Tritium assay in water samples using electrolytic enrichment and liquid scintillation spectrometry

Tritium, produced during atmospheric thermonuclear tests, is used as a global transient tracer for studying dynamics in the hydrological cycle. Because of the very low level of radioactivity involved in those studies, the water samples have to be purified from interfering radionuclides and the tritium is enriched by electrolysis before the measurement can be made. In the given example the uncertainty sources related to the distillation and the electrolytic enrichment are identified and quantified, in addition to the usual uncertainty sources for low level radioactivity measurements, such as counting rate, background radiation and decay correction factors. The example elaborates on two case studies involving different levels of tritium in order to illustrate the relative importance of the different sources of uncertainty as a function of the measured radioactivity. It is shown that for low tritium levels the uncertainty on the count rate is dominating, whereas for higher tritium levels the major contribution to the total uncertainty is due to the enrichment parameter. (author)