Liquid scintillation counting (LSC) techniques can be used for radionuclide standardization when the calculation of detection efficiency is possible. This is done using a model of the physicochemical processes involved in light emission and also of the statistics of photon emission: the free parameter model. This model can then be applied in two ways: by deducing the free parameter from the measurement of a tracer (the CIEMAT/NIST method) or by calculating this free parameter from coincidence ratio in a specific LS counter (the TDCR method). The purpose of this paper is to describe both these models and some practical issues that need to be addressed if LSC is to be effectively used in radionuclide metrology.

Liquid scintillation counting is a very powerful technique for the activity determination of a number of radionuclides. In radionuclide metrology, the TDCR method and the CIEMAT/ NIST efficiency tracing technique are widely used in many laboratories.Both methods require rather complex calculation techniques to derive the counting efficiency of the nuclide under study.This article explores the various sources of uncertainty that should be considered when applying these two techniques, and focuses on possible ways to evaluate them. Concrete examples are provided within the paper.

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The international reference system for radioactivity relies on stable instrumentation for comparing primary standards of radioactivity; these instruments enable national metrology institutes to demonstrate the equivalence of their primary standards. The international bureau of weights and measures (BIPM) holds two such instruments for gamma-ray emitting radionuclides, and is working with the POLATOM, LNE/LNHB, NIM, NPL and PTB to develop a new system called the 'extended international reference system' (ESIR). The ESIR will address pure beta-particle emitting radionuclides as well as other radionuclides (such as 55 Fe) which cannot be measured in the established international systems. The ESIR will be a liquid scintillation system based on three photon-counting channels operating in coincidence. This article reports the results from validation studies carried out using solutions of 3 H, 55 Fe, 63 Ni and 14 C to assess the reproducibility of the results. Several key comparison indicators have been developed and tested to find the best way to obtain a robust and reproducible international reference value. An important conclusion is that the new ESIR can deliver accurate comparison values, immune from changes in detection efficiency or asymmetry of the counting channels. A relative uncertainty better than 0.2% can be expected for radionuclides emitting beta particles with an end-point energy above 150 keV while using commercial liquid scintillation cocktails.

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