NOBLE GAS AND TRITIUM LABORATORY
Tritium, 3H or T, is the radioisotope of hydrogen. It is produced either naturally in the atmosphere by cosmic radiation on atmospheric gases or by CanDU reactors and other nuclear activities and released to the environment from tritium-handling facilities. Atmospheric testing of thermonuclear devices (hydrogen fusion bombs) during the 1950s and 60s also released considerable amounts of tritium to the atmosphere. Tritium decays to 3He with a relatively short half-life of 12.32 years or 388,789,632 seconds. This gives it a decay constant (ln2/T½) of 1.78 · 10-9.
Environmental tritium is generally expressed in tritium units, TU, where 1 TU = 1 T per 1018 H. Higher levels in tritium-contaminated regions are typically expressed by units of activity, or Becquerels, where 1 Bq = 1 decay event per second. In water, 1 TU = 0.119 Bq/L.
Tritium is often measured in water as an indication of modern groundwater and for tritium-helium dating of groundwater. It can also be measured in organics (typically as CH3O) which is referred to as organically-bound tritium or OBT. Tritium is also found in elemental form as hydrogen gas, or HT.
Tritium can be analyzed by measurement of its activity (radiometric) or its concentration (mass spectrometric). Radiometric tritium analysis is done through measurement of the rate of decay by counting decay events in a liquid scintillation counter and through measurement of the product of decay by counting atoms of the daughter nuclide, 3He, after a period of ingrowth. Either way, time is required for decay events to occur for a measurement. Alternatively, tritium concentrations can be measured by AMS. The Tritium Analysis Laboratory is the only one in Canada hosting all three of these methods for the analysis of tritium in environmental samples.
TRITIUM BY AMS
The advantage of tritium analysis by AMS is the very small sample requirement. Less than 1 mg of organic material can be analyzed (right). The level of detection is on the order of 10–15 which is about 100 times higher than natural background in most regions, and so this method is of interest for tracing tritium partitioning in high background regions such as near nuclear power generating stations or tritium handling facilities. Samples are converted to water by combustion, followed by reduction to hydrogen gas and reaction with titanium metal powder to form solid titanium hydride, TiH2 for pressing into the AMS target.
TRITIUM BY ENRICHMENT AND LIQUID SCINTILLATION COUNTING
Most laboratories analyze tritium by liquid scintillation counting of the decay events. However, the low tritium activity of environmental samples generally requires that the water samples are electrolytically-enriched to lower the detection limit. This allows an order of magnitude greater detection limit (<1 TU or 10–18) than by direct counting of water. Samples are mixed with a scintillation cocktail and decay-counted in our low-background Quantulus liquid scintillation counter.
For OBT, samples are first combusted to water using a Parr calorimeter (Parr bomb) shown on the right with the safety shield. Dried organic samples are set in the canister with a fuse wire and then filled with a 300 psi O2 atmosphere and combusted. Combustion water is collected and cleaned by distillation then counted with the Quantulus liquid scintillation counter.
TRITIUM BY INGROWTH OF 3He
Low level tritium samples and small OBT samples can be analyzed by encapsulation and ingrowth of the daughter, 3He. After a period of a few weeks (depending on activity and size of the samples) the ingrown 3H is measured on our Helix SFT. This is a state-of-the-art noble gas mass spectrometer with a split flight tube designed for high precision analysis of helium isotopes. It has an inlet system for purification of He from other reactive and noble gases. Ours is the only such SFT in Canada dedicated to the analysis of helium concentration and isotopes. The Helix SFT is also used for measurements of 3He in groundwaters for T-3He dating, and for measurement of 3He and 3He/4He ratios in crustal fluid studies and 4He dating of groundwaters and minerals.