Lithium-based semiconductor detects thermal neutrons

A new semiconductor made from lithium, indium, phosphorus, and selenium could lead the way to hand-held, portable, sensitive neutron detectors (Nature 2020, DOI: 10.1038/s41586-019-1886-8). Such instruments can spot nuclear materials and play key roles in national security, nuclear medicine, and scientific research.

Neutron detectors rely on the neutron-absorbing properties of a small number of nuclides including ³He, ¹⁰B, and ⁶Li. When materials containing these nuclides absorb a neutron, they produce high-energy charged particles that trigger secondary events that lead to detectable signals.

The most common detectors for thermal neutrons, ones with moderate energies, include so-called proportional counters—tubes filled with ³He or ¹⁰BF₃ gases that generate electrical signals—and scintillation detectors, light-emitting devices typically based on ⁶Li compounds. These instruments tend to be large and have other shortcomings. For example, ³He is rare and the stockpile is dwindling, and boron trifluoride is toxic.

Semiconductors that capture thermal neutrons and directly generate electrical signals could open the door to improved detectors, but researchers have had limited success making them. Paul Sellin, a radiation detector specialist at the University of Surrey, explains that a suitable semiconductor must detect neutrons efficiently and have the right electronic properties—an uncommon combination.

Sellin, who was not involved in the new study, adds that until now, researchers working in this area have used conventional silicon sensors coated with layers of neutron-absorbing materials. Such devices often suffer from poor detection efficiency, he says. “The goal has always been to develop a semiconductor sensor that contains either lithium or boron as an integral part of the semiconductor material.”

That’s exactly what a group of researchers at Northwestern University and Argonne National Laboratory have done. Led by Mercouri G. Kanatzidis, the team devised a strategy for synthesizing LiInP₂Se₆, a seleno-phosphate compound 95% enriched in ⁶Li. The team made the semiconductor by reacting a lithium-indium compound, P₂Se₅, and elemental selenium. They made bulk samples of the product and then used a chemical vapor transport procedure to grow large two-dimensional single crystals of the material suitable for using in a prototype detector.

Initial tests involving a neutron-emitting plutonium source show that the detector responds sensitively and within nanoseconds to low levels of thermal neutrons.

“It’s important to have all sizes of neutron detectors and as many kinds as possible,” Kanatzidis says. “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.”