Bismuth-209 is the "virtually stable" isotopeofbismuth with the longest half-life. It has 83 protons and 126 neutrons, and an atomic mass of 208.9803987 u. All primordial bismuth is of this isotope. It is a decay product from lead-209 decaying by β--decay.
Bismuth-209 was long thought to have the heaviest stable nucleus of any element, but in 2003, Noël Coron and his colleagues at the Institut d’Astrophysique Spatiale in Orsay, France, discovered that 209Bi undergoes alpha decay with a half-life of approximately 600 yottaseconds (1.9×1019 years), over a billion times longer than the current estimated age of the universe. Theory had previously predicted a half-life of 4.6×1019 years. The decay event produces a 3.14 MeV alpha particle and converts the atom to thallium-205.[1][2]
Due to its extraordinarily long half-life, for nearly all applications bismuth-209 can still be treated as if it were non-radioactive. Although bismuth-209 holds the half-life record for alpha decay, bismuth does not have the longest half-life of any radionuclide to be found experimentally—this distinction belongs to tellurium-128 with a half-life estimated at 7.7 x 1024 years by double beta decay.[3]
In the red giant stars of the asymptotic giant branch, the s-process (slow process) is ongoing to produce bismuth-209 by neutron capture as the heaviest element to be formed. All elements heavier than it are formed in the r-process, or rapid process, which occurs during the first fifteen minutes of supernovae.[4]
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