Meas. of 3H activity
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Meas. of 3H activity
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Meas. of 3H activity
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MEASUREMENT OF TRITIUM ACTIVITY
Principle
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:
14
N + 3H + 12C
Tritium atoms decay according to the reaction:
3
H 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.
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Meas. of 3H activity
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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
3
H 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 βparticles 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.
Authored, created: Bogomil Obelic 03/02/2004 01:43 pm; Modified: 10/25/2007 01:58 pm Go to Top
Application
Measurement of concentration of tritium activity in natural waters (precipitation, surface
or ground waters) can be used as a very successful tool in hydrology and related sciences.
Relating to the concentration of tritium activity, waters can be divided into two groups: to those
containing only natural tritium, that were not in contact with the atmosphere after
thermonuclear tests started and those labeled by a higher concentration of artificially produced
tritium. In this way it is possible to follow the path of precipitation and its flow into
http://www.irb.hr/en/str/zef/z3labs/lna/H3/
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Meas. of 3H activity
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underground aquifers and to measure age or the mean residence time of water in the
underground under the supposition that the initial activity of tritium is known and that there
exists no mixing with older or recent water.
For groundwater dating it is necessary to use various models that take into consideration
mean residence or flow time of water in underground. The higher concentration points to the
water recharged by surface waters of the second half of the 20th century.
In order to obtain complete data from analyses of tritium concentration in waters it is
necessary to know the concentration of tritium activity in precipitation in the area where the
activity has been measured. The International Agency for Atomic Energy (IAEA) from Vienna in
collaboration with the World Meteorological Organization (WMO) established a system of
measurement of tritium activity and stable isotopes ratios (2H/1H and 18O/16O) in precipitation at
several hundreds locations all over the Earth (GNIP). The Zagreb station is a part of the global
network since 1976.
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Fig.2: Concentration of 3H activity in precipitation over Zagreb (Croatia) and Ljubljana
(Slovenia)
(measured at Laboratory for Measurements of Low-level Radioactivity, Ruñer Bošković Institute)
€
Measurement of concentration of tritium activity in surface waters, especially in the
vicinity of nuclear power plants is of special interest. IAEA recommends monitoring of 3H activity
in the wider area of nuclear power plants to control radioactivity of water in regular operation
and/or in accidental cases.
Authored, created: Bogomil Obelic 03/02/2004 09:14 pm; Modified: 10/25/2007 02:05 pm Go to Top
© 2003-2009 Ruñer Bošković Institute || last changed: 03/25/2004 03:43 pm (Bogomil Obelic)
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