A newly designed ultrasensitive heat sensor may make monitoring nuclear materials at power plants and weapons facilities easier and more reliable. The device, developed by researchers at the US National Institute of Standards and Technology (NIST), can measure X-rays emitted by radioactive elements with greater accuracy than detectors currently used for this application (Phys. Rev. Lett. 2026, DOI: 10.1103/tcbz-kqk1).
Radioactive atoms can emit energy from their nuclei in the form of γ rays, which scientists can use to identify the element from which they were emitted. But these atoms can also emit X-rays from their electron clouds in nearly the same energy range as the γ rays, interfering with the γ ray signals and complicating the analysis.
New X-ray measurements, which the team analyzed with quantum mechanical calculations, were made for uranium, neptunium, and plutonium. The results may enable researchers to remove the interfering X-ray signals from the overlapping region with greater confidence and up to eight times the accuracy as was possible previously. NIST previously updated these X-ray values more than 20 years ago.
“We hope that presenting this data . . . will help people with low-resolution detectors, where these lines overlap, subtract out those X-ray lines more accurately,” says Jonathan Dean, an author of the study and a NIST postdoctoral associate.
To make the measurements, the team devised an array of ultrasensitive heat sensors—microcalorimeters—capable of detecting single photons. Cooled to just above absolute zero, a film within the devices lies at the transition edge between behaving as a superconductor and behaving as a regular metal. A photon emitted by a radioactive element causes a minuscule rise in temperature, enough to nudge the superconductor over the edge and generate a spike in electrical resistance. From the electrical measurements, researchers can quickly determine the photon energy.
Describing the sensor work as “groundbreaking,” North Carolina State University’s John Mattingly, a nuclear engineer and specialist in γ ray spectroscopy, notes that these types of devices, which are known as transition-edge sensors, have been around for years. Mattingly, who was not involved in the study, adds that researchers have been “pushing the frontier on resolution.”
The publication is timely considering the developments of small modular nuclear reactors, says Cameron Tracy, a senior research scholar at the Risk and Security Lab at the University of California, Berkeley, who also was not involved in the work. Fuel designed for use in some of these reactors is enriched with enough uranium-235 to fall “just below the cusp of where it kind of becomes a major concern for military uses,” he says. “It makes distinguishing civilian and military material more difficult. You need better, more precise sensors to make that distinction reliably.”
The detectors have been installed at several laboratories but have not been commercialized. NIST researchers are working to reduce the size and cost of the refrigeration systems needed to chill the detectors. Yet each still costs about a million dollars, Dean says. Furthermore, the detectors can be finicky, he adds. “It’s not yet at the level where you can send it to a company that just has a technician to run it.”
