Samarium146

Samarium-146 (146Sm) is a radioactive isotope of the lanthanide element samarium (atomic number 62) with a mass number of 146. It decays by alpha emission to neodymium-142 (142Nd) and has a half-life on the order of about 100 million years. Because its half-life is long compared with human timescales but short on geological scales, 146Sm is no longer present in detectable quantities on Earth outside of ancient materials.

In stellar environments, 146Sm is produced by neutron capture processes during nucleosynthesis and was present in the material that formed the solar system. Today it is considered extinct on Earth, though remnants can be studied in primitive meteorites and other early solar-system materials. The decay of 146Sm to 142Nd provides a chronometer for early solar-system differentiation, helping to constrain the timing of planetary heating and crust formation during the first tens of millions of years after solar system formation.

146Sm is part of a broader set of short-lived radionuclides used in cosmochemistry to understand the early evolution of planetary bodies. It is distinct from the longer-lived 147Sm–143Nd dating system, which remains a standard method in geochronology. Chemically, 146Sm behaves similarly to other trivalent lanthanides, but its radiogenic decays make it primarily of interest for studies of early solar-system processes rather than ongoing terrestrial dating.