Primary Standardization of Radionuclides by Anticoincidence Counting

The standardization of radionuclides by primary methods is the foundation for all the standards and services provided by the NIST Radioactivity Group.

These include: SRM's, special calibrations, and proficiency testing. The principle method of primary standardization at NIST is live-timed 4pβ-γ anticoincidence counting (LTAC). During the past two years, NIST researchers have extended this method to perform primary measurements on a variety of radionuclei including alpha, beta and positron/EC emitters: namely ¹⁷⁷Lu, ⁶⁷Ga, ²⁴¹Am, ²⁴¹Pu and ^99mTc. These primary standards have been submitted to international comparisons and have been found to be in excellent agreement with other national metrology institutes. Furthermore, NIST staff calibrates their instruments directly with these primary standards. This powerful tool was also used to make the definitive determination of the Liquid Scintillation (LS) wall-effect. We also began a series of detailed studies of LS background needed for low-energy and low-activity measurements: quantifying volume and source-shape effects on LS background spectra. This past year, the NIST team developed a method for standardizing ²⁴¹Pu using LTAC measurements of the ²⁴¹Am daughter in combination with relative measurements and also by a semi-free-impurity in-growth method.