Ba137

Ba-137, or barium-137, is a radioactive isotope of barium with mass number 137. It is not one of the naturally abundant stable isotopes of barium and occurs primarily as a decay fragment in nuclear processes, most notably in the decay chain of cesium-137.

In the cesium-137 decay sequence, cesium-137 undergoes beta decay to form the metastable isotope Ba-137m (barium-137m). The Ba-137m state then decays to Ba-137 in a short time, emitting a characteristic gamma ray of about 0.661 MeV. The metastable step and the gamma emission are widely used as a signature in gamma spectroscopy and in calibrating detection systems. The Ba-137 ground state, formed after the isomer decays, is long-lived and effectively stable on human timescales, contributing no rapid further decay under ordinary conditions.

Properties and significance: Ba-137 is primarily encountered as part of the Cs-137 decay chain in nuclear fallout, medical and industrial radiography sources, and radiation monitoring. The 0.661 MeV gamma transition from Ba-137m to Ba-137 provides a prominent line used to identify and quantify Cs-137 contamination in environmental samples. Because Cs-137 and its daughters contribute to external gamma dose, Ba-137-bearing decay products are considered in radiological safety analyses.

See also: Cesium-137, Barium isotopes, Radioisotopes, Gamma spectroscopy.