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, an α-particle emitter of interest as the basis for therapeutic radiopharmaceuticals. Solutions of RaCl 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) γ-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 γ ray (I = 4.011(16) per 100 disintegrations of Ra) and of the Ra half-life (T = 3.6313(14) d) are presented and discussed in the context of previous measurements and evaluations.