Tritium (3H) is the radioactive isotope of H that decays by β - decay to 3He (half-life):
= 4500 days = 12.32 years (Lucas and Unterweger, 2000):
3H → 3He λ = ln(2)/τ½
with the decay constant:
= 0.05626 yr-1 = 1.7828 10-9 s-1
For modern water isolated from the atmosphere, 3H is a major source of 3He. For each 3H that decays one 3He atom is produced. The natural abundance of 3H is usually expressed in Tritium Units (TU):
1 TU = 1 3H atoms per 1018 H atoms = 1.100 10-19 mol/gpure water = 66'846 atoms/gpure water
Sometimes 3H concentrations are also expressed in activities (IAEA, 1992):
1 Bq/L = 8.47 TU; 1 pCi/L = 0.313 TU
The conversion from TU into cm3STP/g is given by:
1 cm3STP / gwater = 4.0193 1014 TU / (1-S/100)
where S is the salinity of the water (in ‰).
Since tritium is essential for the 3H/3He dating method, its properties will be discussed here in some detail.