138Ba

138Ba is a stable isotope of the element barium (Z = 56) with a mass number of 138. It contains 56 protons and 82 neutrons, and its ground-state nucleus has spin and parity 0+. As a stable nuclide, it does not undergo radioactive decay.

In nature, 138Ba is the most abundant barium isotope, comprising about 71.7% of natural barium. Other stable isotopes of barium occur in smaller fractions, such as 132Ba, 134Ba, 135Ba, 136Ba, and 137Ba, with varying but significantly lower abundances. The dominance of 138Ba reflects the nuclear structure near the neutron magic number N = 82.

Nucleosynthesis and abundance patterns place 138Ba in the products of stellar processing. It is produced predominantly by the slow neutron capture process (the s-process) in asymptotic giant branch stars. The presence of the neutron shell closure at N = 82 contributes to its relative abundance by creating a bottleneck that tends to accumulate material at this mass region.

Applications of 138Ba largely arise from its status as the principal natural Ba isotope. It serves as a key reference in isotopic ratio measurements and is commonly used for calibration in mass spectrometry techniques such as ICP-MS and TIMS. In geochemical and paleoceanographic studies, Ba isotope systems, including 138Ba, are used to investigate the Ba cycle in rocks and seawater and to support interpretations of past environmental conditions.