New NIST Method Precisely Measures Radioactivity in Tiny Samples

Researchers at the National Institute of Standards and Technology (NIST) have demonstrated a new and faster method for detecting and measuring the radioactivity of minuscule amounts of radioactive material. The innovative technique, known as cryogenic decay energy spectrometry (DES), could have far-reaching impacts, from improving cancer treatments to ensuring the safety of nuclear waste cleanup.

The NIST team has published its results in Metrologia.

The key to this novel technique is a transition-edge sensor (TES), a high-tech device widely used to measure radiation signatures. TES provides a revolutionary capability to record individual radioactive decay events, in which an unstable atom releases one or more particles. By building up data from many individual decays, researchers can then identify which unstable atoms, known as radionuclides, produce the events.

“The TES is much more advanced than a familiar Geiger counter or other detectors used today,” said NIST physicist Ryan Fitzgerald. “Instead of just clicking to indicate radiation, or giving a blurry indication of the decay energy, it gives us a detailed fingerprint of what’s there.”

The TES device operates at extremely low temperatures, near absolute zero. When a radioactive decay occurs in a sample, the energy released is absorbed by the TES. This absorbed energy causes a tiny change in the electrical resistance of the TES. The researchers precisely measure this change in resistance, which provides a high-resolution “energy signature” of the decay event. By analyzing the detailed energy spectrum from multiple events, the researchers can identify the specific radioactive atom undergoing decay. This is possible because different radioactive atoms release unique energy signatures when they decay.

Earlier methods are good at either measuring the amount of radioactivity or identifying which radioactive atoms are present — not both. Fully characterizing a sample once required the use of multiple techniques. In contrast, DES both identifies radioactive elements and quantifies their level of radioactivity.

Rapid Detection and Measurement

When researchers encounter a barrel full of radioactive fluid, they need to identify the mystery substance and measure the amount of radionuclides present to dispose of it safely. Ordinarily, that process could take months, but the transition-edge sensor can significantly cut that time.

“Instead of waiting months for results, we can now get a full radioactivity profile in just a few days from a tiny sample,” said Fitzgerald.

Traditionally, measuring radioactivity required multiple methods and intricate procedures using additional materials called tracers or calibrants. However, the new method offers a streamlined approach, allowing accurate measurement of even tiny samples without needing these extra materials. This allows scientists to better monitor, use and safeguard radioactive materials that affect public health and safety.

Inkjet Precision in Radioactivity Analysis

In their method, the researchers use a specialized inkjet device to carefully dispense tiny amounts, less than 1 millionth of a gram, of a radioactive solution onto thin gold foils. These gold foils have a surface dotted with tiny pores just billionths of a meter in size. These nanopores help to absorb the tiny droplets of the radioactive solution.

By precisely measuring the mass of the solution that was dispensed using the inkjet and then measuring the radioactivity of the dried sample on the gold foils, the researchers can calculate the radioactivity per unit mass, or the “massic activity,” of the sample. This inkjet method allows them to work with extremely small amounts of radioactive material and still get an accurate measurement of its radioactivity.

The potential applications are vast. In medicine, this technology could help ensure the purity and potency of radioactive drugs used in cancer treatments.  For nuclear energy, it could quickly identify radioactive composition of reprocessed fuel, speeding development of new advanced reactors.

TrueBq: Redefining Radioactivity Measurement

The newly reported research is the first step in a larger effort, known as the True Becquerel (TrueBq) project, to transform how we monitor and characterize radioactivity. The project’s name is derived from the unit for measuring radioactive decay, honoring the French physicist Henri Becquerel, who discovered radioactivity.

The broader TrueBq project aims to develop a more comprehensive measurement system that can handle a wide range of radioactive substances, including complex mixtures. It will integrate a precision mass balance system with the TES device to measure the massic activity of radioactive materials with unprecedented accuracy. 

This new approach represents a significant improvement over traditional workflows, which often involve multiple methods, chemical processing, and the use of chemical tracers and standards. By streamlining the measurement process, TrueBq is expected to reduce the time required for analysis while simultaneously increasing accuracy.

The innovations developed through the TrueBq project could enhance NIST’s ability to serve its customers effectively. NIST offers a variety of customer-focused measurement services, including calibrations, standard reference materials (SRMs) and proficiency testing. All of these services stand to benefit from the streamlined workflow, additional information and reduced uncertainties that the TrueBq technology could potentially deliver.

While TrueBq’s current focus is on improving measurements at NIST itself, researchers have long-term aspirations for the technology. In the future, they hope to develop more portable and user-friendly versions of the system that could be deployed outside of NIST for critical applications in fields such as medicine, environmental cleanup and nuclear waste management.

Paper: Ryan P. Fitzgerald, Bradley Alpert, Denis E. Bergeron, Max Carlson, Richard Essex, Sean Jollota, Kelsey Morgan, Shin Muramoto, Svetlana Nour, Galen O’Neil, Daniel R. Schmidt, Gordon Shaw, Daniel Swetz and R. Michael Verkouteren. Primary activity measurement of an Am-241 solution using microgram inkjet gravimetry and decay energy spectrometry. Metrologia. Published online July 8, 2025. DOI: 10.1088/1681-7575/adecaa