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 4ηβ-γ anticoincidence counting (LTAC). Recently NIST researchers have extended this method to perform primary measurements on a variety of radionuclides that decay by alpha, beta and positron and electron-capture, most recently, 99Tc, 131I, 18F, 237Np and 124I. Primary standards have been submitted to international comparisons and have been found to be in excellent agreement with other national metrology institutes. Furthermore, NIST researchers use these primary standards to calibrate their own secondary instruments, including ionization chambers and γ-ray spectrometers. This powerful tool was also recently used to make the definitive determination of the Liquid Scintillation (LS) wall-effect. Ongoing work includes improvement in accuracy by incorporating GEANT simulations into efficiency extrapolations and the inclusion of a digital electronics chain.
MONTE-CARLO SIMULATION OF NIST LIVE-TIMED ANTICOINCIDENCE SYSTEM. SIMULATION AND EXPERIMENT ARE COMBINED TO ACHIEVE HIGH ACCURACY IN THE PRIMARY STANDARDIZATION OF ELECTRON CAPTURE RADIONUCLIDES. GRAPHIC BY RYAN FITZGERALD, NIST.