3H Analysis by Gas Proportional Counting
Tritium assay at very low levels in nature is often given in tritium units (TU), where one TU represents a tritium/hydrogen ratio of 10-18. In water of 1 TU, the specific activity is equal to 3.2 picocuries per liter (pCi L-1) or 7.1 disintegrations per minute per liter (dpm L-1). We measure these very low activities by internal gas proportional counting of H2-gas made from the water sample.
Following conversion to hydrogen gas, “high-level” tritium samples of > 100 TU (300 pCi L-1) may be admitted to counters directly. Low-level hydrological water samples go through an electrolytic enrichment step, in which tritium concentrations are increased about 60-fold through volume reduction. Accuracy of the low-level measurement with enrichment is 0.10 TU (0.3 pCi L-1 of H2O), or 3.5%, whichever is greater; that of low-level counting without enrichment is 3 TU (9.6 pCi L-1 of H2O) or 3.5%, whichever is greater.
Procedures are now available in the Tritium Laboratory for measurement of tritium in samples other than ordinary waters. The Laboratory offers the extraction of water from soils and subsequent measurement of tritium in the extracted water. Tritium concentrations in water found in soils, combined with the local precipitation tritium history can be used to estimate the travel time of rainwater through the soil.