Participation of the Radionuclide Metrology Laboratory in international comparisons

Two international comparisons regarding radioactive standard solutions were initiated by the 'Bureau International des Poids et Mesures' (BIPM) and 'West European Measurement System' (Euromet), in order to quantify the equivalence of the national standards at the international level; our laboratory participated in both of them. The BIPM (Sevres, France), organized a large scale comparison, regarding the absolute standardization of a Tl-204 solution. Tl-204 decays by 97.6 % beta emission and 2.4 % electron capture. The participating laboratories used two types of methods for the standardization: liquid scintillation (LS) methods and proportional counters (p.c.) methods. In applying the 4π LS method we used a home made equipment built on a 9804 EMI Phototube and a CANBERRA MCA 8604A pulse height analyzer; the sources were prepared by dispensing Tl-204 in commercial Packard type vials, containing Packard INSTA-GEL solution. A 20 keV discrimination threshold was set, in order to avoid spurious pulses, and an extrapolation to 100 % detection efficiency (εβ) was applied (a linear extrapolation curve was considered). The final result, radioactivity concentration was: (63.1 ± 1.3)kBqg-1 on the reference date, close to the LS Triple to Double Coincidence Ratio Method values. The 4πp.c. tracer method used the classical 4πp.c.-γ coincidence installation. As a tracer, a Co-60 standard solution (with an uncertainty ± 0.1 %) prepared in our lab, was used. Mixture solution Co-60 (tracer) + Tl-204 (beta emitter) gravimetrically prepared was disposed onto golded VYNS films (sources for absolute standardization). Several efficiency variation intervals were used and several fittings of extrapolation expressions type (Nβ)Tl-204 against (1-εβtracer) were calculated. A linear extrapolation relation valid within the efficiency interval 0.15 ≤ (1-εβtracer) ≤ 0.3 was found. The final result of the radioactivity concentration was (61.8 ± 1.2) kBqg-1 on the reference date; the difference between this result and the proportional counter results obtained in other laboratories was only (+0.3 %). Euromet organized a comparison regarding the absolute standardization of Yb-169, a radionuclide decaying by electron capture. The 4πp.c. -γ coincidence method, in the efficiency extrapolation variant, was used. The main difficulties are: - the main excitation level of Tm-169 (316 keV) has a half life of 0.66 μs, consequently a long coincidence resolving time (5.1 μs) was used; the remaining corrections were due to coincidence losses (0.5 %) and the double registration of an event in the beta channel (-0.15 %); - the high values of conversion coefficients of the low energy levels, resulting in a high decay scheme correction. This latter difficulty an 'equivalent decay schema' was considered and a high gamma discrimination level (150 keV) was used. A second degree extrapolation curve was calculated. The final result, radioactivity concentration, reported to the organizers was (11.99 ± 0.11) MBqg-1 on the reference date, with a deviation from the general mean of (-0.2 %). (authors)