Kr81

Krypton-81 (Kr-81) is a radioactive isotope of the noble gas krypton, with a mass number of 81. It has a long half-life of about 230,000 years and decays by electron capture to bromine-81. In nature, Kr-81 occurs only at trace levels within atmospheric krypton, reflecting its cosmogenic origin and the slow rate of decay.

Natural Kr-81 is produced by nuclear reactions in the atmosphere, including cosmic-ray spallation, and becomes globally distributed in the atmospheric krypton reservoir. Because of its extremely low abundance, Kr-81 is used for radiometric dating only in specialized applications, notably groundwater and ice dating.

Krypton-81 dating relies on measuring the Kr-81/Kr ratio in a sample and comparing it to the atmospheric (modern) Kr-81 abundance. The method is especially suited for dating water and ice in the range of roughly 50,000 to 1,000,000 years, filling a gap between other radiometric clocks. The technique requires extraction and concentration of krypton from dissolved gases or gas samples, followed by precise counting of Kr-81 atoms.

Counting Kr-81 atoms is accomplished with advanced methods such as Atom Trap Trace Analysis (ATTA), a laser-based single-atom counting technique. By determining how much Kr-81 remains relative to stable krypton, researchers compute the age of the water or ice in which the krypton was trapped.

Kr-81 dating has become a valuable tool in hydrogeology and paleoclimatology, providing timing for groundwater recharge and ice-core processes on timescales inaccessible to many other radiometric methods. See also radiometric dating and krypton isotopes.