This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: To monitor the amount and type of nuclear material at power plants and weapons facilities, scientists look for a special signal—the unique pattern of gamma rays emitted by specific radioactive elements. However, some of these elements also emit X-rays in the same energy range as the gamma-ray emissions, masking the signal and making nuclear stockpiles harder to assess.
Now, researchers at the National Institute of Standards and Technology (NIST) have measured the confounding X-ray emissions from plutonium, uranium and neptunium (a nuclear decay product of uranium) with unprecedented accuracy. This achievement allows scientists to filter out the X-ray background noise so they can more precisely evaluate the accumulation of nuclear materials. "Our measurements support international nuclear safeguards by enabling more precise accounting of material in nuclear facilities," said Jonathan Dean, a physicist at NIST and the University of Colorado Boulder.
Dean and his colleagues, including researchers from NIST, the University of Colorado Boulder, Los Alamos National Laboratory in New Mexico, Houghton University in New York and the Kastler Brossel Laboratory at Sorbonne University in Paris, reported their work in Physical Review Letters. To make the X-ray measurements, the team employed an array of quantum sensors developed at NIST, which act as miniature, exquisitely sensitive thermometers. These devices, known as transition edge sensors (TESs), consist of a superconducting film held at a temperature just a fraction of a degree above absolute zero.
At this temperature, the film lies right at the transition region between a zero-resistance superconductor and an ordinary metal with measurable resistance. When an individual particle of light, such as an X-ray photon, strikes the sensor, it imparts a minuscule amount of heat—but just enough to sharply increase the resistance of the superconducting film. The resistance is directly proportional to the energy of the photon, enabling the sensor to make high-resolution energy measurements.
The researchers measured the X-rays emitted by uranium, plutonium and neptunium in the energy range where they overlap with gamma-ray emissions. The sensitivity of the TES reduced the uncertainty of the X-ray energy measurements by one-third to one-eighth compared with previous measurements. By precisely measuring and effectively subtracting the obscuring X-ray radiation, TESs and other detectors that record gamma rays can more accurately characterize nuclear materials.
For instance, atoms of a single element can contain different numbers of neutrons in their nuclei—distinct forms called isotopes. Measuring the ratio between these isotopes is critical because it can indicate whether a material is intended for use in a nuclear power plant or a nuclear weapon. For example, the isotope uranium-235 accounts for only 0.7% of the total abundance of uranium in nature but must be enriched to a relative abundance of a few percent for fuel and 90% for weapons-grade material.
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