Tritium (T or 3H), a hydrogen isotope with a half-life of 12.43 years is being produced naturally by the impact of cosmic neutrons on nitrogen nuclei (cosmic ray spallation) by the reaction:
14 7 N+10n → 126C+31H
Free 3H collides with O2 to form 1H3HO via 3HO2 and enters the water cycle. Production in the upper atmosphere causes a steady state on the order of 4 to 25 tritium units (TU) in precipitation. Since 1952, tritium produced by open air thermonuclear tests has overshadowed natural production, at times by 2 or 3 orders of magnitude.
Liquid scintillation counting (LSC) is the technique used for the detection and quantification of tritium. Tritium measurements have a lower limit of detection of approximately 6 tritium units for water counted directly. Samples with a tritium content near this level are enriched approximately 15 times by electrolysis and then counted. The detection limit for enriched samples is 0.6 ± 0.8 TU.
Tritium measurements (counts per minute [cpm]) are converted directly into absolute concentrations where:
1 TU = 1-3H/1018-1H = 7.19 dpm/l = 3.26 pCi/l = 0.11815 Bq/l
The sample is processed as follows:
A Canberra-Packard Pico-fluor LLT (low level tritium) cocktail is used, which has a high carrying capacity for water with high efficiency and low background characteristics.
The laboratory standard is NBS-4361 tritium reference material diluted with background water which is then calibrated to NBS-4926C. The background water is from a well with Radiocarbon activity older than 3500 years and a conductivity of less than 300 µmho.