The sample contains 4 atoms of krypton-85 and 12 atoms of rubidium-85 is 21.52 years
Krypton-85 is a radioactive gas found in small quantities in the atmosphere. It is naturally produced when stable Krypton-84 reacts with the incoming cosmic rays. Almost 1 cm3 of Krypton-84 is present per cubic meter of atmosphere. Atmospheric Krypton-85 is mostly produced by volcanoes, earthquakes, nuclear power plants, and nuclear explosions.
Since the mid-1940s, much greater quantities of Krypton-85 than that found in nature have been manufactured as a byproduct of nuclear fission. When Uranium-235 or some other fissile nucleus goes through a fissile reaction, it gets split into two portions or fragments having 2 or 3 neutrons and mass numbers between 90 and 140. The fission yield of Krypton-85 is around 0.3%; i.e., approximately 3 atoms of Krypton-85 are generated per 1000 fissions. Most of the mass of Krypton-85 is converted directly to Rubidium-85 and does not go through the long-standing state of nuclear isomer.
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