A team of scientists from Sandia National Laboratories in the U.S. has improved the science of scintillators, objects that detect nuclear threats, with the aim of making it harder to smuggle nuclear materials through America’s ports and borders.
The Sandia Labs team developed a scintillator made of an organic glass which is more effective than the best-known nuclear threat detection material while being much easier and cheaper to produce. Material scientist Patrick Feng and his team synthesized scintillators able to indicate the difference between nuclear materials that could be potential threats and normal, non-threatening sources of radiation, like those used for medical treatments or the radiation naturally present in the atmosphere.
Scintillators behave a lot like light-emitting diodes (LEDs). With LEDs, a known source and amount of electrical energy is applied to a device to produce a desired amount of light. In contrast, scintillators produce light in response to the presence of an unknown radiation source material. Depending on the amount of light produced and the speed with which the light appears, the source can be identified.
Despite these differences in the ways that they operate, both LEDs and scintillators harness electrical energy to produce light. Fluorene is a light-emitting molecule used in some types of LEDs. The team found it was possible to achieve the most desirable qualities — stability, transparency and brightness — by incorporating fluorene into their scintillator compounds.
The gold standard scintillator material for the past 40 years has been the crystalline form of a molecule called trans-stilbene, despite intense research to develop a replacement. Trans-stilbene is highly effective at differentiating between two types of radiation: gamma rays, which are ubiquitous in the environment, and neutrons, which emanate almost exclusively from controlled threat materials such as plutonium or uranium. Trans-stilbene is very sensitive to these materials, producing a bright light in response to their presence.
However, it takes a lot of energy and several months to produce a trans-stilbene crystal only a few inches long. The crystals are incredibly expensive, around $1,000 per cubic inch, and they’re fragile, so they aren’t commonly used in the field.
Instead, the most commonly used scintillators at borders and ports of entry are plastics. They’re comparatively inexpensive at less than a dollar per cubic inch, and they can be molded into very large shapes, which is essential for scintillator sensitivity. “The bigger your detector, the more sensitive it’s going to be, because there’s a higher chance that radiation will hit it,” says Feng.
Despite these positives, plastics aren’t able to efficiently differentiate between types of radiation — a separate helium tube is required for that. The type of helium used in these tubes is rare, non-renewable and significantly adds to the cost and complexity of a plastic scintillator system. And plastics aren’t particularly bright, at only two-thirds the intensity of trans-stilbene, which means they do not do well detecting weak sources of radiation.
Feng’s team found the glass scintillators surpass even the trans-stilbene in radiation detection tests — they are brighter and better at discriminating between types of radiation. The initial glass compounds the team made weren’t stable. If the glasses got too hot for too long, they would crystallize, which affected their performance. Feng’s team found that blending compounds containing fluorene to the organic glass molecules made them indefinitely stable. The stable glasses could then also be melted and cast into large blocks, which is an easier and less expensive process than making plastics or trans-stilbene.
The researchers plan to cast a large prototype organic glass scintillator for field testing. Feng and his team want to show that organic glass scintillators can withstand the humidity and other environmental conditions found at ports.