Ra-224 Activity, Half-Life, And 241 Kev Gamma Ray Absolute Emission Intensity: A Nist-Npl Bilateral Comparison

The national metrology institutes for the United Kingdom (UK) and the United States of America (USA) have compared activity standards for Ra-224, an a-particle emitter of interest as the basis for therapeutic radiophar-maceuticals. Solutions of (RaCl2)-Ra-224 were assayed by absolute methods, including digital coincidence counting and triple-to-double coincidence ratio liquid scintillation counting. Ionization chamber and high-purity germanium (HPGe) gamma-ray spectrometry calibrations were compared; further, a solution was shipped between laboratories for a direct comparison by HPGe spectrometry. New determinations of the absolute emission intensity for the 241 keV gamma ray (I gamma = 4.011(16) per 100 disintegrations of Ra-224) and of the Ra-224 half-life (T-1/2 = 3.6313(14) d) are presented and discussed in the context of previous measurements and evaluations.