Made to scintillate - nuclear radiation detector crystal development
The ability to detect nuclear radiation effectively and reliably is of unarguable importance in a range of applications, from oil drilling to national security. A common way to detect the gamma rays and subatomic particles emitted by nuclear materials is to use devices based on scintillation, where the rays or particles strike a crystal of sodium iodide (NaI) (or similar material) and create flashes of light that are converted to electrical pulses to help identify the type of radiation.
The technology has drawbacks, though. The NaI crystal needs to be large to give good resolution, and is usually fragile, difficult to produce and highly vulnerable to humidity. Researchers at Georgia Tech Research Institute, USA, have unveiled a prototype detector crystal that overcomes these problems.
The prototype consists of a glass containing nanoparticles (about 20nm in size) of cerium-doped gadolinium. The gadolinium is essential for scintillation-based detection because of its ability to absorb gamma rays, but it is not an efficient light emitter, so this role is taken by the cerium.
The size of the particles was crucial, as anything larger would have scattered the luminescence created by incoming gamma rays, making readings unreliable. Initially, the researchers looked at using a plastic matrix for the nanoparticles, but found that doing so made it difficult to disperse them uniformly, so a glass matrix was chosen instead.
To make the detector, the team mixed batches of high-purity powders of gadolinium and cerium halides (fluoride or bromide) with silica and alumina, heated it to about 1,400ºC, poured the molten mixture into moulds and then allowed it to cool. The samples were then annealed at 450–750ºC to release thermal stress and promote precipitation of the gadolinium and cerium nanocrystals.
Preliminary results are promising, but field trials are still about a year away. The team’s co-principal investigator, Brent Wagner, explains, ‘Although it does not currently perform as well as standard scintillators, we have been making steady progress in improving the performance. So far, we have matched the gamma ray efficiency of similarly sized NaI scintillators but we need to make improvements in resolution.’ The new crystal will also detect neutrons and X-rays, he adds.
‘It is certainly robust compared to typical scintillator crystals such as NaI. It is not sensitive to humidity. In general it is much simpler to produce than the single-crystal scintillators, so it will be less expensive to make.’
Co-sponsored by the US Department of Homeland Security, a prime use for the technology is in ports, border crossings and airports, but Wagner claims it has other uses. ‘Other areas include oil drilling, geothermal and so on, where robustness at high temperatures is needed, as well as environmental monitoring for radiation protection.’
‘This is an interesting development,’ says Neil Hyatt, Professor of Nuclear Materials Chemistry at the University of Sheffield, UK, ‘and could open the door to novel detectors based on flexible glass scintillator fibres. This would be useful in decommissioning operations to characterise hard-to-reach locations. ‘The key, though, will be to demonstrate reproducible performance so the potential benefits of low-cost and high-volume production, compared to conventional NaI scintillators, can be realised,’ he says.