Accounting for alpha radiation

Remember the Fukushima nuclear accident in 2011? The Tōhoku earthquake, east of Japan, caused a tsunami that disabled three reactors at the Fukushima Daiichi nuclear power plant. Nuclear emergency response officials needed to know immediately if the reactors were leaking radioactive material, and if so, which ones, how much, and where.

One way this information was determined was by using alpha spectrometers. Alpha spectrometers measure alpha radiation, which is energy emitted by actinide elements, including plutonium and uranium. Measuring alpha radiation is one way to determine the presence and quantity of actinide elements.

Alpha spectroscopy has traditionally required extracting a sample, transporting it to a specialized laboratory, and carefully preparing it using a time-consuming process that involves strong chemicals and generates radioactive waste. In addition, workers separating and purifying the sample risk exposure to radiation and chemicals.  What would have been useful at Fukushima is a portable, reliable, remotely operated alpha spectrometer, which could have quickly and safely provided information at the scene of the accident.

Fast-forward 13 years, and scientists at Los Alamos National Laboratory have developed NDAlpha, the first field-deployable alpha spectrometer capable of point-and-shoot scanning to immediately measure on-site alpha radiation. The “ND” in the name stands for “nondestructive:” there is no need to remove a sample for evaluation.

The NDAlpha device is slightly larger than a soda can and has a small (5 by 25 millimeter) thin plastic window on one end. The operator points the window at a nuclear material or contaminated surface. Alpha particles pass through the window and deposit their energy in a silicon detector, which produces an energy spectrum. The details of the energy spectrum are analyzed with software designed to identify materials and quantify their composition.

“All the operator needs to do is position the device near a spent fuel for a few seconds to get an answer,” says NDAlpha developer Mark Croce, who notes that NDAlpha can also be used remotely—mounted on a remotely operated vehicle, for example—which protects operators from hazardous environments.

Croce explains that many materials also emit beta particles and gamma rays—often with great intensity—which can sometimes make alpha radiation difficult to measure. With this in mind, NDAlpha was built with a magnetic filter to redirect beta particles away from the internal silicon detector, and the thin active region of the silicon minimizes sensitivity to gamma rays.

Croce also notes that alpha radiation can sometimes be tricky to measure because alpha particles lose energy every time they travel through material—even air—which is why careful sample preparation is usually required. But the software that NDAlpha uses to assess a material accounts for this loss of energy. “The algorithm we developed can handle it,” Croce says. “We’re able to accurately measure any thick piece of material, such as fragments of nuclear fuel in the field.”

Scientist Katherine Schreiber, who helped develop NDAlpha, says the technology will be particularly helpful to the nuclear emergency response community. “It could also become an important part of process monitoring in a nuclear fuel facility, allowing on-site alpha spectroscopy,” she continues, “and it could even be used in the decommissioning and cleanup of nuclear facilities.” ★

Learn about other new nuclear forensic technologies here.