A new technique could see radiation detection of cargo and baggage become more effective and less costly.
Spectral shape discrimination (SDD) – developed at Sandia National Laboratories – uses metal organic frameworks (MOFs) to distinguish between high energy neutron particles and background gamma rays. Adding a doping agent to the MOFs – which are filled with tiny nanopores – leads to the emission of red and blue light when they interact with high-energy particles from nuclear material.
“We are approaching the problem from a materials-chemistry perspective,” said Mark Allendorf, a materials scientist. “Fundamentally it’s easier to monitor the colour of light emissions rather than the rate at which light is emitted. That’s the crux of this new approach.”
Current radiation detection uses time to discriminate between neutrons and gamma rays – and requires complex and costly electronics. They are limited in terms of speed and sensitivity, but this new technology monitors the colour of light emissions – making the screening process easier and more reliable.
The technology works with plastic scintillators – materials that fluoresce when struck by charged particles or high-energy photons. The porosity of the MOFs allows researchers to add other materials to fine-tune the scintillation. Researchers discovered combining MOFs with OLEDs – organic light-emitting diodes – produces not only more light, but light of another colour.
When high energy particle interact with the organic material, the material becomes excited. When this ionised state collapses back to its ground state, it emits light in the blue part of the spectrum. Adding a second molecule that interacts with the excited material leads to the emission of a second colour.
The trick is to add just the right amount of dopant so that scavenged light from the excited-state energy and fluorescence from the excited MOF is emitted, said materials scientist Patrick Doty. The ratio of the intensities at the two wavelengths is a function of the type of high-energy particle interacting with the material.
“That’s the critical thing,” Doty said. “SSD allows one particle type to be distinguished from another on the basis of the colour emitted.”
The researchers suggest the threshold for detecting neutrons produced by fissionable material could be lowered substantially using SSD and are seeking commercial partners to license the technology.