Bismuth-209

Bismuth-209, ²⁰⁹Bi

General
Symbol	209Bi
Names	bismuth-209, 209Bi, Bi-209
Protons (Z)	83
Neutrons (N)	126
Nuclide data
Natural abundance	100%
Half-life (t1/2)	2.01×1019 years
Isotope mass	208.9803987 Da
Spin	9/2−
Excess energy	−18258.461±2.4 keV
Binding energy	7847.987±1.7 keV
Parent isotopes	209Pb (β−) 209Po (β+) 213At (α)
Decay products	205Tl
Decay modes
Decay mode	Decay energy (MeV)
Alpha emission	3.1373
Isotopes of bismuth Complete table of nuclides

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.

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 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 4.6×10¹⁹ years. 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:

²⁰⁹
₈₃Bi
→ ²⁰⁵
₈₁Tl
+ ⁴
₂He

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 in thallium-205 case above, once fully ionized, again reverts to lead.

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 slighter 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 (2.01±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. 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 (double beta decay would produce energy lines only in neutrinoless transitions, which has not been observed yet).

Uses

²¹⁰Po can be manufactured by bombarding ²⁰⁹Bi with neutrons in a nuclear reactor. Only some 100 grams of ²¹⁰Po are produced each year. Astatine can also be produced by bombarding ²⁰⁹Bi with alpha particles. Bismuth-209 has been used as a target for the creation of several superheavy elements such as roentgenium and nihonium.

Formation

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.

See also

• Isotopes of bismuth