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Tritium (3H, T) is the radioactive isotope of hydrogen with atomic mass 3 and half-life of 12.43 years (3.866·108 s) that is formed in higher layers of the atmosphere by interaction of thermal neutrons, products of interaction of primary cosmic radiation, to atoms of atmospheric nitrogen:
14N + → 3H + 12C
Tritium atoms decay according to the reaction:
3H → 3 He + β + ν
The energy is 18 keV, distributed between electron and antineutrino.
Concentration of tritium activity is commonly expressed by so called tritium units (TU), where one tritium unit represents the ration of one atom of 3H to 1018 atoms of 1H, corresponding to the concentration of activity:
1 TU = 7.2 desintegration/min/L water = 0.118 Bq/L
The mean rate of tritium production is estimated to be 0.25 atoms cm-3s-1, which corresponds to the yearly production of about 70,000 TBq or 200 g of tritium. Two thirds of tritium atoms are produced in the stratosphere and one third in the troposphere. The whole amount of naturally produced tritium in the atmosphere in about 1.3·106 TBa or 3.6 kg. Most (99%) of natural tritium oxidizes into water (HTO) and this comes to the Earth surface in the form of precipitation and atmospheric moisture, forming a part of hydrologic cycle. Production of tritium depends on geographic latitude. Natural distribution of tritium activity in precipitation is not uniform: on the Northen Hemisphere it is between 5 and 10 TU (0.5 to 1 Bq/L), increasing from Equator to the North Pole, while in tropical region and on the Southern Hemisphere (except on Antarctica) it is less than 5 TU.
Greater amount of tritium present in the atmosphere and in the whole hydrological cycle is of anthropogenic origin, formed by a series of thermonuclear explosions in the atmosphere since 1952 forward. The maximum concentration of 3H activity, being 6000 TU (550 Bq/L), was recorded in 1963 in precipitation on the Northern Hemisphere. After the ban of atmospheric nuclear tests concentration of tritium activity started to decrease due to the relatively short half life. Today it is less than 10 TU and is approaching the natural level of the first half of the last century.
Fig.1: Tritium
concentration in precipitation since 1950 at four IAEA stations (IAEA/WMO,
2006):
Ottawa, Canada (Northern Hemisphere, continental);
Valentia, Ireland (Northern Hemisphere, marine);
Harare, Zimbabwe (Southern
Hemisphere, continental); Kaitoke, New Zealand (Southern Hmisphere,
marine).
Main characteristics of the natural temporal and spatial distribution of 3 H activity concentration are:
- Seasonal effect: Concentration of tritium activity in monthly precipitation samples (for moderate belt of the Northern Hemisphere) is lower in winter and higher in summer, when the exchange of air masses between the troposphere and the stratosphere reaches its maximum.
- Ocean-continental effect: Concentration of tritium activity at the stations close to oceans is lower than that at continental stations (increase with distance from the ocean); also, seasonal variations increase with the distance from oceans.
- Latitudinal effect: Concentration of tritium activity increases with geographic latitude, going from Equator to the poles.
Due to these characteristics tritium became one of the most important natural tracers and has an important role in hydro(geo)logical, ecological, climatological and meteorological investigations.
A more and more notable proportion of technogenic tritium in the atmosphere is a result of its production in nuclear reactors, experimental fusion reactors and hospitals, as well as its presence in various consumer products. Technogenic tritium changes the spatial distribution of 3H in the atmosphere on a local and/or regional level.
Measurement of tritium activity belongs to the category of low-activity measurements. Therefore a very sensitive technique is needed. Due to the low energy and the low range of β-particles emitted by tritium decay the sample should be introduced into the detector. Most often gas proportional counters or liquid scintillation counters are used. Very low concentration of tritium activity should be determined by using of electrolytic enrichment of the sample prior to the measurement in a counter.
