The National Institute of Standards and Technology (NIST), Radioactivity Group, and the National Physical Laboratory (NPL) of the UK, and the national metrology laboratories of their respective countries, exchanged 210Pb solution sources for a blind measurement comparison.
The solutions from each laboratory were linked to their primary standardizations and national standards. Measurements by NIST on both solutions were performed by three methods: viz., liquid scintillation (LS) standardizations (with 3H-standard efficiency tracing); with relative photonic emission rates with a 4ηγ(NaI) sandwich detector; and by high-resolution Si(Li) spectrometry. All three methods were linked to extant NIST standards, which are disseminated as Standard Reference Material SRM 4337. The NPL measurements were based on four methods: high resolution HPGe gamma-ray spectrometry; Čerenkov counting of the 210Bi progeny; LS counting with efficiency tracing with a 3H standard; and measuring the 210Po daughter, in equilibrium, by alpha spectrometry, where the efficiency was traced with 208Po. For both solutions, the massic activity measurements from the two laboratories agreed to about 0.5 %, which was well within the 1.1 % propagated standard uncertainty assigned to the two sets of measurements. These findings add support to an earlier suspicion that the measurement uncertainty previously assigned to the certified massic activity values for both the NIST 210Pb SRM and that for the NPL disseminated standards may be overestimated