Rubidium-87 (${}^{87}\mathrm{Rb}$)

Rubidium-87 is a long-lived radioactive isotope with thirty-seven protons and fifty neutrons. Its beta decay to strontium-87 supports rubidium-strontium geochronology.

It is also a common isotope in laser-cooled atomic-gas experiments. Its nuclear spin and hyperfine states provide experimentally accessible internal structure, while the whole atom can behave as a boson. These atomic roles are distinct from the geological use of its slow nuclear decay.

Ground-state electronic angular momentum $J=1/2$ couples to nuclear spin $I=3/2$, giving $F=1,2$. For contact hyperfine interaction $A\mathbf{I}\cdot \mathbf{J}/{\hslash }^2$, their zero-field energies are $-5A/4$ and $3A/4$. The splitting is $2A$. Electronic weak-field contributions give $g_{F=1}\simeq -1/2$ and $g_{F=2}\simeq +1/2$; the small nuclear contribution refines these numbers.