Perfect perovskite crystals detect gamma rays

By combining advances in crystal-growth technology and engineering know-how, researchers have developed a γ-ray detector with the potential to provide better performance at lower cost than the commercial radiation detectors used for applications including counterterrorism, nuclear plant safety, and medical diagnostics.

Developed by scientists at Northwestern University, the University of Michigan, and Argonne National Laboratory, the devices are enabled by large, high-quality crystals of cesium lead bromide (CsPbBr₃) a perovskite material widely studied for use in solar cells, light-emitting diodes, and lasers (Nat. Photonics. 2020, DOI: 10.1038/s41566-020-00727-1).

γ Rays are high-energy photons released by uranium, plutonium, and other radioactive atoms undergoing fission. Detectors of these rays therefore play key roles in nuclear safety, national security, medical diagnostics, and scientific research. But there are only a couple of types of such detectors available commercially, because very few materials do a satisfactory job of detecting high-energy radiation, says Arnold Burger, an expert in radiation detectors at Fisk University who was not involved in the study.

The ideal materials for such detectors require high stopping power—the ability to prevent high-energy γ rays from whizzing through unobstructed and undetected. They also need the right set of electronic properties to register the captured γ rays as a detectable signal. Materials must be sensitive enough to detect the low numbers of γ rays emitted by most radioactive sources. On top of that, these materials must have properties that allow the detector to distinguish the energies of γ rays well enough to uniquely identify the emitting isotope. For example, the detector should distinguish the low-level emissions from potassium-40 isotopes in a cargo container of bananas from those made by a smuggled uranium device.

Among semiconductors, the only materials that fit the bill and are considered commercially viable are germanium, which is the gold-standard detector, and cadmium telluride-based materials, such as CdZnTe.

But germanium detectors must be cooled to cryogenic temperatures and require a lot of power, making them expensive, bulky instruments typically not suited to portable, battery-powered operation, which would be helpful in many settings. And both materials are costly because they must be prepared as extremely pure crystals.

CsPbBr₃–based detectors can tolerate more crystal impurities than ones based on germanium and CdZnTe. Even so, various researchers, including Northwestern’s Mercouri G. Kanatzidis, who led the new study, have tried unsuccessfully for nearly 10 years to produce perovskite crystals of sufficient quality and size to reliably detect γ-rays.

Now they’ve done it. By carefully controlling the heating and cooling rate in a vacuum-based crystal-growth method, the team produced defect-free CsPbBr₃ crystals measuring up to nearly 4 cm in diameter. The group incorporated the crystals in detectors with various types of device architectures and showed that they exhibit the necessary resolution and sensitivity for typical applications, operate reliably from roughly 0–70 °C, and remained stable during an 18-month test.

“This is very impressive work,” Burger says. These new crystals may supersede other materials used as γ-ray detectors with the potential to do a better job at lower cost, he adds.

Ju Li, an expert in detector technology at the Massachusetts Institute of Technology agrees, remarking that the quality of the crystals and the extraordinary degree to which they capture γ rays and convert them to measureable signals “may fundamentally change the game of radiation detection.”

Kanatzidis and colleagues have founded a company, Actinia, to commercialize the new detectors.