Standardization of 204Tl and 89Sr at VNIIM

D.I. Mendeleyev Institute for Metrology (VNIIM) participated in two international comparisons of 204Tl and 89Sr solutions organized by the BIPM. The activity concentration was measured using the 4π PC β-counting method. Special attention was paid for investigating the dependence of the result on the inner diameter of the source support ring. For thin Al rings (100–200 μm), no dependence was detected. The β-impurities in the 89Sr solution were determined using a scintillation plastic spectrometer. The combined standard uncertainty of the standardization of the 89Sr solution is 0.2%, and that for 204Tl is 0.4%.