Bismuth-209

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^[2] of 126 neutrons,^[2] 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),^[3][4] over a billion times longer than the current estimated age of the universe.^[5] 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. It had been suspected to be radioactive for a long time.^[6] The decay event produces a 3.14 MeV alpha particle and converts the atom to thallium-205.^[3][4]

Bismuth-209 will eventually form ²⁰⁵Tl if unperturbed:

20983Bi → 20581Tl + 42He^[7]

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.^[8]

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).^[9][10][11]

The half-life of bismuth-209 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 half-life for alpha decay of bismuth-209 to the first excited state of thallium-205 (at 204 keV), was estimated to be 1.66×10²¹ years.^[12] 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^[13] (double beta decay would produce energy lines only in neutrinoless transitions, which has not been observed yet).

Because primordial bismuth is entirely bismuth-209, bismuth-209 is used for all of the normal applications attributed to bismuth, such as being used as a replacement for lead,^[14][15] in cosmetics,^[16][17] in paints,^[18] and in several medicines such as Pepto-Bismol.^[5][19][20] Alloys containing bismuth-209 such as bismuth bronze have been used for thousands of years.^[21]

²¹⁰Po can be manufactured by bombarding ²⁰⁹Bi with neutrons in a nuclear reactor.^[22] Only around 100 grams of ²¹⁰Po are produced each year.^{[23][22] 209}Po and ²⁰⁸Po can be made through the proton bombardment of ²⁰⁹Bi in a cyclotron.^[24] Astatine can also be produced by bombarding forms of ²⁰⁹Bi with alpha particles.^[25][26][27] Traces of ²⁰⁹Bi have also been used to create gold in nuclear reactors.^[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.^[41][42][43]

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,^[44] and the latter quickly decays.^[44] All elements heavier than it are formed in the r-process, or rapid process, which occurs during the first fifteen minutes of supernovas.^[45][44] Bismuth-209 is also created during the r-process.^[44]

Some bismuth-209 was created radiogenically as a result of the neptunium series decay chain.^[46] Neptunium-237 is an extinct radionuclide, but it can be found in traces in uranium ores because of neutron capture reactions.^[46][47] Americium-241, which is used in smoke detectors,^[48] decays to neptunium-237.

• Isotopes of bismuth
• Primordial radionuclide
• List of elements by stability of isotopes

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