Researchers at Sandia National Laboratories in Livermore have found a way to keep radiation-detecting plastic, used by the Department of Homeland Security to detect nuclear material at U.S. ports of entry, from “fogging,” thus extending the life of the sensitive devices by decades.
The 2-inch thick sheets of polyvinyltoluene (PVT) plastic contain a molecule that reacts to radiation. Sensitive “collectors” at the top of the 6-by-8-foot sheets of plastic gather the small amount of light given off by the molecules, indicating the amount and energy of radiation reaching the material.
However, after a few years of use, the plastic sheets had begun showing signs of “fogging” that reduced their sensitivity.
“For reliable radiation measurements, it’s of the utmost importance that the material is optically transparent and remains that way for decades,” said Sandia materials scientist Nick Myllenbeck.
Visual inspection suggested that “fog droplets” were forming inside the plastic, scattering and preventing some of the light from the glowing molecules from reaching the collectors. But when researchers examined the “droplets” under optical microscopes, they realized they were actually microscale defects in the plastic caused by condensed water absorbed from the air.
By subjecting samples of the radiation-detecting plastic to simulated cycles of daytime and nighttime temperatures, Sandia researchers, working with colleagues at Lawrence Livermore, Pacific Northwest and Oak Ridge national laboratories, determined the tiny defects formed in two phases.
During the first few temperature cycles, the fog-like defects appeared to be completely reversible, disappearing when the plastic was warmed or dried. However, if tiny amounts of condensed water remained in the plastic through enough cycles, permanent defects developed. Once the researchers determined how the fog formed, they began experimenting with additives to stabilize the water and keep it from causing defects in the plastic.
At Sandia, Myllenbeck and his colleagues found a single commercially available additive that interacted with both the water and the plastic matrix. When they tested the new formula under accelerated temperature and humidity conditions, the researchers did not see any sign of fogging after tens of cycles. In contrast, the standard plastic would fog severely after just one cycle. Researchers suspect that moisture inside the plastic clings to the additive rather than other water molecules, which prevents the formation of droplets.
“This one ingredient change is a huge advantage to manufacturers,” Myllenbeck explained. Because manufacturers only have to add a small amount of the compound to their existing formula, with minor process modifications, they should be able to scale up production over the next few months to make large sheets of the plastic to replace fogged detectors.