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Doping MOFs leads to red, blue emission

Photonics Spectra
Sep 2012
LIVERMORE, Calif. – 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 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.


“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 technology employs plastic scintillators – materials that fluoresce when high-energy particles collide with them. MOFs, which are porous in nature and have 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 cause a different color to be emitted.


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


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. With this arrangement, 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 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 must be made before the technology moves into the marketplace, but Sandia is currently seeking commercial partners to license the technology.


GLOSSARY
fluorescence
The emission of light or other electromagnetic radiation of longer wavelengths by a substance as a result of the absorption of some other radiation of shorter wavelengths, provided the emission continues only as long as the stimulus producing it is maintained. In other words, fluorescence is the luminescence that persists for less than about 10-8 s after excitation.
photonics
The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
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