Warning light

An international collaboration involving research laboratories in the UK, the US and South Africa has begun a three-year project to create more accurate radiation detectors to guard ports and airports. The research coincided with a recent report from the International Atomic Energy Authority (IAEA) that confirmed 149 known cases of illicit trafficking of radioactive materials worldwide in 2006.

The team, led by Imperial College London, will seek to find better materials to replace those now used to make radiation detectors. They hope to find a composition that will pick up even faint hints of radiation and quickly convert that energy into a bright warning light.

Some existing detector materials, such as titanium-doped sodium iodide, offer some of these properties. When radioactive material is detected by a sodium iodide crystal, it emits bright blue fluorescence. The researchers believe they can find a material combination that is not only bright but denser, therefore able to capture and detect super-penetrative gamma rays more efficiently.

They believe they can obtain those improvements with the use of rare earth oxides, which are denser, brighter halides. They will test this theory by using computers to simulate the behaviour of the materials' individual atoms and assess their ability to sense radioactive materials with a series of laboratory experiments.

The atomic-scale assessment of these compositions is crucial because the efficiency of a radiation detector entirely depends on the character of its compositions' electrons. The majority of materials used for detectors require that radiation excites an electron, which is released from an ion embedded in the crystal structure of the material. Subsequently, the electron returns to its ground state and emits light, which is picked up by a photo-multiplier, collected into dynodes, converted to a signal, then run through a multichannel analyser to identify radiological signatures. That information is then fed to a computer, where a program compares it against an isotope database.

The researchers will spend the next three years becoming acquainted with the nature of their compositions' electrons and the local environment of the ion in its crystal structure. They will also study the materials' tendency for detection failure because electrons can become trapped at defect sites in the crystal.

'You don't want an electron to get trapped because that destroys the efficiency of the detector — either it doesn't light up as much, or it takes a lot longer for the response to occur,' said the project's principal investigator, Robin Grimes, an expert in radiation detector materials at Imperial College. 'It's trying to manipulate the concentration of those bad defects that is at the heart of this project.'

If the researchers are able to find a reliable material, it could be a candidate for future fixed and mobile radiation screening devices used to guard the 1,000 points of entry into the UK.

The devices will have to be ready to detect relatively small amounts of radioactive material. For instance, it takes 10kg of Uranium 235 to construct a first-generation Chinese or Pakistani-designed weapon. However, no matter what improvements are made with detectors, no scanner can be expected to detect all threats.

As shown by the death of the former Russian spy Alexander Litvinenko in London last year, ingesting only one microgram of Polonium-210 can be fatal. The substance is highly radioactive but as long as it is housed in a sealed capsule of glass or metal, it will elude radiation detectors because it emits alpha rays, not gamma which most devices are looking for.

The researchers hope their 'defect engineering' research leads to more sensitive detectors that will at least be an additional hurdle for possible weapons smugglers.

'We are trying to create a situation where it is a bit more difficult for people to smuggle these materials,' said Grimes. 'It might be possible through this judicious combination of defects and dopants in these materials to make a detector that is particularly sensitive to particular wavelengths of radiation.

'If you can have it so it is tuned to a particular frequency then you have the possibility of really making a big gain in sensitivity. [Our research will] put in place the methodology for which you might be able to do that.'

The challenge that remains for all future detectors is the ability to distinguish between benign, commercial-use radioactive products found in items such as medical radioisotopes, smoke detectors and ceramic pots, and potentially dangerous nuclear materials such as cesium, strontium, uranium and plutonium that could be intended for a terrorist bomb.

'It might be hard for some materials to tell the difference between americium (which is found in smoke detectors) and plutonium because they are in the same part of the periodic table. Americium is 95 and Plutonium is 94,' said Grimes. 'Although, they do give off different wavelengths, so theoretically it is possible to tell them apart.'

In addition to Imperial College, the Los Alamos National Laboratory in the US and South Africa's University of the Witwatersrand are participating in the project.

Grimes said in three years time he hopes his studies on the defects of rare earth oxides and halides will prove an effective way to optimise materials. 'I'd love to have identified optimum materials within this class of materials,' he said. 'But I think more important is that we have to demonstrate that this approach works, so that other people can look at other systems in the same way and hit upon really good material for radiation detection in the future.'