Doping MOFs Leads to Red, Blue Emission

A new technique that employs a nanoporous material to efficiently identify neutrons could provide a more effective and less costly way for homeland security inspectors to detect radiation in cargo and baggage.

Current neutron detection methods are expensive and technically challenging because of the difficulty in distinguishing neutrons from ubiquitous background gamma rays. In addition, traditional radiation detection techniques are limited in terms of speed and sensitivity, which are crucial elements for dynamic scenarios such as border crossings and nuclear treaty verification.

The new method, developed at Sandia National Laboratories, can monitor the color of light emissions, which potentially could offer a screening process that is easier and more reliable than measuring the rate of light emissions. Called spectral shape discrimination (SSD), the technique takes advantage of a new class of nanoporous materials known as metallorganic frameworks (MOFs). By adding a doping agent to an MOF, the scientists discovered, the material emits red and blue light when it interacts with high-energy particles emanated from nuclear or radiological materials.

Crystals of a metallorganic framework (left) emit light in the blue (middle) when exposed to ionizing radiation. Infiltrating them with an organometallic compound causes the crystals to emit red light as well (right), creating a new way to differentiate fission neutrons from background gamma particles. (Image: Sandia National Laboratories)

“We are approaching the problem from a materials chemistry perspective,” said Mark Allendorf, a materials scientist at Sandia. “Fundamentally, it is easier to monitor the color of light emissions rather than the rate at which that light is emitted. That’s the crux of this new approach.”

The new technology employs plastic scintillators — materials that fluoresce when high-energy particles collide with them. MOFs, which are porous in nature with exceptional scintillation properties themselves, facilitate the addition of other materials to fine-tune the inherent scintillation.

The scientists used the heavy-metal compound iridium, used as a dopant in organic LEDs, to increase light output. They discovered that the dopants not only increase the brightness of the emitted light by scavenging the high-energy electrons that were not converted to light, but also caused a different color to be emitted.

From left, Sandia researchers Patrick Doty, Patrick Feng and Mark Allendorf have created a new type of scintillator using metallorganic framework or plastic scintillator hosts combined with heavy metal dopants, shown in Doty’s hand. This material enables detection of neutrons using spectral- or pulse-shape discrimination techniques that could transform radiation detection. (Image: Dino Vournas, Sandia National Laboratories)

Team member Patrick Doty hypothesized that this discovery could be applied to radiation detection. The trick, he said, is to add just the right amount of dopant so that both the scavenged light and the fluorescence from the excited MOF itself are emitted. This enables the ratio of the intensities at the two wavelengths to function as 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 color of the emitted light.”

Because the ratio of neutrons to gamma rays is so low, the threshold at which current detectors can see neutrons is fairly high. Sandia calculations suggest that the threshold for detecting neutrons produced by fissionable material could be substantially lowered using the new technique.

Improvements will need to be made before the technology moves into the marketplace, but Sandia is currently seeking commercial partners to license the technology.

For more information, visit: www.sandia.gov