Natural tritium

Tritium is formed in the upper atmosphere by bombardment of nitrogen, oxygen and carbon by high energy protons and by capture of cosmic-ray generated neutrons by nitrogen 

The rate of production depends on solar activity and ranges from 0.4 to 0.8 kg (4 to 8 MCi) per annum (National Council on Radiation Protection and Measurement, 1979). Most of the tritium is formed in the stratosphere, and remains there for some months, giving an inventory of 0.24 kg (2.3 MCi) (UNSCEAR, 1977). 

Harteck (1954) measured the amount of T in the lower atmosphere before H-bomb tests had added significantly to natural activity, and found 4000 and 3 TU in hydrogen and water vapour respectively. The amount of H2 in the atmosphere has increased in recent years due to industrial production (Schmidt, 1974). Circa 1950 there were about 1.5 x 1011 kg of H as H2, compared with 1.4 x 1015 kg as H20. Thus Harteck s values for natural T correspond to tropospheric inventories of 1.8 g as HT and 13 g as HTO. Because HTO is deposited in rain and by vapour transfer to the sea, its residence time in the troposphere is only about a week, similar to that of 137Cs (Fig. 2.8), and more than 90% of the atmospheric inventory is in the stratosphere. By contrast, the residence time of HT in the troposphere is several years (Mason Ostlund, 1979), and most of the atmospheric inventory of HT is in the troposphere. It is only necessary for a small fraction of naturally produced T to form HT to account for the high specific activity of hydrogen gas. 

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The principal source of natural tritium is the nuclear reactions induced by cosmic radiation in the upper atmosphere, where fast neutrons, protons, and deuterons coUide with components of the stratosphere to produce tritium ... 

Provided the natural production of 5TU is the only source of tritium in the groundwater, we would have at hand a water dating tool. For example, water pumped from a well with 3TU has preserved 3 x 100/5 = 60% of its natural tritium content, equivalent to an age of 9 years (readable from Fig. 10.1). However, a hydrologist s life is not that simple. Most aquifers have a capacity equal to several annual recharges, or, in other words, water accumulates in the aquifer for many years and the age we have just calculated from the tritium concentration is not a real age, but rather an... 

The manmade tritium, which reached several thousand TU in precipitation during 1963, completely masked the natural tritium production discussed in the previous section. The awareness of the potential importance of tritium to hydrology arose only after the nuclear tests began. By that time the natural tritium content in precipitation could no longer be measured, but a unique solution was found—measurements in stored and dated wine bottles,... 

Before 1952, natural tritium was about 5 TU in precipitation. How much would be left in groundwater recharged in 1951 and emerging in a spring in 1988 The answer can be found by using the following steps ... 

Grosse etal measured highly concentrated heavy water samples produced from surface waters that contained concentrations of natural tritium approximately one million times higher than the original water. Observations have shown that concentrated samples are indeed radioactive to a level corresponding to a natural abundance of tritium of about 10 by atomic ratio. 

Tritiated-water standards have been distributed by the National Bureau of Standard (NBS), USA, and IAEA, meeting the demands from laboratories measuring the natural tritium. 

In order to make use of this transient, the contributions of the natural and bomb radiocarbon had to be separated. Three sets of observations went into this separation. First, use was made of measurements on surface waters collected very early in the nuclear testing era (Broecker etal., 1960) and also with results of measurements on prenuclear growth ring-dated corals and mollusks (Drulfel and Linick, 1978 Drulfel, 1981, 1989). Second, use was made of the tritium released to the atmosphere during nuclear tests. As this bomb-test tritium swamped the natural tritium present in the ocean, the vertical distribution of tritium in the sea could be used to establish the limit of penetration of bomb radiocarbon. Finally, based on the radiocarbon analyses made on thermocline waters free of bomb tritium, it was shown that there was a close correlation between the natural ratio and the dissolved silica... 

The average global production of tritium is —2,500 atom m s (Solomon and Cook, 2000). The deposition rate of the tritium varies with latitude, but it is also mixed with the bulk of precipitation originating from the ocean (which has a very low tritium content), and thus the average tritium content of precipitation tends to vary inversely with annual precipitation. Natural tritium in precipitation varies from —1 TU in oceanic high-precipitation regions to as high as 10 TU in arid inland areas. 

Libby W. F. (1953) The potential usefulness of natural tritium. Proc. Natl. Acad. Sci. 39, 245 - 247. 

Deuterium Deuterium is a stable isotope occurring naturally tritium is radioactive. These isotopes are and tritium used in research and in thermonuclear weapons. 

A nuclear application of lithium is in thermonuclear weapons and fusion research. In a weapon or fusion reactor, nuclear fusion occurs between two isotopes of hydrogen—deuterium and tritium. Deuterium occurs naturally and has an abundant supply in the worlds oceans (it is present in about 0.015 percent of water molecules). Tritium, on the other hand, is radioactive, has a relatively short half-life, and does not occur naturally. Tritium can be manufactured, however, by bombarding lithium 6 with neutrons. 

Natural tritium concentrations in ocean surface waters and in young groundwater are of the order of 1 tritium atom per 10 hydrogen atoms. This is the reason that tritium concentrations are reported as TU (Tritium Units). One TU stands for a tritium to hydrogen ratio [ H]/[H] of 10. The activity of a water sample with a tritium concentration of 1 TU is equivalent to 3.2 pCi or 0.12 Bq per liter of H2O. The production rate of natural tritium is about 0.5 0.3 atoms cm s (Craig and Lai 1961) leading to natural tritium values in ocean surface waters of about 0.2 TU (Dreisigacker and Roether 1978). 

Before natural tritium could be fully exploited for studies of natural water systems, tritium from anthropogenic sources (mainly nuclear weapon tests) was added to the atmosphere in considerable amounts. By the mid 1960s the natural background of tritium in precipitation was practically masked by so-called bomb tritium (e.g., Weiss et al. 1979 Fig. 1). For the past 4 to 5 decades, bomb tritium severely limited the use of natural tritium as a tracer because only few uncontaminated tritium data are available from the pre-bomb era. However, bomb tritium offered a new tool for studies of water movement in natural system. It is equivalent to a dye that was introduced into the environment on a global scale at a relatively well-known rate. Most of the bomb tritium was added to the environment in three pulses during 1954, 1958-1959 and, predominantly, 1963. 

Clarke WB, Jenkins WJ, Top Z (1976) Determination of tritium by mass spectrometric measurement of He. Inti J Appl Rad Isotopes 27 515-522 Craig H, Lai D (1961) The production rate of natural tritium. Tellus 13 85-105... 

The northern component reflects the more immediate injection of bomb tritium into the northern hemispheric hydrologic system because virtually all of the major detonations occurred in the Northern Hemisphere. Prior to the bomb tests, the concentration of natural tritium in rainfall was of the order of 5-10 tritium units (1 TU = 1 tritium atom per 10 normal hydrogen atoms). During the mid-1960s, tritium concentrations of more than several thousand TU were recorded in higher latitude, mid-continental locales such as Chicago, USA or Ottawa, Canada. The southern component, on the other hand, is much weaker in amplitude and more smeared out in time... 

Tritium arrives at the earth s surface in rain and is present as HTO in the hydrological cycle. The natural tritium content is, however, very low, 1 atom T in 10 atoms of hydrogen. The atmospheric testing of nuclear weapons during the 1950s and 1960s made the contents considerably higher. Tritium is itself an important constituent of... 

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