A team of nanomaterials researchers at Sandia National Laboratories has developed a new technique that could make radiation detection in cargo and baggage more effective and less costly for homeland security inspectors. Known as spectral shape discrimination (SSD), the method takes advantage of a new class of nanoporous materials known as metal-organic frameworks (MOFs).

Crystals of a metal organic 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.
Researchers discovered that adding a doping agent to a MOF leads to the emission of red and blue light when the MOF interacts with high-energy particles emanated from radiological or nuclear material, enabling more effective detection of neutrons. Neutron detection is currently a costly and technically challenging endeavor due to the difficulty in distinguishing neutrons from ubiquitous background gamma rays. The new technology works with plastic scintillators, materials that fluoresce when struck by charged particles or high-energy photons, making it suitable for commercialization by companies who produce plastic and other organic scintillators used in radiation detection devices.

Current radiation detection methods are limited in terms of speed and sensitivity because they use time to discriminate between neutrons and gamma rays, requiring complex and costly electronics. This new technology monitors the color of light emissions, which have the potential to make the screening process easier and more reliable.

Sandia’s team of researchers has been working with MOFs for more than five years. Early on, they discovered a fluorescent, porous MOF with superb scintillation properties, an important breakthrough and the first new class of scintillators found in decades. The MOF’s porosity is a key feature because it allows researchers to add other materials to fine-tune the scintillation. The MOF’s nanoporosity triggered a new idea when one team member read about the use of dopants to increase the efficiency of organic light-emitting diodes (OLEDs). These dopants, usually compounds containing heavy metals such as iridium, dramatically increase OLED brightness by “scavenging” the excited-state energy in the device that was not converted to light. This energy represents as much as 75 percent of the possible light output.

Combining MOFs with OLED dopants led to a second breakthrough. By filling MOF pores with dopants, the team created a material that not only produces more light, but light of another color. This led one team member to hypothesize that the discovery could be applied to radiation detection. The trick is to add just the right amount of dopant so that both the scavenged light and fluorescence from the excited MOF itself are emitted. The ratio of the intensities at the two wavelengths is then a function of the type of high-energy particle interacting with the material. In other words, it is possible to distinguish one particle type from another based on the color of the emitted light.

Because the ratio of neutrons to gamma rays is so low — on the order of one neutron to 105 gamma rays — 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 lowered substantially using SSD, perhaps improving the “figure of merit” by a factor of 10 compared to the current standards. For more information, visit http://info.hotims.com/40435-306.