Krypton-81 Krypton-81 (half-life 230,000 years) is useful in determining how old the water beneath the ground is. Radioactive krypton-81 is the product of
spallation reactions with
cosmic rays striking gases present in the Earth atmosphere, along with the six stable or nearly stable krypton
isotopes. The long half-life ensures that the isotope has a uniform concentration in the atmosphere and in surface water; when the water goes underground is supply is no longer replenished and decays, allowing dating of the
residence time in deep
aquifers in a range of 20,000 to a million years, bridging the gap where other isotopic methods (e.g.
carbon-14 dating) lose sensitivity. The same long half-life renders detection of its decay impossible and, therefore, demands some form of
mass spectrometry. Even so, technical limitations of the method have traditionally required the sampling of very large volumes of water: several hundred liters or a few cubic meters of water (about a milligram of krypton). This is particularly challenging for dating
pore water in deep
clay aquitards with very low
hydraulic conductivity. More recently, it has been announced that samples an order of magnitude less can be used successfully. Because cosmic ray production in the atmosphere creates a globally fairly uniform 81Kr/Kr concentration, one can assume a known initial ratio in meteoric water before recharge. There are essentially no significant anthropogenic or in situ geological sources (in typical crustal settings) that would confound the decay clock, making krypton-81 a relatively "clean" choice for geological dating. The short-lived isomer krypton-81m (half-life 13 seconds) has medical uses but is often considered impractical for use as it must be
generated from the rare rubidium-81. It almost entirely decays to the ground state with a monochromatic gamma ray.
Krypton-85 Krypton-85 (half-life 10.728 years) is produced by the
nuclear fission of
uranium and
plutonium in
nuclear weapons testing and in
nuclear reactors, as well as by cosmic rays. An important goal of the
Limited Nuclear Test Ban Treaty of 1963 was to eliminate the release of such radioisotopes into the atmosphere, and since 1963 much of that krypton-85 has had time to decay. However, it is almost inevitable that krypton-85 is released during the
reprocessing of
fuel rods from nuclear reactors, which is far larger-volume than was ever nuclear testing.
Atmospheric concentration The atmospheric concentration of krypton-85 around the
North Pole is about 30 percent higher than that at the
South Pole because nearly all of the world's nuclear reactors and all of its major nuclear reprocessing plants are located in the
Northern Hemisphere, well north of the
equator and transfer of air between the hemispheres is slow. The
nuclear reprocessing plants with significant capacities are located in the
United States, the
United Kingdom, the
French Republic, the
Russian Federation,
Mainland China (PRC),
Japan,
India, and
Pakistan.
Krypton-86 Krypton-86 was formerly used to
define the meter from 1960 until 1983, when the definition of the meter was based on the wavelength of the 606 nm (orange) spectral line of a krypton-86 atom. == See also ==