Radiacode — Pocket-Sized Next-Generation Geiger Counter and Gamma Spectrometer

Terbium-155 (Tb-155) and Terbium-156 (Tb-156) are radioactive isotopes of terbium with similar half-lives of about 5.3 days. Tb-155 decays by electron capture and emits characteristic gamma rays, particularly around 87 and 105 keV. Tb-156 has a more complex decay scheme and produces numerous gamma-ray lines, including prominent emissions around 199, 534, and 1222 keV. This makes the two isotopes readily distinguishable by gamma spectroscopy.

Tb-155 is primarily studied for medical imaging, especially single-photon emission computed tomography (SPECT), due to its suitable gamma emissions and half-life. Tb-156 has less direct medical application and is often encountered in research related to radionuclide production. In particular, it is an important radionuclidic impurity to identify and control during the production of Tb-155 for nuclear medicine.

Neither Tb-155 nor Tb-156 occurs naturally in significant quantities. They are produced artificially through nuclear reactions, typically by irradiating gadolinium targets with accelerated particles. Both isotopes can therefore be detected in irradiated target materials and at facilities involved in accelerator-based isotope production and nuclear medicine research.