The moment nuclear material is produced, processed or purified, it sets off a hidden countdown, marked by the half-life of its radioactive atoms as they begin to decay. For scientists tracking the origins of these substances, decoding this natural clock is crucial for verifying material histories in support of global security efforts.
In a new study published in the Journal of Radioanalytical and Nuclear Chemistry, researchers at Lawrence Livermore National Laboratory (LLNL) and collaborators at the University of New Mexico and the University of Michigan offer a novel approach for measuring the age of nuclear materials. Relying on ultra-cold microcalorimeters operating at 0.01 Kelvin, the team successfully determined the age of a 100-day old plutonium sample that weighed only 26 trillionths of a gram by measuring the decay-rate ratio of plutonium-241 (²⁴¹Pu) to its decay product americium-241 (²⁴¹Am).
Accurate nuclear age-dating helps determine when nuclear material was made or last processed, which is important for investigating the origin of samples for nuclear forensics and safeguards. Organizations like the International Atomic Energy Agency can use these results, along with other measurements, to verify whether nuclear materials match what states have declared under international safeguards, or to identify any undeclared activities.
Traditional techniques, such as mass spectrometry, determine the age of a sample by quantifying its content of plutonium-241 and americium-241 atoms. Mass spectrometry often requires costly and time-intensive chemical separation procedures since isobaric interferences, where atoms from different elements share similar masses, can affect accuracy.
"Mass spectrometry is extremely precise, but it can require complex preparation and careful laboratory work," said Geon-Bo Kim, LLNL staff physicist. "Traditional radiation-based (radiometric) techniques, such as gamma or alpha spectrometry, are often simpler to perform, but they can become less accurate when only tiny amounts of material are available."
Gamma-ray spectrometry identifies radiation emitted by these isotopes without requiring chemical separation. However, since plutonium-241 produces notably weak gamma emissions, gamma-ray spectrometry generally demands substantially larger sample sizes than those used in this study to achieve reliable results.
Cryogenic decay energy spectrometry (DES) takes a fundamentally different approach from traditional radiation-based methods, enabling highly precise measurements even for extremely small amounts of plutonium. The method relies on one of LLNL's quantum sensing technologies, Magnetic Microcalorimetry (MMC), which has ultra-sensitive sensors cooled to near-absolute zero. When a radioactive decay happens inside the detector, it releases energy that produces an incredibly small temperature rise and a corresponding change in the material's magnetism. This change in magnetism is measured by a quantum magnetometer with extreme precision.
In the DES experiment, the team embedded the plutonium sample directly in the microcalorimeter so each decay event could be measured one by one. The ratio of the plutonium-241 and the americium-241 becomes the "radioactive clock," used to date the age of the sample with an uncertainty of only a few days.
"It's a new approach directly counting individual nuclear decays with 100% efficiency. We believe that it can complement today's state-of-the-art methods by providing an independent, orthogonal measurement for added confidence," said Kim.
DES has the potential to not only verify results but provide faster analysis and increased cost-efficiency. By eliminating the need for chemical purification that involves precise quantification of separation and recovery yields, the technique can reduce sample preparation steps and the risk of procedural errors.
These advantages position MMC and DES as a valuable tool for nuclear forensics and safeguards, enabling organizations to obtain critical information about nuclear materials more rapidly and with fewer resources.
This work was supported by the U.S. Department of Energy by Lawrence Livermore National Laboratory and funded by the National Nuclear Security Administration of the Department of Energy, Office of International Nuclear Safeguards, Office of Defense Nuclear Nonproliferation Research and Development and the Consortium for Nuclear Forensics.
