Bismuth-209

Decay properties

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 now given more precisely as 2.01×10¹⁹ years (20.1 quintillion years), over 10⁹ times longer than the estimated age of the universe. The heaviest nucleus considered to be stable is now lead-208 and the heaviest stable monoisotopic element is gold (gold-197).

Theory had previously predicted a half-life of 4.6×10¹⁹ years. It had been suspected to be radioactive for a long time. The decay produces a 3.14 MeV alpha particle plus thallium-205.

Due to its extremely long half-life, ²⁰⁹Bi can be treated as non-radioactive for nearly all applications. It is much less radioactive than human flesh, so it poses no real radiation hazard. Though ²⁰⁹Bi holds the half-life record for alpha decay, it does not have the longest known half-life of any nuclide; this distinction belongs to tellurium-128 (¹²⁸Te) with a half-life estimated at 7.7×10²⁴ years by double beta decay.

The half-life of ²⁰⁹Bi was confirmed in 2012 by an Italian team in Gran Sasso who reported (2.01±0.08)×10¹⁹ years. They also reported an even longer partial half-life for alpha decay of ²⁰⁹Bi to the first excited state of ²⁰⁵Tl (at 204 keV), estimated at 1.66×10²¹ years. 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 ${\displaystyle \mathrm{\Delta }E=\frac{\hslash }{2T_{1/2}}}$ (beta or double beta decay would produce energy lines only in neutrinoless transitions, which have never been observed).