Toward a New Primary Standardization of Radionuclide Massic Activity Using Microcalorimetry and Quantitative Milligram-Scale Samples.

We present a new paradigm for the primary standardization of radionuclide activity per mass of solution (Bq/g). Two key enabling capabilities are 4π decay-energy spectrometry using chip-scale sub-Kelvin microcalorimeters and direct realization of mass by gravimetric inkjet dispensing using an electrostatic force balance. In contrast to traditional traceability, which typically relies on chemical separation of single-radionuclide samples, 4π integral counting, and additional spectrometry methods to verify purity, the system described here has both 4π counting efficiency and spectroscopic resolution sufficient to identify multiple radionuclides in the same sample at once. This enables primary standardization of activity concentrations of mixed-radionuclide samples. A major benefit of this capability, beyond metrology, is in assay of environmental and forensics samples, for which the quantification of multiplenuclide samples can be achieved where presently inhibited by interferences. This can be achieved without the need for chemical separations or efficiency tracers, thereby vastly reducing time, radioactive waste, and resulting measurement uncertainty.

Natural Uranium Radioactivity Solution Standard: SRM 4321d.

A new natural uranium solution standard has been produced and will be disseminated by the National Institute of Standards and Technology (NIST) as Standard Reference Material 4321d. The standard is certified for the massic activities of 234U, 235U, and 238U in solution, and it is based on isotopic mass data for the metallic Certified Reference Material (CRM) 112-A (originally issued as SRM 960) that was obtained from THE U.S. Department of Energy, New Brunswick Laboratory. The metallic CRM was chemically cleaned, dissolved, and gravimetrically diluted to prepare a master solution, which was quantitatively dispensed into 5 mL aliquots that were contained within flame-sealed glass ampoules for each SRM unit. Homogeneity among SRM units, verifying solution homogeneity, was substantiated by photonic-emission integral counting with a NaI(Tl) well counter. Confirmatory measurements were performed by liquid scintillation counting for the total massic activity, and by isotope dilution α spectrometry for the 234U and 238U massic activities.

Gravimetric deposition of microliter drops with radiometric confirmation.

A manual microliter gravimetric dispensing technique is demonstrated using a micropipettor modified for use with removeable microcapillaries. Liquid scintillation sources were prepared from a well-characterized 241Am reference solution, providing a radiometric check of dispensed masses. Further experiments confirmed controlled dispensing of drops onto gold foils with losses ≤0.34(4) % of the total drop activity. A detailed measurement equation for the weighing technique, including the corrections for evaporation, is presented with a full accounting of associated uncertainties.

Preparation and evaluation of new reference materials for naturally occurring radioactive materials (NORM): Zirconium silicate, bauxite, and phosphogypsum.

New reference materials (RMs), zirconium silicate, bauxite and phosphogypsum, were produced and characterized according to an ISO guide. The homogeneity of the three RMs was evaluated using X-ray fluorescence (XRF), and characterizations of the three candidate materials were performed through a collaborative study with nine expert radioanalytical laboratories. The assigned radionuclides are 230Th, 232Th, 234U, 235U, and 238U for zirconium silicate; 230Th, 232Th, 234U, and 238U for bauxite; and 226Ra, 230Th, 234U, and 238U for phosphogypsum.

Preparation and calibration of a 231Pa reference material.

A 231Pa reference material has been characterized for amount of protactinium. This reference material is primarily intended for calibration of 233Pa tracers produced for 235U-231Pa model age measurements associated with nuclear forensics and nuclear safeguards. Primary measurements for characterization were made by isotope dilution mass spectrometry of a purified 231Pa solution using a 233Pa isotopic spike. The spike was calibrated by allowing multiple aliquots of the 233Pa spike solution to decay to 233U and then measuring the ingrown 233U by isotope dilution mass spectrometry using a certified uranium assay and isotopic standard as a reverse-spike. The new 231Pa reference material will simplify calibration of the 233Pa isotope dilution spikes, provide metrological traceability, and potentially reduce the overall measurement uncertainty of model ages.

234Th analysis of marine sediments via extraction chromatography and liquid scintillation counting.

234Th is widely used as a natural tracer for study of biological mixing and particle scavenging processes in the ocean. This naturally occurring nuclide serves this purpose due to its convenient half-life (24.1 days), constant rate of production from 238U dissolved in seawater, and its strong tendency to attach to particles in seawater. As a beta/gamma emitter, 234Th may be determined using low-level gas-flow proportional counting, gamma spectrometry, and liquid scintillation counting (LSC). We describe here a technique which combines Cerenkov counting to evaluate 234Th (via 234Pa) with LSC alpha counting of 230Th added to the samples as a yield tracer. Our separation approach is based on a sample preparation procedure for marine sediments using nitric acid leaching in a "hot block", and extraction chromatography (TEVA x Resin) for Th isolation. Samples are counted in plastic LSC vials, using Ultima Gold AB cocktail, in 1 M H3PO4 media. A series of sediment samples spiked with known amounts of 234Th yielded activities within a few percent of the anticipated values.

Emergency radiobioassay preparedness exercises through the NIST radiochemistry intercomparison program.

The present challenge for the international emergency radiobioassay community is to analyze contaminated samples rapidly while maintaining high quality results. The National Institute of Standards and Technology (NIST) runs a radiobioassay measurement traceability testing program to evaluate the radioanalytical capabilities of participating laboratories. The NIST Radiochemistry Intercomparison Program (NRIP) started more than 10 years ago, and emergency performance testing was added to the program seven years ago. Radiobioassay turnaround times under the NRIP program for routine production and under emergency response scenarios are 60 d and 8 h, respectively. Because measurement accuracy and sample turnaround time are very critical in a radiological emergency, response laboratories' analytical systems are best evaluated and improved through traceable Performance Testing (PT) programs. The NRIP provides participant laboratories with metrology tools to evaluate their performance and to improve it. The program motivates the laboratories to optimize their methodologies and minimize the turnaround time of their results. Likewise, NIST has to make adjustments and periodical changes in the bioassay test samples in order to challenge the participating laboratories continually. With practice, radioanalytical measurements turnaround time can be reduced to 3-4 h.

Ultra-low level plutonium isotopes in the NIST SRM 4355A (Peruvian Soil-1).

For more than 20 years, countries and their agencies which monitor radionuclide discharge sites and storage facilities have relied on the National Institute of Standards and Technology (NIST) Standard Reference Material (SRM) 4355 Peruvian Soil. Its low fallout contamination makes it an ideal soil blank for measurements associated with terrestrial-pathway-to-man studies. Presently, SRM 4355 is out of stock, and a new batch of the Peruvian soil is currently under development as future NIST SRM 4355A. Both environmental radioanalytical laboratories and mass spectrometry communities will benefit from the use of this SRM. The former must assess their laboratory procedural contamination and measurement detection limits by measurement of blank sample material. The Peruvian Soil is so low in anthropogenic radionuclide content that it is a suitable virtual blank. On the other hand, mass spectrometric laboratories have high sensitivity instruments that are capable of quantitative isotopic measurements at low plutonium levels in the SRM 4355 (first Peruvian Soil SRM) that provided the mass spectrometric community with the calibration, quality control, and testing material needed for methods development and legal defensibility. The quantification of the ultra-low plutonium content in the SRM 4355A was a considerable challenge for the mass spectrometric laboratories. Careful blank control and correction, isobaric interferences, instrument stability, peak assessment, and detection assessment were necessary. Furthermore, a systematic statistical evaluation of the measurement results and considerable discussions with the mass spectroscopy metrologists were needed to derive the certified values and uncertainties. The one sided upper limit of the 95% tolerance with 95% confidence for the massic (239)Pu content in SRM 4355A is estimated to be 54,000 atoms/g.