A Crystal-Clear Solution

Posted: July 22, 2013 - 3:27pm | Y-12 Report | Volume 10, Issue 1 | 2013

When he’s not analyzing uranium, Ashley Stowe is finding new ways to detect it. His project to grow lithium semiconductor crystals suitable for radiation detection is the first step in solving a global shortage of helium-3, the most common element used in current detectors.

In 2008, the U.S. government announced a critical shortage of helium-3, a material decidedly more important to national security than the helium-4 used to inflate party balloons. The helium-3 isotope is the most common element used in the radiation detectors that monitor U.S. ports and borders for the possible trafficking of nuclear materials.

Because the world’s helium-3 supply is diminishing and demand is increasing, scientists have been searching for new ways to detect fissile materials such as highly enriched uranium. Chemist Ashley Stowe may have the answer.

“I’ve spent the last four years trying to develop a material that will interact strongly with neutrons and act as a surrogate or replacement for helium-3,” he said. One of those materials is lithium-6, a special isotope used as the fusion fuel in thermonuclear weapons. The goal was to create a semiconductor crystal structure that would contain lithium throughout. As part of a radiation detector, the lithium will interact with neutrons emitted from radioactive sources, indicating the potential presence of highly enriched uranium or other nuclear materials.

Stowe’s project to grow semiconductor crystals suitable for radiation detection is a collaboration with an acclaimed university researcher. “He’s one of the world’s most renowned crystal growth scientists for radiation detection,” Stowe said. “And we have the materials, instruments and capabilities to do the work here at Y‑12. It’s been an excellent partnership.”

They knew the crystal structure would need three things: a neutron absorber; a strong, stable material; and good semiconductor properties. Y‑12 already holds a patent for earlier work identifying a certain compound class that fulfills those requirements. After experimenting with various combinations, maintaining lithium-6 as the neutron absorber, the team ultimately settled on lithium, indium and selenium.

Previously, neutron detection with semiconductors required a separate, thin neutron-absorbing layer on the surface of the semiconductor device, which dramatically limited the detector’s efficiency in detecting neutrons. To Stowe’s knowledge, no one had ever used a device like his, which incorporates lithium within the semiconductor crystals, to detect ionizing radiation — until now.

“We have seen a radiation response, including a neutron response,” Stowe said. “This is the first step in solving the helium-3 shortage problem. We can now optimize the crystals for multiple homeland security or neutron science applications.”

When the technology fully matures, Y‑12 will work with a private-industry partner to create hand-held neutron detectors. With the right partner, Stowe estimates these detectors could be commercially available in three to four years.

Continuous Evolution

But Stowe’s work is never finished. There are always new projects on the horizon. “Each question we answer leads to five more,” he said. “Research is a continuous evolution, a journey to a better understanding or a more useful device.”

Whatever he’s working on, Stowe keeps an open mind about his research. He lets the data guide his questions, never forcing research down a certain path. That philosophy, he said, has exposed him to many amazing discoveries and experiences in research and in life.