Transition edge sensors (TESs) developed at NIST have measured Kα X-ray emissions from plutonium, uranium, and neptunium at uncertainty values one-third to one-eighth those of prior data — a 3× to 8× improvement that lets safeguards analysts subtract the X-ray background that previously masked gamma-ray signatures from these actinides.

The work, published online Sept. 10, 2026 in Physical Review Letters (DOI: 10.1103/tcbz-kqk1) with A. Wessels as first author, brings together NIST and University of Colorado Boulder physicist Jonathan Dean with coauthors from Los Alamos National Laboratory, Houghton University, and the Kastler Brossel Laboratory at Sorbonne University.

"Our measurements support international nuclear safeguards by enabling more precise accounting of material in nuclear facilities," Dean said.

How does a transition edge sensor work?

A TES is a superconducting thin film held just a fraction of a kelvin above absolute zero, poised on the phase boundary between zero-resistance superconductivity and measurable metallic resistance. A single X-ray photon deposits a tiny heat pulse; the resulting resistance jump scales linearly with photon energy, so the device acts as both a calorimeter and a high-resolution energy spectrometer.

The team recorded X-ray photons in the energy band where actinide Kα lines overlap the gamma-ray emissions inspectors track for isotope-ratio analysis. The TES resolution lets analysts model and remove the obscuring X-ray component from the spectrum.

Why does isotope ratio matter?

Uranium-235 occurs at 0.7% natural abundance. Reactor fuel enrichment raises that figure to a few percent; weapons-grade uranium reaches 90%. Gamma spectroscopy is the standard assay, but its accuracy collapses when X-ray continuum from the same sample drowns the relevant photopeaks.

Tighter X-ray data sharpens the background model behind every gamma-spectrum fit, improving isotope-ratio estimates drawn from a single detector readout.

Where is the hardware deployed?

In partnership with Los Alamos, NIST has installed TES arrays at three Department of Energy sites to monitor nuclear material. Additional instruments operate at:

• SLAC National Accelerator Laboratory
• Advanced Photon Source, Argonne National Laboratory
• National Synchrotron Light Source, Brookhaven National Laboratory
• CERN, Switzerland

The team is also working on installing a new detector at NASA Goddard to study the most extreme astrophysical environments. Separately, it collaborates with CERN to push the detectors' accuracy to new levels for searches of new fundamental particles.

What is the practical adoption barrier?

The sensor stage requires sub-kelvin cooling; the necessary refrigeration gear is too bulky and power-hungry for handheld or in-pipe use. Two U.S. firms already manufacture a compact, NIST-designed refrigeration unit, and NIST continues to push for simpler, smaller, and cheaper cryogenics.

"Our instruments are compatible with both approaches," Dean said, referring to on-site TES readout at facilities with reliable power, or sampling and shipment to a NIST-class lab where cryogenics are available.

For commercial nuclear power, the gain is operational as well as regulatory. Generating electricity from fission is a multistep process, and operators must reassess fuel composition before each step. Faster isotopic verification should shorten the hold-up time between steps and trim costs at each licensed power plant.