Bismuth-209 (209Bi) 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 209Bi undergoes alpha decay with a half-life of 20.1 exayears (2.01×1019, or 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 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 205Tl: 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 209Bi can be treated as non-radioactive. 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 β-decay (double beta decay). The half-life of 209Bi was confirmed in 2012 by an Italian team in Gran Sasso who reported 2.01 years. They also reported an even longer half-life for alpha decay of 209Bi to the first excited state of 205Tl (at 204 keV), was estimated at 1.66 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 ΔΕ5.5×10−43 eV and ΔΕ1.3×10−44 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

210Po can be manufactured by bombarding 209Bi with neutrons in a nuclear reactor. Only around 100 grams of 210Po are produced each year. 209Po and 208Po can be made through the proton bombardment of 209Bi in a cyclotron. Astatine can also be produced by bombarding 209Bi with alpha particles. Traces of 209Bi have also been used to create gold in nuclear reactors. 209Bi 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 209Bi 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.