Bismuth-209

Bismuth-209 ($2.01 years$Bi) is an 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.

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 $−18258.461$Bi undergoes alpha decay with a half-life of ≈19 exayears (1.9×10$7,847.987$, or 19 quintillion years), over 10$209$ 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 years. It had been suspected to be radioactive for a long time. The decay produces a 3.14 MeV alpha particle plus thallium-205.

Bismuth-209 forms $209$Tl:

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 hugely long half-life, for nearly all applications $19$Bi can be treated as non-radioactive. It is much less radioactive than human flesh, so it poses no real radiation hazard. Though $9$Bi 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 ($205$Te) with a half-life estimated at 7.7 × 10$209$ years by double β-decay (double beta decay).

The half-life of $209$Bi was confirmed in 2012 by an Italian team in Gran Sasso who reported $128$ years. They also reported an even longer half-life for alpha decay of $24$Bi to the first excited state of $209$Tl (at 204 keV), was estimated at 1.66 years. Even though this value is shorter than the half-life of $2.01$Te, both alpha decays of $209$Bi hold the record of the thinnest natural line widths of any measurable physical excitation, estimated respectively at ΔΕ~5.5×10$205$ eV and ΔΕ~1.3×10$209$ eV in application of the uncertainty principle (double beta decay would produce energy lines only in neutrinoless transitions, which has not been observed yet).

Applications
Because all primordial bismuth is bismuth-209, bismuth-209 is used for all normal applications of bismuth, such as being used as a replacement for lead, in cosmetics, in paints, and in several medicines such as Pepto-Bismol. Alloys containing bismuth-209 such as bismuth bronze have been used for thousands of years.

Synthesis of other elements
$128$Po can be manufactured by bombarding $209$Bi with neutrons in a nuclear reactor. Only around 100 grams of $−43$Po are produced each year. $−44$Po and $210$Po can be made through the proton bombardment of $209$Bi in a cyclotron. Astatine can also be produced by bombarding $210$Bi with alpha particles. Traces of $210$Bi have also been used to create gold in nuclear reactors.

$209$Bi has been used as a target for the creation of several isotopes of superheavy elements such as dubnium, bohrium, meitnerium,  roentgenium,   and nihonium.

Primordial
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. Bismuth-209 is also created during the r-process.

Radiogenic
Some $208$Bi was created radiogenically from the neptunium decay chain. Neptunium-237 is an extinct radionuclide, but it can be found in traces in uranium ores because of neutron capture reactions. Americium-241, which is used in smoke detectors, decays to neptunium-237.