There are 33 known isotopes of krypton (Kr) with atomic mass numbers from 69 through 101. Naturally occurring krypton is made of six stable isotopes, two of which might theoretically be slightly radioactive, plus traces of radioisotopes that are produced by cosmic rays in the atmosphere.
The spectral signature of krypton can be observed to have several very sharp lines. When krypton is placed into an electric discharge tube, it emits visible light with a distinctive orange-red color.
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 605 nm (orange) spectral line of a krypton-86 atom.
Radioactive krypton-81 is the product of reactions with cosmic rays that strike the atmosphere, along with some of the other isotopes of krypton. Krypton-81 has a half-life of about 229,000 years.
Krypton-81 has been used for dating old (50,000- to 800,000-year-old) groundwater. Krypton-85 is a radioisotope of krypton that has a half-life of about 10.75 years. This isotope 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 inevitable that krypton-85 is released during the reprocessing of fuel rods from nuclear reactors.
The atmospheric concentration of krypton-85 around the North Pole is about 30 percent higher than that at the Amundsen–Scott South Pole Station because nearly all of the world's nuclear reactors and all of its major nuclear reprocessing plants are located in the Northern Hemisphere, and also well-north of the equator. To be more specific, those 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. See the article on nuclear reprocessing for more information.
All of the other radioisotopes of krypton have half-lives of less than one day, except for krypton-79, which has a half-life of about 35.0 hours. This isotope decays by the emission of positrons and thus becoming bromine.
Relative atomic mass: 83.798.