Isotopes of terbium
Naturally occurring terbium is composed of one stable isotope, 159Tb. Thirty-seven radioisotopes have been characterized, with the most stable being 158Tb with a half-life of 180 years, 157Tb with a half-life of 71 years, and 160Tb with a half-life of 72.3 days. All of the remaining radioactive isotopes have half-lives that are less than one week, and the majority of these have half-lives that are less than 24 seconds. This element also has 27 meta states, with the most stable being 156m2Tb, 154m2Tb and 154m1Tb.
The primary decay mode before the most abundant stable isotope, 159Tb, is electron capture to gadolinium isotopes, and the primary mode after is beta decay to dysprosium isotopes.