Naturally occurring terbium (65Tb) 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 (t1/2 = 24.4 hours), 154m2Tb (t1/2 = 22.7 hours) and 154m1Tb (t1/2 = 21.5 hours).
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.
It is also the heaviest element with no observationally stable isotopes that are predicted to be radioactive excluding the possibilities of proton decay (never observed so far) and spontaneous fission (not observed until the actinides).
List of isotopes
|-id=Terbium-135 | 135Tb | style="text-align:right" | 65 | style="text-align:right" | 70 | 134.96452(43)# | style="text-align:center" | 2004 | 1.01(28) ms | p | 134Gd | (7/2−) | |-id=Terbium-136 | 136Tb | style="text-align:right" | 65 | style="text-align:right" | 71 | 135.96146(54)# | style="text-align:center" | 2026 | 200# ms[>310 ns]
| 5−# | |-id=Terbium-137 | 137Tb | style="text-align:right" | 65 | style="text-align:right" | 72 | 136.95602(43)# | style="text-align:center" | 2026 | 600# ms[>310 ns]
| 3/2+# | |-id=Terbium-138 | 138Tb | style="text-align:right" | 65 | style="text-align:right" | 73 | 137.95319(32)# | style="text-align:center" | 2026 | 800# ms[>310 ns]
| 2+# | |-id=Terbium-139 | 139Tb | style="text-align:right" | 65 | style="text-align:right" | 74 | 138.94833(32)# | style="text-align:center" | 1999 | 1.6(2) s | β+ | 139Gd | 5/2−# | |-id=Terbium-140 | rowspan=3|140Tb | rowspan=3 style="text-align:right" | 65 | rowspan=3 style="text-align:right" | 75 | rowspan=3|139.94581(86) | rowspan=3 style="text-align:center" | 1986 | rowspan=3|2.29(15) s | β+ (99.74%) | 140Gd | rowspan=3|(7+) | rowspan=3| |- | EC (<3%) | 140Gd |- | β+, p (0.26%) | 139Eu |-id=Terbium-141 | 141Tb | style="text-align:right" | 65 | style="text-align:right" | 76 | 140.94145(11) | style="text-align:center" | 1986 | 3.5(2) s | β+ | 141Gd | (5/2−) | |-id=Terbium-141m | style="text-indent:1em" | 141mTbOrder of ground state and isomer is uncertain.Discovery of this isotope is disputed. | colspan="3" style="text-indent:2em" | 0(200)# keV | style="text-align:center" | (1988)Only published in a conference proceeding and not a refereed journal | 7.9(6) s | β+ | 141Gd | 11/2−# | |-id=Terbium-142 | rowspan=3|142Tb | rowspan=3 style="text-align:right" | 65 | rowspan=3 style="text-align:right" | 77 | rowspan=3|141.93928(75) | rowspan=3 style="text-align:center" | 1991 | rowspan=3|597(17) ms | β+ (96.8%) | 142Gd | rowspan=3|1+ | rowspan=3| |- | EC (3.2%) | 142Gd |- | β+, p (0.0022%) | 141Eu |-id=Terbium-142m1 | style="text-indent:1em" | 142m1Tb | colspan="3" style="text-indent:2em" | 279.7(4) keV | style="text-align:center" | 1991 | 303(17) ms | IT | 142Tb | 5− | |-id=Terbium-142m2 | style="text-indent:1em" | 142m2Tb | colspan="3" style="text-indent:2em" | 652.1(6) keV | style="text-align:center" | 2009 | 26(1) μs | IT | 142Tb | 8+ | |-id=Terbium-143 | 143Tb | style="text-align:right" | 65 | style="text-align:right" | 78 | 142.935137(55) | style="text-align:center" | 1985 | 12(1) s | β+ | 143Gd | (11/2−) | |-id=Terbium-143m | style="text-indent:1em" | 143mTb | colspan="3" style="text-indent:2em" | 0(100)# keV | style="text-align:center" | (1986)Half-life not measured, not included in discovery database | 17(4) s
| 5/2+# | |-id=Terbium-144 | 144Tb | style="text-align:right" | 65 | style="text-align:right" | 79 | 143.933045(30) | style="text-align:center" | 1982 | ~1 s | β+ | 144Gd | 1+ | |-id=Terbium-144m1 | rowspan=2 style="text-indent:1em" | 144m1Tb | rowspan=2 colspan="3" style="text-indent:2em" | 396.9(5) keV | rowspan=2 style="text-align:center" | 1982 | rowspan=2|4.25(15) s | IT (66%) | 144Tb | rowspan=2|6− | rowspan=2| |- | β+ (34%) | 144Gd |-id=Terbium-144m2 | style="text-indent:1em" | 144m2Tb | colspan="3" style="text-indent:2em" | 476.2(5) keV | style="text-align:center" | 1996 | 2.8(3) μs | IT | 144Tb | (8−) | |-id=Terbium-144m3 | style="text-indent:1em" | 144m3Tb | colspan="3" style="text-indent:2em" | 517.1(5) keV | style="text-align:center" | 1996 | 670(60) ns | IT | 144Tb | (9+) | |-id=Terbium-144m4 | style="text-indent:1em" | 144m4Tb | colspan="3" style="text-indent:2em" | 544.5(6) keV | style="text-align:center" | (1996) | <300 ns | IT | 144Tb | (10+) | |-id=Terbium-145 | 145Tb | style="text-align:right" | 65 | style="text-align:right" | 80 | 144.92872(12) | style="text-align:center" | 1981 | 30.9(6) s | β+ | 145Gd | (11/2−) | |-id=Terbium-145m | style="text-indent:1em" | 145mTb | colspan="3" style="text-indent:2em" | 860(230) keV | style="text-align:center" | 1993
| (3/2+) | |-id=Terbium-146 | 146Tb | style="text-align:right" | 65 | style="text-align:right" | 81 | 145.927253(48) | style="text-align:center" | 1974 | 8(4) s | β+ | 146Gd | 1+ | |-id=Terbium-146m1 | style="text-indent:1em" | 146m1Tb | colspan="3" style="text-indent:2em" | 150(100)# keV | style="text-align:center" | 1982 | 24.1(5) s | β+ | 146Gd | 5− | |-id=Terbium-146m2 | style="text-indent:1em" | 146m2Tb | colspan="3" style="text-indent:2em" | 930(100)# keV | style="text-align:center" | 1984 | 1.18(2) ms | IT | 146Tb | 10+ | |-id=Terbium-147 | 147Tb | style="text-align:right" | 65 | style="text-align:right" | 82 | 146.9240546(87) | style="text-align:center" | 1969 | 1.64(3) h | β+ | 147Gd | (1/2+) | |-id=Terbium-147m | style="text-indent:1em" | 147mTb | colspan="3" style="text-indent:2em" | 50.6(9) keV | style="text-align:center" | 1969 | 1.87(5) min | β+ | 147Gd | (11/2−) | |-id=Terbium-148 | 148Tb | style="text-align:right" | 65 | style="text-align:right" | 83 | 147.924275(13) | style="text-align:center" | 1960 | 60(1) min | β+ | 148Gd | 2− | |-id=Terbium-148m1 | style="text-indent:1em" | 148m1Tb | colspan="3" style="text-indent:2em" | 90.1(3) keV | style="text-align:center" | 1973 | 2.20(5) min | β+ | 148Gd | (9)+ | |-id=Terbium-148m2 | style="text-indent:1em" | 148m2Tb | colspan="3" style="text-indent:2em" | 8618.6(10) keV | style="text-align:center" | 1980 | 1.310(7) μs | IT | 148Tb | (27+) | |-id=Terbium-149 | rowspan=2|149Tb | rowspan=2 style="text-align:right" | 65 | rowspan=2 style="text-align:right" | 84 | rowspan=2|148.9232538(39) | rowspan=2 style="text-align:center" | 1950 | rowspan=2|4.118(25) h | β+ (83.3%) | 149Gd | rowspan=2|1/2+ | rowspan=2| |- | α (16.7%) | 145Eu |-id=Terbium-149m | rowspan=2 style="text-indent:1em" | 149mTb | rowspan=2 colspan="3" style="text-indent:2em" | 35.78(13) keV | rowspan=2 style="text-align:center" | 1962 | rowspan=2|4.16(4) min | β+ (99.98%) | 149Gd | rowspan=2|11/2− | rowspan=2| |- | α (0.022%) | 145Eu |-id=Terbium-150 | 150Tb | style="text-align:right" | 65 | style="text-align:right" | 85 | 149.9236648(79) | style="text-align:center" | 1959 | 3.48(16) h | β+ | 150Gd | (2)− | |-id=Terbium-150m | style="text-indent:1em" | 150mTb | colspan="3" style="text-indent:2em" | 461(27) keV | style="text-align:center" | 1972 | 5.8(2) min | β+ | 150Gd | 9+ | |-id=Terbium-151 | rowspan=2|151Tb | rowspan=2 style="text-align:right" | 65 | rowspan=2 style="text-align:right" | 86 | rowspan=2|150.9231090(44) | rowspan=2 style="text-align:center" | 1953 | rowspan=2|17.609(1) h | β+ (99.99%) | 151Gd | rowspan=2|1/2+ | rowspan=2| |- | α (0.0095%) | 147Eu |-id=Terbium-151m | rowspan=2 style="text-indent:1em" | 151mTb | rowspan=2 colspan="3" style="text-indent:2em" | 99.53(5) keV | rowspan=2 style="text-align:center" | 1978 | rowspan=2|25(3) s | IT (93.4%) | 151Tb | rowspan=2|11/2− | rowspan=2| |- | β+ (6.6%) | 151Gd |-id=Terbium-152 | rowspan=3|152Tb | rowspan=3 style="text-align:right" | 65 | rowspan=3 style="text-align:right" | 87 | rowspan=3|151.924082(43) | rowspan=3 style="text-align:center" | 1959 | rowspan=3|17.8784(95) h | EC (83%) | rowspan=2|152Gd | rowspan=3|2− | rowspan=3| |- | β+ (17%) |- | α (<7×10−7%) | 148Eu |-id=Terbium-152m1 | style="text-indent:1em" | 152m1Tb | colspan="3" style="text-indent:2em" | 342.15(16) keV | style="text-align:center" | 1980 | 960(10) ns | IT | 152Tb | 5− | |-id=Terbium-152m2 | rowspan=2 style="text-indent:1em" | 152m2Tb | rowspan=2 colspan="3" style="text-indent:2em" | 501.74(19) keV | rowspan=2 style="text-align:center" | 1959 | rowspan=2|4.2(1) min | IT (78.9%) | 152Tb | rowspan=2|8+ | rowspan=2| |- | β+ (21.1%) | 152Gd |-id=Terbium-153 | 153Tb | style="text-align:right" | 65 | style="text-align:right" | 88 | 152.9234417(42) | style="text-align:center" | 1957 | 2.34(1) d | β+ | 153Gd | 5/2+ | |-id=Terbium-153m | style="text-indent:1em" | 153mTb | colspan="3" style="text-indent:2em" | 163.175(5) keV | style="text-align:center" | 1965 | 186(4) μs | IT | 153Tb | 11/2− | |-id=Terbium-154 | 154Tb | style="text-align:right" | 65 | style="text-align:right" | 89 | 153.924684(49) | style="text-align:center" | 1950 | 9.994(39) h | β+ | 154Gd | 3− | |-id=Terbium-154m1 | style="text-indent:1em" | 154m1Tb | colspan="3" style="text-indent:2em" | 130(50)# keV | style="text-align:center" | 1955 | 21.5(4) h | β+ | 154Gd | 0− | |-id=Terbium-154m2 | style="text-indent:1em" | 154m2Tb | colspan="3" style="text-indent:2em" | 200(150)# keV | style="text-align:center" | 1971 | 22.7(5) h | β+ | 154Gd | 7− | |-id=Terbium-154m3 | style="text-indent:1em" | 154m3Tb | colspan="3" style="text-indent:2em" | 405(150)# keV | style="text-align:center" | 1982 | 513(42) ns | IT | 154Tb
|-id=Terbium-155 | 155Tb | style="text-align:right" | 65 | style="text-align:right" | 90 | 154.923510(11) | style="text-align:center" | 1957 | 5.2346(36) d | EC | 155Gd | 3/2+ | |-id=Terbium-156 | 156Tb | style="text-align:right" | 65 | style="text-align:right" | 91 | 155.9247542(40) | style="text-align:center" | 1950 | 5.35(10) d | β+ | 156Gd | 3− | |-id=Terbium-156m1 | style="text-indent:1em" | 156m1Tb | colspan="3" style="text-indent:2em" | 88.4(2) keV | style="text-align:center" | 1955 | 5.3(2) h | IT | 156Tb | (0+) | |-id=Terbium-156m2 | style="text-indent:1em" | 156m2Tb | colspan="3" style="text-indent:2em" | 100(50)# keV | style="text-align:center" | 1970 | 24.4(10) h | IT | 156Tb | (7−) | |-id=Terbium-157 | 157Tb | style="text-align:right" | 65 | style="text-align:right" | 92 | 156.9240319(11) | style="text-align:center" | 1960 | 71(7) y | EC | 157Gd | 3/2+ | |-id=Terbium-158 | rowspan=2|158Tb | rowspan=2 style="text-align:right" | 65 | rowspan=2 style="text-align:right" | 93 | rowspan=2|157.9254199(14) | rowspan=2 style="text-align:center" | 1957 | rowspan=2|180(11) y | β+ (83.4%) | 158Gd | rowspan=2|3− | rowspan=2| |- | β− (16.6%) | 158Dy |-id=Terbium-158m1 | style="text-indent:1em" | 158m1Tb | colspan="3" style="text-indent:2em" | 110.3(12) keV | style="text-align:center" | 1957 | 10.70(17) s | IT | 158Tb | 0− | |-id=Terbium-158m2 | style="text-indent:1em" | 158m2Tb | colspan="3" style="text-indent:2em" | 388.39(11) keV | style="text-align:center" | 1967 | 0.40(4) ms | IT | 158Tb | 7− | |-id=Terbium-159 | 159TbFission product | style="text-align:right" | 65 | style="text-align:right" | 94 | 158.9253537(12) | style="text-align:center" | 1933 | colspan=3 align=center|Stable | 3/2+ | 1.0000 |-id=Terbium-160 | 160Tb | style="text-align:right" | 65 | style="text-align:right" | 95 | 159.9271746(12) | style="text-align:center" | 1943 | 72.3(2) d | β− | 160Dy | 3− | |-id=Terbium-161 | 161Tb | style="text-align:right" | 65 | style="text-align:right" | 96 | 160.9275768(13) | style="text-align:center" | 1949 | 6.948(5) d | β− | 161Dy | 3/2+ | |-id=Terbium-162 | 162Tb | style="text-align:right" | 65 | style="text-align:right" | 97 | 161.9292754(22) | style="text-align:center" | 1965 | 7.60(15) min