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Isotopes

Naturally occurring terbium is composed of its only stable isotope, terbium-159; the element is thus mononuclidic and monoisotopic. Thirty-nine radioisotopes have been characterized, with the heaviest being terbium-174 and lightest being terbium-135 (both with unknown exact mass). The most stable synthetic radioisotopes of terbium are terbium-158, with a half-life of 180 years, and terbium-157, with a half-life of 71 years. All of the remaining radioactive isotopes have half-lives that are less than three months, and the majority of these have half-lives that are less than half a minute. The primary decay mode before the most abundant stable isotope, Tb, is electron capture, which results in production of gadolinium isotopes, and the primary mode after is beta minus decay, resulting in dysprosium isotopes.

The element also has 31 nuclear isomers, with masses of 141–154, 156, 158, 162, and 164–168 (not every mass number corresponds to only one isomer). The most stable of them are terbium-156m, with a half-life of 24.4 hours, and terbium-156m2, with a half-life of 22.7 hours; this is longer than half-lives of most ground states of radioactive terbium isotopes, except those with mass numbers 155–161.

Terbium-149, with a half-life of 4.1 hours, is a promising candidate in targeted alpha therapy and positron emission tomography.