# Bismuth-209

Bismuth-209 ($^{209}$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 $^{209}$Bi undergoes alpha decay with a half-life of 20.1 exayears (2.01×10$^{19}$, or 20.1 quintillion years), over 10$^9$ 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 4.6×10$^{19}$ 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 $^{205}$Tl:
    $^{209}$ $_{83}$Bi → $^{205}$ $_{81}$Tl + $^4$ $_2$He
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 $^{209}$Bi can be treated as non-radioactive. It is much less radioactive than human flesh, so it poses no real radiation hazard. Though $^{209}$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 ($^{128}$Te) with a half-life estimated at 7.7 × 10$^{24}$ 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 (2.01±0.08)×10$^{19}$ years. They also reported an even longer half-life for alpha decay of $^{209}$Bi to the first excited state of $^{205}$Tl (at 204 keV), was estimated at 1.66×10$^{21}$ years. Even though this value is shorter than the half-life of $^{128}$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$^{−43}$ eV and ΔΕ~1.3×10$^{−44}$ 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

$^{210}$Po can be manufactured by bombarding $^{209}$Bi with neutrons in a nuclear reactor. Only around 100 grams of $^{210}$Po are produced each year. $^{209}$Po and $^{208}$Po can be made through the proton bombardment of $^{209}$Bi in a cyclotron. Astatine can also be produced by bombarding $^{209}$Bi with alpha particles. Traces of $^{209}$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.

## Formation
### 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 $^{209}$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.