Stable lithium could lead to new class of neutron detectors

According to researchers at Northwestern University and Argonne National Laboratory, the new material could be integrated into a handheld device for field inspections or incorporated into current wall-sized devices that monitor trucks for radioactive contraband. The study appears in Nature.

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"People have imagined semiconductor neutron detectors for a long time," said Northwestern's Mercouri Kanatzidis, who led the research. "The idea was there, but no one had the right material to do it."

Heavy elements like uranium and plutonium eject neutrons from their nuclei as they decay. Most neutron detectors are scintillators that sense ejected neutrons and emit light to alert the user. In contrast, the new material detects electrical signals induced by the neutrons.

Lithium has been touted as a promising material for neutron detecting devices as it is excellent at absorbing neutrons but integrating the element into a semiconductor and making it stable has presented challenges.

"You can find good semiconductors, but they don't have lithium," Kanatzidis said in a statement. "Or you can find stable lithium compounds that are not good semiconductors. We found the best of both worlds. The specific lithium-6 isotope, which is reasonably abundant and low cost, is a strong neutron absorber."

In their study, Kanatzidis and his team discovered the right combination of materials to make a working device that also keeps lithium stable. Their new material - lithium-indium-phosphorous-selenium - is layered in structure and enriched with the lithium-6 isotope.

"The crystal structure is special," Kanatzidis said. "The lithium is inside the layers, so water cannot reach it. That's a big, important feature of this material."

The resulting semiconductor neutron detector can very rapidly detect thermal neutrons from even a very weak source. The team said it can also discriminate between neutrons and other types of nuclear signals, such as gamma rays, thereby preventing false alarms.

The material also contains a very high amount of lithium, so a smaller fraction of the material can absorb the same amounts of neutrons as a very large device, leading to devices small enough to fit in a person’s hand.

"It's important to have all sizes of neutron detectors and as many kinds as possible, such as our new semiconductor," Kanatzidis said. "You want ones that are as big as a wall, where you can pass a truck right by it. But you also want small ones that can be portable for inspections out in the field."