Quantum Sensors Sharpen Nuclear Material Assessments

Researchers at the National Institute of Standards and Technology have measured confounding X-ray emissions from key nuclear materials with unprecedented accuracy. The work, reported in Physical Review Letters, uses ultra-cold quantum sensors to cut measurement uncertainty, enabling more precise assessments of nuclear stockpiles and power plant fuel.

According to the source, monitoring nuclear material involves detecting the unique gamma-ray signatures of radioactive elements like plutonium and uranium. These signals are often masked by X-rays emitted in the same energy range. Physicist Jonathan Dean of NIST and the University of Colorado Boulder stated that the new measurements "support international nuclear safeguards by enabling more precise accounting of material in nuclear facilities."

How the Quantum Sensors Work

The team employed an array of transition edge sensors, or TESs, developed at NIST. These devices act as miniature, exquisitely sensitive thermometers. Each sensor consists of a superconducting film held at a temperature just a fraction of a degree above absolute zero, placing it at the transition between a superconductor and an ordinary metal.

When an individual X-ray photon strikes the sensor, it imparts a minuscule amount of heat. This causes a sharp, measurable increase in the film's electrical resistance. The resistance change is directly proportional to the photon's energy, allowing for high-resolution energy measurements. The sensitivity of the TES array reduced the uncertainty of X-ray energy measurements for uranium, plutonium, and neptunium by one-third to one-eighth compared to previous methods.

Applications in Nuclear Safeguards

Filtering out the precise X-ray background allows gamma-ray detectors to characterize nuclear materials more accurately. A critical task is measuring the ratio of different isotopes within a material. For instance, natural uranium contains only 0.7% of the fissile isotope uranium-235. This must be enriched to a few percent for reactor fuel and to 90% for weapons-grade material.

The improved accuracy also speeds up assessments. Dean noted this could shorten hold-up times between processing steps at nuclear power plants, potentially increasing efficiency and reducing costs. The sensors are already in use. In partnership with Los Alamos National Laboratory, NIST has installed TES detectors at three Department of Energy laboratories to monitor on-site nuclear material.

Deployment and Future Directions

The sensors require extreme cooling, making the necessary equipment too bulky for handheld use. However, TES arrays can operate anywhere with sufficient electricity to power the refrigeration system. Alternatively, radioactive samples can be sent to a lab equipped with the technology. "Our instruments are compatible with both approaches," Dean said.

NIST has deployed detectors for research at several major facilities, including the SLAC and Argonne national laboratories in the U.S. And the CERN particle accelerator in Switzerland. Current work aims to push detector accuracy further for fundamental particle searches at CERN and for astrophysics missions with NASA. Efforts are also underway to simplify and miniaturize the cooling systems, with two U.S. Companies now manufacturing a compact NIST-designed refrigerator.