Bismuth-209

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 approximately 19 exayears (1.9×10¹⁹, approximately 19 quintillion years), over a billion times longer than the current estimated age of the universe. The heaviest nucleus considered to be stable is now lead-208 and the heaviest stable monoisotopic element is gold as the ¹⁹⁷Au isotope.

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.[2][3]

Bismuth-209 will eventually form ²⁰⁵Tl if unperturbed:

²⁰⁹
₈₃Bi
→ ²⁰⁵
₈₁Tl
+ ⁴
₂He
[4]

If perturbed, it would join in lead-bismuth neutron capture cycle from lead-206/207/208 to bismuth-209, despite low capture cross sections. Even in thallium-205 case above, once fully ionized, again reverts to lead.

Due to its extraordinarily long half-life, for nearly all applications ²⁰⁹Bi can still be treated as if it were non-radioactive. Its radioactivity is much slighter than that of human flesh, so it poses no meaningful hazard from radiation. Although ²⁰⁹Bi 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 (¹²⁸Te) with a half-life estimated at 7.7 × 10²⁴ years by double β-decay (double beta decay).[5]

The half-life value of bismuth-209 was confirmed in 2012 by an Italian team in Gran Sasso who reported (2.01±0.08)×10¹⁹ years, and an even longer half-life, for bismuth-209 alpha decay to the first excited state of thalium-205 at 204 keV, was estimated to be 1.66×10²¹ years.[6] Even though this value is shorter than the measured half-life of tellurium-128, both alpha decays of bismuth-209 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 of Heisenberg[7] (double beta decay would produce energy lines only in neutrinoless transitions, which has not been observed yet).

²¹⁰Po can be manufactured by bombarding ²⁰⁹Bi with neutrons in a nuclear reactor. Only some 100 grams of ²¹⁰Po are produced each year.[8] Astatine can also be produced by bombarding ²⁰⁹Bi with alpha particles.

In the red giant stars of the asymptotic giant branch, the s-process (slow process) is ongoing to produce bismuth-209 and polonium-210 by neutron capture as the heaviest elements to be formed, and the latter quickly decays. All elements heavier than it are formed in the r-process, or rapid process, which occurs during the first fifteen minutes of supernovas.[9]

• Isotopes of bismuth