Basic Metrology: True Becquerel - A New Paradigm for 21st Century Radioactivity Measurements

Summary

Expanding applications of radioactivity in medicine, energy, and national security demand quantification of complex radionuclide mixtures at uncertainty levels that are currently unachievable (sometimes by a factor of 10). The NIST “True Bq” project meets this need by combining Decay Energy Spectrometry (DES) using ultra-sensitive, cryogenic, Transition Edge Sensors (TESs) with quantitative source preparation using drop-on-demand inkjet.

Recent results include the primary standardizations of solutions of Am-241 and Am-243, the latter work including assay of radionuclide impurities (ICRM2025, paper in progress). Our inkjet deposition work has spun off to other applications including quantitative preparation of autoradiography phantoms.

Our team includes experts in TES design and fabrication, milligram mass and inkjet metrology, radioactive source preparation, detector modelling, and spectral analysis.

We are presently seeking postdoctoral candidates and other collaborators on multiple aspects of the project. Please contact Ryan Fitzgerald (ryan.fitzgerald [at] nist.gov (ryan[dot]fitzgerald[at]nist[dot]gov)) and see our NRC postdoc opportunity.

Description

Close-up of a superconducting sensor board containing multiple transition edge sensors (top row of squares), which detect energy released by individual radioactive decay events.

Credit: M. Carlson/NIST

A diverse user community needs to both: identify radionuclides present in a material and quantify the massic activity (Bq/g) of each radionuclide, in a manner that’s traceable to the SI. At present, there is no method to achieve both objectives simultaneously for all types of decay. In many cases, analysis requires a complex chain of chemical separation and reference materials, with results too resource-intensive and uncertain.

Recent advances in multiple fields of metrology present an opportunity to innovate. For one, ultra-cold sensors, such as Transition Edge Sensors (TESs) and Magnetic Microcalorimeters (MMCs) offer exquisite sensitivity to thermal pulses. Our efforts focus on TESs. By embedding radioactive material in energy-absorber material and placing that material in thermal contact with the sensors, Decay Energy Spectrometry (DES) can be achieved with exquisite energy resolution. In this way, radionuclides can be identified and quantified by their DES signatures with high detection efficiency, reaching 100 % for alpha decay. Geant4-based Monte Carlo simulations are incorporated into multi-nuclide spectral analysis.

inkjet liquid deposit and drying on nanoporous gold

Inkjet deposition of an aqueous solution containing dissolved strontium chloride. Substrate is 1.7 um nanoporous gold (np-Au) on 100 nm Au, on 1 nm Ti, on a Si wafer. A total of 200 droplets amounting to 12 nL, containing 1.2 pg strontium is deposited. Video shows wicking and evaporation without liquid creep. Afterwards, Sr remains within the nanoporous gold layer. Credit: Mike Verkouteren (NIST)

Measuring mass is critical for True Bq since many applications require assay of the massic activity (Bq/g) of a starting aqueous solution. Since ultra-cold sensors need to be kept as small as possible, only small (<1 mg) masses of solution can be sampled. We have developed a manual method for masses of about 5 mg, an inkjet method for masses as low as about 20 µg with relative standard uncertainty of about 0.5 % and is being leveraged for imaging phantoms as well. Meanwhile, we are pursuing an electrostatic-force balance with integrated dispenser to further reduce uncertainty.

Applications of interest include radioactive dating for security and forensics applications, deployable all-in-one analysis for environmental; trustworthy assay of emerging radiopharmaceuticals, and a new tool to understand radiation-induced errors in critical computing applications (including quantum computing).