Standardization of tritiated water and 204Tl by TDCR liquid scintillation counting

The triple to double coincidence ratio method was used with a combined uncertainty of 1-2% to standardize tritiated water and a solution of 204Tl. The vial with liquid scintillator, in which the sample to be measured was dissolved, is optically coupled to three photomultipliers. The electronic module MAC-3 assures the selection of double and triple coincidences count rates, D and T, from the three counting channels. It contains the gating circuits, necessary to obtain the livetime value and the extended deadtime circuit. Specific computer programs were used to calculate the free parameter value, the efficiency of D and so, the value of the activity. The optimal value of the Birks ionisation-quenching parameter, kB, was evaluated by changing the detection efficiency with grey filters. Three types of liquid scintillators, namely InstaGel, PPO+POPOP+Triton in toluene and Ultima Gold, were employed. 204Tl was measured in the frame of an international comparison organized by BIPM. For tritiated water a comparison was made with LNHB-Saclay; the relative difference between the obtained values for the massic activity was only 0.2%.