Bismuth-209

Bismuth-209 (209Bi) is an isotope of bismuth, with the longest known half-life of any radioisotope that undergoes α-decay (alpha decay).

Bismuth-209 was long thought to have the heaviest stable nucleus of any element, but in 2003, a research team at the Institut d’Astrophysique Spatiale in Orsay, France, discovered that 209Bi undergoes alpha decay with a half-life of 20.1 exayears (2.01×1019, or 20.1 quintillion years),[3][4] over 109 times longer than the estimated age of the universe.

They also reported an even longer half-life for alpha decay of 209Bi to the first excited state of 205Tl (at 204 keV), was estimated at 1.66×1021 years.

[12] Even though this value is shorter than the half-life of 128Te, both alpha decays of 209Bi hold the record of the thinnest natural line widths of any measurable physical excitation, estimated respectively at ΔΕ~5.5×10−43 eV and ΔΕ~1.3×10−44 eV in application of the uncertainty principle[13] (double beta decay would produce energy lines only in neutrinoless transitions, which has not been observed yet).

[28][29] 209Bi has been used as a target for the creation of several isotopes of superheavy elements such as dubnium,[30][31][32][33] bohrium,[30][34] meitnerium,[35][36][37] roentgenium,[38][39][40] and nihonium.

[44] All elements heavier than it are formed in the r-process, or rapid process, which occurs during the first fifteen minutes of supernovas.