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


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). Starting in FY2021, NIST began a multidisciplinary project to develop a new capability for primary standardization of radionuclides. This “TrueBq” project focuses on developing Decay Energy Spectrometery (DES) of quantitatively-prepared sources using ultra-sensitive, cryogenic, Transition Edge Sensors (TESs).

Our team draws from 5 Divisions and 3 Operating Units at NIST with expertise in TES design and fabrication, milligram mass and inkjet metrology, radioactive source preparation, detector electronics and modeling, and algorithms and optimization.

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


True Bq
Credit: Dan Schmidt/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, in collaboration with Los Alamos National Laboratory. By embedding radioactive material in energy-absorber material and placing that material in thermal contact with the sensors, Decay Energy Spectrometery (DES) can be achieved with exquisite energy resolution. In this way, radionuclides can be identified and quantified by their DES signatures. High detection efficiency, reaching 100 % for alpha decay is expected.

A second recent advance is the redefinition of the kilogram in the new SI, adopted by the international community in 2020. The redefinition of mass in the SI allows primary calibrations to be performed at arbitrary mass scales using various electrical metrology paths. 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. Accurately measuring such small masses of dispensed solution is challenging. We are pursuing a solution based on electrical metrology and inkjet technology to quantitatively dispense radionuclide solutions.

By combining 100 % efficiency DES, advanced TES pulse analysis to account for pileup and deadtime, and accurately-measured radionuclide sample masses, we will achieve absolute DES (ADES) for a wide range of radionuclides. This new capability will provide: 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).

inkjet liquid deposit and drying on nanoporous gold
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)

An illustration of the quantitative DES process is shown in Fig. 1 for a simulated use case of measuring massic activity 231Pa in the presence of an 227Ac impurity, for possible nuclear forensics standards applications.

measurement outline illustration
Fig. 1. Measurement outline. a. Mass of radionuclide solution (pink) deposited on substrate (gold) is determined by gravimetric ink-jet dispenser. b. Top layer of substrate is added for 4π decay-energy spectrum (mixed 231Pa/227Ac source simulated). c. Nuclide identification and quantification using library of measured and modeled radionuclide decay signatures.
Credit: Ryan Fitzgerald/NIST

NRC Postdoc Opportunity

Created March 2, 2021, Updated April 1, 2022