Survey of systems for concentration and low background counting of tritium in water

This paper surveys systems used for concentrating and counting tritium activities in the range 0.1-5 000 TU. It should serve as a guide to the factors involved in such measurements, for scientists interested in practical applications of natural and bomb-produced tritium as a tracer, and for those who wish to construct similar apparatus. In most tritium-counting systems the criterion to be applied in comparing methods is the time required to achieve the desired rms relative precision. This time decreases with increasing sample activity, increasing amount of sample in the counter and with decreasing background. Counting systems compared are gas counters and the filling gases, scintillation counters and scintillators. The best proportional counters are considerably better than the best scintillation counters, but benzene synthesis and assay by liquid scintillation counting is as good as the best Geiger counters. To concentrate a single sample by a factor of 10, thermal diffusion is quicker and gives a more accurate enrichment factor. However, as thermal diffusion columns are bulky, it is easier with electrolysis to run simultaneously a larger number of samples, and the reproducibility is adequate for most purposes. The absolute accuracy of measurements at different laboratories is compared and it is shown that the standard deviation is generally better than about ±2%. (author)