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... Wikipedia