The standardization of a 204Tl solution

Two methods were used for the standardization of the 204Tl solution received in the framework of the 1997 BIPM comparison. The first method, 4pi liquid scintillation (LS), consisted of the extrapolation of the 4pi counting rate to the zero discrimination level. A home-made, single channel instrument and a commercial Packard INSTA-GEL scintillator were used. The radioactivity concentration on the reference date 1 April 1997 was 63.1+/-1.3 kBq g(-1). The second method was the efficiency tracer technique, using a 60Co standard solution; the radioactivity concentration was calculated as the mean of several extrapolated values of the curve NbetaTl-204 = N0Tl-204f(1 - epsilon(betaCo-60)). The preliminary reported value was 62.8+/-0.8 kBq g(-1); the revised value is 61.8+/-1.2 kBq g(-1), in good agreement with other 4pi (p.c.) tracer results in the comparison. Some remarks are made regarding our measurements and the comparison results.