barium137m

Barium-137m (Ba-137m) is a metastable nuclear isomer of barium-137. It is produced predominantly as the immediate daughter of cesium-137 in beta decay, a common fission product found in spent nuclear fuel and environmental fallout.

Ba-137m has an excitation energy of about 661.7 keV above the ground state and decays to ground-state Ba-137 by emitting a gamma ray of 661.7 keV. Its half-life is short, on the order of a few minutes (commonly cited as about 2.6 minutes), after which the nucleus returns to the stable ground state.

Because the 661.7 keV gamma line is intense and well-isolated, Ba-137m is widely used in gamma-ray spectroscopy and in calibrating gamma-ray detectors and imaging equipment. In practice, calibration sources contain 137Cs, which decays to Ba-137m, providing the line without needing a separate Ba-137m source.

Chemically, Ba-137m behaves as a daughter of cesium; it is not a long-lived nuclide on its own. The primary safety concern is the parent 137Cs, which persists with a half-life of 30.17 years and poses long-term radiological hazard in sealed sources and contaminated environments.

See also: Cesium-137, gamma-ray spectroscopy, nuclear isomer.