87Rb

Rubidium-87 (87Rb) is an isotope of the alkali metal rubidium, with atomic number 37 and a mass number of 87. In natural rubidium, about 28% is 87Rb and the remainder consists mainly of 85Rb. 87Rb is long-lived but radioactive, decaying by beta emission to 87Sr with a half-life of about 4.88 × 10^10 years. Because of its long decay time, 87Rb is widely used as a radiometric chronometer in Rubidium–Strontium dating of geological and planetary materials.

Nuclear and atomic properties of 87Rb make it especially important in physics. The nucleus has spin I = 3/2 and a relatively large magnetic moment, which leads to hyperfine structure in its electronic ground state. The two hyperfine levels, F = 1 and F = 2, are separated by a frequency of about 6.834 GHz, a transition used in rubidium atomic clocks and other precision measurements. The principal optical transitions for laser cooling and trapping occur near the D-line wavelengths: the D1 line around 795 nm and the D2 line around 780 nm, enabling efficient laser cooling and magneto-optical trapping experiments.

Applications and significance extend across metrology, quantum optics, and geochronology. 87Rb is a standard choice for vapor-cell atomic clocks and sensitive magnetometers, and it is commonly used in Bose-Einstein condensation experiments and other quantum technologies. In geology and archaeology, the 87Rb–87Sr decay system provides a tool for dating ancient materials. As a reactive alkali metal, rubidium is handled with care in laboratory and industrial settings.