Nuclear data as touchstone and measurand in the establishment, comparison, and dissemination of activity standards for $^{\mathrm{224}}$Ra*

Radium-224 decays by alpha emission with a half-life of 3.631(2) d, approaching secular equilibrium with its progeny approximately 6 d after separation from its $^{\mathrm{228}}$Th parent. Like some other alpha-emitting nuclides, $^{\mathrm{224}}$Ra is currently being investigated for applications in molecular radiotherapy of various forms of cancer. In such applications, understanding dose-response relationships requires accurate activity assays. We address some challenges in assaying a nuclide approaching or at equilibrium with short-lived progeny, with focus on nuclear data considerations. Half-lives govern the relative abundances of the progeny, while the gamma-ray emission probabilities underlie common clinical activity measurements. We treat the $\gamma_{\mathrm{1,0}}$(Rn) (241 keV) emission as both a point of reference for a bilateral intercomparison and, then, as a measurand. We assess equivalence between the USA and UK standards for $^{\mathrm{224}}$Ra activity and report new values for the $^{\mathrm{224}}$Ra half-life and $\gamma_{\mathrm{1,0}}$(Rn) emission probability.