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Concept
Bismuth-209
Summary
Bismuth-209 (209Bi) is the isotope of bismuth with the longest known half-life of any radioisotope that undergoes α-decay (alpha decay). It has 83 protons and a magic number of 126 neutrons, and an atomic mass of 208.9803987 amu (atomic mass units). Primordial bismuth consists entirely of this isotope.
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 years. It had been suspected to be radioactive for a long time. The decay event produces a 3.14 MeV alpha particle and converts the atom to thallium-205.
Bismuth-209 will eventually form 205Tl if unperturbed:
→ +
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 thallium-205, the decay product of bismuth-209, reverts to lead when fully ionized.
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 less 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).
The half-life of bismuth-209 was confirmed in 2012 by an Italian team in Gran Sasso who reported 2.01e19 years. They also reported an even longer half-life for alpha decay of bismuth-209 to the first excited state of thallium-205 (at 204 keV), was estimated to be 1.
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