Naturally occurring gadolinium (64Gd) is composed of six stable isotopes, 154Gd, 155Gd, 156Gd, 157Gd, 158Gd and 160Gd, and one long-lived radioisotope, 152Gd, with 158Gd being the most abundant (24.84% natural abundance). The predicted double beta decay of 160Gd has not been observed.
In all, 32 radioisotopes of gadolinium have been characterized, with the three most stable being alpha emitters: 152Gd (naturally occurring) with a half-life of 1.08×1014 years, 150Gd with a half-life of 1.79×106 years, and 148Gd (theoretically not beta-stable) with a half-life of 86.9 years. All of the remaining radioactive isotopes have half-lives less than a year, the majority of these having half-lives less than two minutes. There are also 16 metastable isomers, with the most stable being 143mGd (t1/2 = 110 seconds), 145mGd (t1/2 = 85 seconds) and 141mGd (t1/2 = 24.5 seconds).
The isotopes with atomic masses lower than the most abundant stable isotope, 158Gd, primarily decay by electron capture to isotopes of europium. For higher atomic masses, the primary decay mode is beta decay to isotopes of terbium.
List of isotopes
|-id=Gadolinium-133 | 133Gd | style="text-align:right" | 64 | style="text-align:right" | 69 | 132.96129(54)# | style="text-align:center" | 2026 | 10# ms[>310 ns]
| 5/2+#
|-id=Gadolinium-134 | 134Gd | style="text-align:right" | 64 | style="text-align:right" | 70 | 133.95542(43)# | style="text-align:center" | 2026 | 400# ms[>310 ns]
| 0+
|-id=Gadolinium-135 | rowspan=2|135Gd | rowspan=2 style="text-align:right" | 64 | rowspan=2 style="text-align:right" | 71 | rowspan=2|134.95250(43)# | rowspan=2 style="text-align:center" | 1996 | rowspan=2|1.1(2) s | β+ (98%) | 135Eu | rowspan=2|(5/2+) | rowspan=2| | rowspan=2| |- | β+, p (98%) | 134Sm |-id=Gadolinium-136 | rowspan=2|136Gd | rowspan=2 style="text-align:right" | 64 | rowspan=2 style="text-align:right" | 72 | rowspan=2|135.94730(32)# | rowspan=2 style="text-align:center" | 2026 | rowspan=2|1# s [>310 ns] | β+? | 136Eu | rowspan=2|0+ | rowspan=2| | rowspan=2| |- | β+, p? | 135Sm |-id=Gadolinium-137 | rowspan=2|137Gd | rowspan=2 style="text-align:right" | 64 | rowspan=2 style="text-align:right" | 73 | rowspan=2|136.94502(32)# | rowspan=2 style="text-align:center" | 1999 | rowspan=2|2.2(2) s | β+ | 137Eu | rowspan=2|(7/2)+# | rowspan=2| | rowspan=2| |- | β+, p? | 136Sm |-id=Gadolinium-138 | 138Gd | style="text-align:right" | 64 | style="text-align:right" | 74 | 137.94025(22)# | style="text-align:center" | 1985 | 4.7(9) s | β+ | 138Eu | 0+
|-id=Gadolinium-138m | style="text-indent:1em" | 138mGd | colspan="3" style="text-indent:2em" | 2232.6(11) keV | style="text-align:center" | 1997 | 6.2(0.2) μs | IT | 138Gd | (8−)
|-id=Gadolinium-139 | rowspan=2|139Gd | rowspan=2 style="text-align:right" | 64 | rowspan=2 style="text-align:right" | 75 | rowspan=2|138.93813(21)# | rowspan=2 style="text-align:center" | 1983 | rowspan=2|5.7(3) s | β+ | 139Eu | rowspan=2|9/2−# | rowspan=2| | rowspan=2| |- | β+, p? | 138Sm |-id=Gadolinium-139m | rowspan=2 style="text-indent:1em" | 139mGdOrder of ground state and isomer is uncertain. | rowspan=2 colspan="3" style="text-indent:2em" | 250(150)# keV | rowspan=2 style="text-align:center" | 1999 | rowspan=2|4.8(9) s | β+ | 139Eu | rowspan=2|1/2+# | rowspan=2| | rowspan=2| |- | β+, p? | 138Sm |-id=Gadolinium-140 | rowspan=2|140Gd | rowspan=2 style="text-align:right" | 64 | rowspan=2 style="text-align:right" | 76 | rowspan=2|139.933674(30) | rowspan=2 style="text-align:center" | 1985 | rowspan=2|15.8(4) s | β+ (67(8)%) | rowspan=2|140Eu | rowspan=2|0+ | rowspan=2| | rowspan=2| |- | EC (33(8)%) |-id=Gadolinium-141 | rowspan=2|141Gd | rowspan=2 style="text-align:right" | 64 | rowspan=2 style="text-align:right" | 77 | rowspan=2|140.932126(21) | rowspan=2 style="text-align:center" | 1986 | rowspan=2|14(4) s | β+ (99.97%) | 141Eu | rowspan=2|(1/2+) | rowspan=2| | rowspan=2| |- | β+, p (0.03%) | 140Sm |-id=Gadolinium-141m | rowspan=2 style="text-indent:1em" | 141mGd | rowspan=2 colspan="3" style="text-indent:2em" | 377.76(9) keV | rowspan=2 style="text-align:center" | 1989 | rowspan=2|24.5(5) s | β+ (89%) | 141Eu | rowspan=2|(11/2−) | rowspan=2| | rowspan=2| |- | IT (11%) | 141Gd |-id=Gadolinium-142 | rowspan=2|142Gd | rowspan=2 style="text-align:right" | 64 | rowspan=2 style="text-align:right" | 78 | rowspan=2|141.928116(30) | rowspan=2 style="text-align:center" | 1986 | rowspan=2|70.2(6) s | EC (52(5)%) | rowspan=2|142Eu | rowspan=2|0+ | rowspan=2| | rowspan=2| |- | β+ (48(5)%) |-id=Gadolinium-143 | rowspan=3|143Gd | rowspan=3 style="text-align:right" | 64 | rowspan=3 style="text-align:right" | 79 | rowspan=3|142.92675(22) | rowspan=3 style="text-align:center" | 1975 | rowspan=3|39(2) s | β+ | 143Eu | rowspan=3|1/2+ | rowspan=3| | rowspan=3| |- | β+, p? | 142Sm |- | β+, α? | 139Pm |-id=Gadolinium-143m | rowspan=3 style="text-indent:1em" | 143mGd | rowspan=3 colspan="3" style="text-indent:2em" | 152.6(5) keV | rowspan=3 style="text-align:center" | 1975 | rowspan=3|110.0(14) s | β+ | 143Eu | rowspan=3|11/2− | rowspan=3| | rowspan=3| |- | β+, p? | 142Sm |- | β+, α? | 139Pm |-id=Gadolinium-144 | 144Gd | style="text-align:right" | 64 | style="text-align:right" | 80 | 143.922963(30) | style="text-align:center" | 1968 | 4.47(6) min | β+ | 144Eu | 0+
|-id=Gadolinium-144m | style="text-indent:1em" | 144mGd | colspan="3" style="text-indent:2em" | 3433.1(5) keV | style="text-align:center" | 1978 | 145(30) ns | IT | 144Gd | (10+)
|-id=Gadolinium-145 | 145Gd | style="text-align:right" | 64 | style="text-align:right" | 81 | 144.921710(21) | style="text-align:center" | 1959 | 23.0(4) min | β+ | 145Eu | 1/2+
|-id=Gadolinium-145m | rowspan=2 style="text-indent:1em" | 145mGd | rowspan=2 colspan="3" style="text-indent:2em" | 749.1(2) keV | rowspan=2 style="text-align:center" | 1969 | rowspan=2|85(3) s | IT (94.3%) | 145Gd | rowspan=2|11/2− | rowspan=2| | rowspan=2| |- | β+ (5.7%) | 145Eu |-id=Gadolinium-146 | 146Gd | style="text-align:right" | 64 | style="text-align:right" | 82 | 145.9183185(44) | style="text-align:center" | 1957 | 48.27(9) d | EC | 146Eu | 0+
|-id=Gadolinium-147 | 147Gd | style="text-align:right" | 64 | style="text-align:right" | 83 | 146.9191010(20) | style="text-align:center" | 1957 | 38.06(12) h | β+ | 147Eu | 7/2−
|-id=Gadolinium-147m | style="text-indent:1em" | 147mGd | colspan="3" style="text-indent:2em" | 8587.8(5) keV | style="text-align:center" | 1978 | 510(20) ns | IT | 147Gd | 49/2+
|-id=Gadolinium-148 | 148Gd | style="text-align:right" | 64 | style="text-align:right" | 84 | 147.9181214(16) | style="text-align:center" | 1953 | 86.9(39) y | αTheorized to also undergo β+β+ decay to 148Sm or EC to 148Eu | 144Sm |0+ | |-id=Gadolinium-149 | rowspan=2|149Gd | rowspan=2 style="text-align:right" | 64 | rowspan=2 style="text-align:right" | 85 | rowspan=2|148.9193477(36) | rowspan=2 style="text-align:center" | 1951 | rowspan=2|9.28(10) d | β+ | 149Eu | rowspan=2|7/2− | rowspan=2| | rowspan=2| |- | α (4.3×10−4%) | 145Sm |-id=Gadolinium-150 |150Gd |style="text-align:right" | 64 | style="text-align:right" | 86 | 149.9186639(65) | style="text-align:center" | 1953 |1.79(8)×106 y | αTheorized to also undergo β+β+ decay to 150Sm | 146Sm |0+
|-id=Gadolinium-151 | rowspan=2|151Gd | rowspan=2 style="text-align:right" | 64 | rowspan=2 style="text-align:right" | 87 | rowspan=2|150.9203549(32) | rowspan=2 style="text-align:center" | 1950 | rowspan=2|123.9(10) d | EC | 151Eu | rowspan=2|7/2− | rowspan=2| | rowspan=2| |- | α (1.1×10−6%) | 147Sm |-id=Gadolinium-152 | 152Gdprimordial radionuclide | style="text-align:right" | 64 | style="text-align:right" | 88 | 151.9197984(11) | style="text-align:center" | 1938 | 1.08(8)×1014 y | αTheorized to also undergo β+β+ decay to 152Sm | 148Sm | 0+ | 0.0020(1) | |-id=Gadolinium-153 | 153Gd | style="text-align:right" | 64 | style="text-align:right" | 89 | 152.9217569(11) | style="text-align:center" | 1947 | 240.6(7) d | EC | 153Eu | 3/2−
|-id=Gadolinium-153m1 | style="text-indent:1em" | 153m1Gd | colspan="3" style="text-indent:2em" | 95.1737(8) keV | style="text-align:center" | 1979 | 3.5(4) μs | IT | 153Gd | 9/2+
|-id=Gadolinium-153m2 | style="text-indent:1em" | 153m2Gd | colspan="3" style="text-indent:2em" | 171.188(4) keV | style="text-align:center" | 1967 | 76.0(14) μs | IT | 153Gd | (11/2−)
|-id=Gadolinium-154 | 154Gd | style="text-align:right" | 64 | style="text-align:right" | 90 | 153.9208730(11) | style="text-align:center" | 1938 | colspan="3" style="text-align:center;"|Observationally StableBelieved to undergo α decay to 150Sm | 0+ | 0.0218(2) | |-id=Gadolinium-155 | 155GdFission product | style="text-align:right" | 64 | style="text-align:right" | 91 | 154.9226294(11) | style="text-align:center" | 1933 | colspan="3" style="text-align:center;"|Observationally StableBelieved to undergo α decay to 151Sm | 3/2− | 0.1480(9) | |-id=Gadolinium-155m | style="text-indent:1em" | 155mGd | colspan="3" style="text-indent:2em" | 121.10(19) keV | style="text-align:center" | 1967 | 31.97(27) ms | IT | 155Gd | 11/2−
|-id=Gadolinium-156 | 156Gd | style="text-align:right" | 64 | style="text-align:right" | 92 | 155.9221301(11) | style="text-align:center" | 1933 | colspan="3" style="text-align:center;"|Stable | 0+ | 0.2047(3) | |-id=Gadolinium-156m | style="text-indent:1em" | 156mGd | colspan="3" style="text-indent:2em" | 2137.60(5) keV | style="text-align:center" | 1969 | 1.3(1) μs | IT | 156Gd | 7-
|-id=Gadolinium-157 | 157Gd | style="text-align:right" | 64 | style="text-align:right" | 93 | 156.9239674(10) | style="text-align:center" | 1933 | colspan="3" style="text-align:center;"|Stable | 3/2− | 0.1565(4) | |-id=Gadolinium-157m1 | style="text-indent:1em" | 157m1Gd | colspan="3" style="text-indent:2em" | 63.916(5) keV | style="text-align:center" | 1964 | 460(40) ns | IT | 157Gd | 5/2+
|-id=Gadolinium-157m2 | style="text-indent:1em" | 157m2Gd | colspan="3" style="text-indent:2em" | 426.539(23) keV | style="text-align:center" | 1967 | 18.5(23) μs | IT | 157Gd | 11/2−
|-id=Gadolinium-158 | 158Gd | style="text-align:right" | 64 | style="text-align:right" | 94 | 157.9241112(10) | style="text-align:center" | 1933 | colspan="3" style="text-align:center;"|Stable | 0+ | 0.2484(8) | |-id=Gadolinium-159 | 159Gd | style="text-align:right" | 64 | style="text-align:right" | 95 | 158.9263958(11) | style="text-align:center" | 1949 | 18.479(4) h | β− | 159Tb | 3/2−
|-id=Gadolinium-160 | 160Gd | style="text-align:right" | 64 | style="text-align:right" | 96 | 159.9270612(12) | style="text-align:center" | 1933 | colspan="3" style="text-align:center;"|Observationally StableBelieved to undergo β−β− decay to 160Dy with a half-life over 3.1×1019 years | 0+ | 0.2186(3) | |-id=Gadolinium-161 | 161Gd | style="text-align:right" | 64 | style="text-align:right" | 97 | 160.9296763(16) | style="text-align:center" | 1949 | 3.646(3) min | β− | 161Tb | 5/2−
|-id=Gadolinium-162 | 162Gd | style="text-align:right" | 64 | style="text-align:right" | 98 | 161.9309918(43) | style="text-align:center" | 1967 | 8.4(2) min | β− | 162Tb | 0+
|-id=Gadolinium-162m | style="text-indent:1em" | 162mGd | colspan="3" style="text-indent:2em" | 1448.6 keV | style="text-align:center" | 2021 | 99(3) μs | IT | 162Gd | (6−)
|-id=Gadolinium-163 | 163Gd | style="text-align:right" | 64 | style="text-align:right" | 99 | 162.93409664(86) | style="text-align:center" | 1982 | 68(3) s | β− | 163Tb | 7/2+
|-id=Gadolinium-163m | rowspan=2 style="text-indent:1em" | 163mGd | rowspan=2 colspan="3" style="text-indent:2em" | 138.22(20) keV | rowspan=2 style="text-align:center" | 2014 | rowspan=2|23.5(10) s | IT? | 163Gd | rowspan=2|1/2− | rowspan=2| | rowspan=2| |- | β− | 163Tb |-id=Gadolinium-164 | 164Gd | style="text-align:right" | 64 | style="text-align:right" | 100 | 163.9359162(11) | style="text-align:center" | 1988 | 45(3) s | β− | 164Tb | 0+
|-id=Gadolinium-164m | style="text-indent:1em" | 164mGd