn of stray radioactivity. Gamma rays react with the germanium and leave a charge, which is then collected with an electric field with a voltage across the detector.

Prof Paul Nolan, head of physics at Liverpool University and project leader, said the key advance of the technology is developing a germanium detector that can measure the exact amounts of the substances as well as identifying them.

‘Other detectors only have a broad-brush approach, so we can’t identify what exactly is giving out the radiation. You can’t identify the various components unambiguously,’ he said. ‘In our detector we read out the information using very fast electronics; we’re measuring how the charge in the detector is collected in real time.’

‘What that allows us to do is discover a detailed pattern as the gamma radiation hits the detector that we then invert back to where it came from in the first place to give an image.’

The device could also be used to detect radioactive material, said Nolan.

A key problem with today’s radiation detectors used to screen for dirty bombs and nuclear material at ports is the number of false alarms generated by natural gamma ray emissions from substances such as cat litter or even bananas).

The EPSRC project is due to begin this summer and the team hopes to test a prototype within three years. ‘We’re very much at the lead of this imaging technology,’ said Nolan. ‘We think we will have a better detector with fewer false positives and background effects.’

Elsewhere, in the US this month, researchers revealed a sensitive handheld recorder and walkthrough scanner. The technology, developed at the University of Arizona, measures the movement of ions released from a dangerous substance in an electric field to identify it. Ions from different chemicals have varying speeds in the field that distinguish them. Fast readout electronics adapted from infrared astronomy improved the detection sensitivity significantly compared with conventional ion spectrometers, said the researchers.