Speaker
Description
Authors (affiliation): 1. Denis E. Bergeron (NIST, USA), 2. Richard Essex (NIST, USA), 3. Svetlana Nour (NIST, USA), 4. Gordon A. Shaw (NIST, USA), 5. R. Michael Verkouteren (NIST, USA), 6. Ryan P. Fitzgerald (NIST, USA).
The aspirating pycnometer method for gravimetric preparation of radioactive sources routinely achieves relative mass uncertainties ≈ 0.05 % when several 10 mg to 25 mg drops of aqueous material are dispensed. To support new research into decay energy spectrometry with cryocalorimeters, our team is working to prepare sources with just 1 mg to 5 mg (< 5 μL) of aqueous material. Here, we demonstrate a manual gravimetric dispensing technique using a micropipettor modified for use with removeable microcapillaries. A well-characterized Am-241 standard reference material (SRM) was used to prepare sources for liquid scintillation counting, providing a precise radiometric check of the gravimetric dispensing.
Microliter sources prepared by direct deposition into vials containing liquid scintillation cocktail gave the same massic activity as sources similarly prepared with larger masses using the traditional aspirating pycnometer method to within the counting uncertainties (< 0.15 %). Using the same method, microliter sources were prepared on gold foils and then transferred to vials containing liquid scintillation cocktail; the massic activity of these sources was also consistent within uncertainties, but some evidence for activity losses (typically < 0.1 % of the total activity) was observed by measuring gel "placemats" upon which the target foils had been placed.
We present a detailed measurement equation for the weighing technique, including the corrections for evaporation while the microcapillary is on the ultramicrobalance and during dispensing and addressing attendant uncertainties. We continue to pursue alternative methods for drop deposition (including an inkjet-based approach) and for uncertainty characterization (including isotope dilution mass spectrometry).
