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There are 34 known isotopes of krypton (₃₆Kr) with atomic mass numbers from 67 to 103. Naturally occurring krypton is made of five stable isotopes and one (⁷⁸
Kr) which is slightly radioactive with an extremely long half-life, plus traces of radioisotopes that are produced by cosmic rays in the atmosphere. Atmospheric krypton today is, however, considerably radioactive due almost entirely to artificial ⁸⁵Kr.^[5]

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.^[13] 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.^[14] More recently, it has been announced^[15] 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.^{[citation needed]}

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.^[16] It almost entirely decays to the ground state with a monochromatic gamma ray.

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,^[17] which is far larger-volume than was ever nuclear testing.

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^[18] 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 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.^[19]

Daughter products other than krypton

• Isotopes of rubidium
• Isotopes of bromine
• Isotopes of selenium

• Brookhaven National Laboratory: Krypton-101 information Archived 2017-10-18 at the Wayback Machine