# Isotopes of thallium

Thallium ($_{81}$Tl) has 41 isotopes with atomic masses that range from 176 to 216. $^{203}$Tl and $^{205}$Tl are the only stable isotopes and $^{204}$Tl is the most stable radioisotope with a half-life of 3.78 years. $^{207}$Tl, with a half-life of 4.77 minutes, has the longest half-life of naturally occurring Tl radioisotopes. All isotopes of thallium are either radioactive or observationally stable, meaning that they are predicted to be radioactive but no actual decay has been observed.
Thallium-202 (half-life 12.23 days) can be made in a cyclotron while thallium-204 (half-life 3.78 years) is made by the neutron activation of stable thallium in a nuclear reactor.
In the fully ionized state, the isotope $^{205}$Tl$^{81+}$ becomes beta-radioactive, undergoing bound-state β$^−$ decay to $^{205}$Pb$^{81+}$ with a half-life of 291+33 −27 days, but $^{203}$Tl remains stable.
$^{205}$Tl is the decay product of bismuth-209, an isotope that was once thought to be stable but is now known to undergo alpha decay with an extremely long half-life of 2.01×10$^{19}$ y. $^{205}$Tl is at the end of the neptunium series decay chain.

## List of isotopes

| Nuclide | Historic name | Z | N | Isotopic mass (Da) | Half-life | Decay mode | Daughter isotope | Spin and parity | Natural abundance (mole fraction) | Natural abundance (mole fraction) |
| Nuclide | Historic name | Excitation energy | Excitation energy | Excitation energy | Half-life | Decay mode | Daughter isotope | Spin and parity | Normal proportion | Range of variation |
| $^{176}$Tl | | 81 | 95 | 176.00059(21)# | 2.4+1.6 −0.7 ms | p (~63%) | $^{175}$Hg | (3−, 4−, 5−) | | |
| $^{176}$Tl | | 81 | 95 | 176.00059(21)# | 2.4+1.6 −0.7 ms | α (~37%) | $^{172}$Au | (3−, 4−, 5−) | | |
| $^{176m}$Tl | | ~671 keV | ~671 keV | ~671 keV | 290+200 −80 μs | p (~50%) | $^{175}$Hg | | | |
| $^{176m}$Tl | | ~671 keV | ~671 keV | ~671 keV | 290+200 −80 μs | α (~50%) | $^{172m}$Au | | | |
| $^{177}$Tl | | 81 | 96 | 176.996427(27) | 18(5) ms | α (73%) | $^{173}$Au | (1/2+) | | |
| $^{177}$Tl | | 81 | 96 | 176.996427(27) | 18(5) ms | p (27%) | $^{176}$Hg | (1/2+) | | |
| $^{177m}$Tl | | 807(18) keV | 807(18) keV | 807(18) keV | 230(40) μs | p (51%) | $^{176}$Hg | (11/2−) | | |
| $^{177m}$Tl | | 807(18) keV | 807(18) keV | 807(18) keV | 230(40) μs | α (49%) | $^{173}$Au | (11/2−) | | |
| $^{178}$Tl | | 81 | 97 | 177.99490(12)# | 255(9) ms | α (62%) | $^{174}$Au | (4-,5-) | | |
| $^{178}$Tl | | 81 | 97 | 177.99490(12)# | 255(9) ms | β$^+$ (38%) | $^{178}$Hg | (4-,5-) | | |
| $^{178}$Tl | | 81 | 97 | 177.99490(12)# | 255(9) ms | β$^+$, SF (0.15%) | (various) | (4-,5-) | | |
| $^{179}$Tl | | 81 | 98 | 178.99109(5) | 437(9) ms | α (60%) | $^{175}$Au | (1/2+) | | |
| $^{179}$Tl | | 81 | 98 | 178.99109(5) | 437(9) ms | β$^+$ (40%) | $^{179}$Hg | (1/2+) | | |
| $^{179m1}$Tl | | 825(10)# keV | 825(10)# keV | 825(10)# keV | 1.41(2) ms | α | $^{175}$Au | (11/2−) | | |
| $^{179m1}$Tl | | 825(10)# keV | 825(10)# keV | 825(10)# keV | 1.41(2) ms | IT (rare) | $^{179}$Tl | (11/2−) | | |
| $^{179m1}$Tl | | 825(10)# keV | 825(10)# keV | 825(10)# keV | 1.41(2) ms | β$^+$ (rare) | $^{179}$Hg | (11/2−) | | |
| $^{179m2}$Tl | | 904.5(9) keV | 904.5(9) keV | 904.5(9) keV | 119(14) ns | IT | $^{179}$Tl | (9/2−) | | |
| $^{180}$Tl | | 81 | 99 | 179.98991(13)# | 1.09(1) s | β$^+$ (93%) | $^{180}$Hg | 4-# | | |
| $^{180}$Tl | | 81 | 99 | 179.98991(13)# | 1.09(1) s | α (7%) | $^{176}$Au | 4-# | | |
| $^{180}$Tl | | 81 | 99 | 179.98991(13)# | 1.09(1) s | β$^+$, SF (0.0032%) | $^{100}$Ru, $^{80}$Kr | 4-# | | |
| $^{181}$Tl | | 81 | 100 | 180.986257(10) | 2.9(1) s | β$^+$ (91.4%) | $^{181}$Hg | 1/2+# | | |
| $^{181}$Tl | | 81 | 100 | 180.986257(10) | 2.9(1) s | α (8.6%) | $^{177}$Au | 1/2+# | | |
| $^{181m}$Tl | | 834.9(4) keV | 834.9(4) keV | 834.9(4) keV | 1.40(3) ms | IT (99.60%) | $^{181}$Tl | (9/2−) | | |
| $^{181m}$Tl | | 834.9(4) keV | 834.9(4) keV | 834.9(4) keV | 1.40(3) ms | α (0.40%) | $^{177}$Au | (9/2−) | | |
| $^{182}$Tl | | 81 | 101 | 181.98567(8) | 2.0(3) s | β$^+$ (96%) | $^{182}$Hg | 2−# | | |
| $^{182}$Tl | | 81 | 101 | 181.98567(8) | 2.0(3) s | α (4%) | $^{178}$Au | 2−# | | |
| $^{182m1}$Tl | | 100(100)# keV | 100(100)# keV | 100(100)# keV | 2.9(5) s | α | $^{178}$Au | (7+) | | |
| $^{182m1}$Tl | | 100(100)# keV | 100(100)# keV | 100(100)# keV | 2.9(5) s | β$^+$ (rare) | $^{182}$Hg | (7+) | | |
| $^{182m2}$Tl | | 600(140)# keV | 600(140)# keV | 600(140)# keV | | | | 10− | | |
| $^{183}$Tl | | 81 | 102 | 182.982193(10) | 6.9(7) s | β$^+$ (98%) | $^{183}$Hg | 1/2+# | | |
| $^{183}$Tl | | 81 | 102 | 182.982193(10) | 6.9(7) s | α (2%) | $^{179}$Au | 1/2+# | | |
| $^{183m1}$Tl | | 630(17) keV | 630(17) keV | 630(17) keV | 53.3(3) ms | IT (99.99%) | $^{183}$Tl | 9/2−# | | |
| $^{183m1}$Tl | | 630(17) keV | 630(17) keV | 630(17) keV | 53.3(3) ms | α (.01%) | $^{179}$Au | 9/2−# | | |
| $^{183m2}$Tl | | 976.8(3) keV | 976.8(3) keV | 976.8(3) keV | 1.48(10) μs | | | (13/2+) | | |
| $^{184}$Tl | | 81 | 103 | 183.98187(5) | 9.7(6) s | β$^+$ | $^{184}$Hg | 2−# | | |
| $^{184m1}$Tl | | 100(100)# keV | 100(100)# keV | 100(100)# keV | 10# s | β$^+$ (97.9%) | $^{184}$Hg | 7+# | | |
| $^{184m1}$Tl | | 100(100)# keV | 100(100)# keV | 100(100)# keV | 10# s | α (2.1%) | $^{180}$Au | 7+# | | |
| $^{184m2}$Tl | | 500(140)# keV | 500(140)# keV | 500(140)# keV | 47.1 ms | IT (99.911%) | | (10−) | | |
| $^{184m2}$Tl | | 500(140)# keV | 500(140)# keV | 500(140)# keV | 47.1 ms | α (.089%) | $^{180}$Au | (10−) | | |
| $^{185}$Tl | | 81 | 104 | 184.97879(6) | 19.5(5) s | α | $^{181}$Au | 1/2+# | | |
| $^{185}$Tl | | 81 | 104 | 184.97879(6) | 19.5(5) s | β$^+$ | $^{185}$Hg | 1/2+# | | |
| $^{185m}$Tl | | 452.8(20) keV | 452.8(20) keV | 452.8(20) keV | 1.93(8) s | IT (99.99%) | $^{185}$Tl | 9/2−# | | |
| $^{185m}$Tl | | 452.8(20) keV | 452.8(20) keV | 452.8(20) keV | 1.93(8) s | α (.01%) | $^{181}$Au | 9/2−# | | |
| $^{185m}$Tl | | 452.8(20) keV | 452.8(20) keV | 452.8(20) keV | 1.93(8) s | β$^+$ | $^{185}$Hg | 9/2−# | | |
| $^{186}$Tl | | 81 | 105 | 185.97833(20) | 40# s | β$^+$ | $^{186}$Hg | (2−) | | |
| $^{186}$Tl | | 81 | 105 | 185.97833(20) | 40# s | α (.006%) | $^{182}$Au | (2−) | | |
| $^{186m1}$Tl | | 320(180) keV | 320(180) keV | 320(180) keV | 27.5(10) s | β$^+$ | $^{186}$Hg | (7+) | | |
| $^{186m2}$Tl | | 690(180) keV | 690(180) keV | 690(180) keV | 2.9(2) s | | | (10−) | | |
| $^{187}$Tl | | 81 | 106 | 186.975906(9) | ~51 s | β$^+$ | $^{187}$Hg | (1/2+) | | |
| $^{187}$Tl | | 81 | 106 | 186.975906(9) | ~51 s | α (rare) | $^{183}$Au | (1/2+) | | |
| $^{187m}$Tl | | 335(3) keV | 335(3) keV | 335(3) keV | 15.60(12) s | α | $^{183}$Au | (9/2−) | | |
| $^{187m}$Tl | | 335(3) keV | 335(3) keV | 335(3) keV | 15.60(12) s | IT | $^{187}$Tl | (9/2−) | | |
| $^{187m}$Tl | | 335(3) keV | 335(3) keV | 335(3) keV | 15.60(12) s | β$^+$ | $^{187}$Hg | (9/2−) | | |
| $^{188}$Tl | | 81 | 107 | 187.97601(4) | 71(2) s | β$^+$ | $^{188}$Hg | (2−) | | |
| $^{188m1}$Tl | | 40(30) keV | 40(30) keV | 40(30) keV | 71(1) s | β$^+$ | $^{188}$Hg | (7+) | | |
| $^{188m2}$Tl | | 310(30) keV | 310(30) keV | 310(30) keV | 41(4) ms | | | (9−) | | |
| $^{189}$Tl | | 81 | 108 | 188.973588(12) | 2.3(2) min | β$^+$ | $^{189}$Hg | (1/2+) | | |
| $^{189m}$Tl | | 257.6(13) keV | 257.6(13) keV | 257.6(13) keV | 1.4(1) min | β$^+$ (96%) | $^{189}$Hg | (9/2−) | | |
| $^{189m}$Tl | | 257.6(13) keV | 257.6(13) keV | 257.6(13) keV | 1.4(1) min | IT (4%) | $^{189}$Tl | (9/2−) | | |
| $^{190}$Tl | | 81 | 109 | 189.97388(5) | 2.6(3) min | β$^+$ | $^{190}$Hg | 2(−) | | |
| $^{190m1}$Tl | | 130(90)# keV | 130(90)# keV | 130(90)# keV | 3.7(3) min | β$^+$ | $^{190}$Hg | 7(+#) | | |
| $^{190m2}$Tl | | 290(70)# keV | 290(70)# keV | 290(70)# keV | 750(40) μs | | | (8−) | | |
| $^{190m3}$Tl | | 410(70)# keV | 410(70)# keV | 410(70)# keV | >1 μs | | | 9− | | |
| $^{191}$Tl | | 81 | 110 | 190.971786(8) | 20# min | β$^+$ | $^{191}$Hg | (1/2+) | | |
| $^{191m}$Tl | | 297(7) keV | 297(7) keV | 297(7) keV | 5.22(16) min | β$^+$ | $^{191}$Hg | 9/2(−) | | |
| $^{192}$Tl | | 81 | 111 | 191.97223(3) | 9.6(4) min | β$^+$ | $^{192}$Hg | (2−) | | |
| $^{192m1}$Tl | | 160(50) keV | 160(50) keV | 160(50) keV | 10.8(2) min | β$^+$ | $^{192}$Hg | (7+) | | |
| $^{192m2}$Tl | | 407(54) keV | 407(54) keV | 407(54) keV | 296(5) ns | | | (8−) | | |
| $^{193}$Tl | | 81 | 112 | 192.97067(12) | 21.6(8) min | β$^+$ | $^{193}$Hg | 1/2(+#) | | |
| $^{193m}$Tl | | 369(4) keV | 369(4) keV | 369(4) keV | 2.11(15) min | IT (75%) | $^{193}$Tl | 9/2− | | |
| $^{193m}$Tl | | 369(4) keV | 369(4) keV | 369(4) keV | 2.11(15) min | β$^+$ (25%) | $^{193}$Hg | 9/2− | | |
| $^{194}$Tl | | 81 | 113 | 193.97120(15) | 33.0(5) min | β$^+$ | $^{194}$Hg | 2− | | |
| $^{194}$Tl | | 81 | 113 | 193.97120(15) | 33.0(5) min | α (10$^{−7}$%) | $^{190}$Au | 2− | | |
| $^{194m}$Tl | | 300(200)# keV | 300(200)# keV | 300(200)# keV | 32.8(2) min | β$^+$ | $^{194}$Hg | (7+) | | |
| $^{195}$Tl | | 81 | 114 | 194.969774(15) | 1.16(5) h | β$^+$ | $^{195}$Hg | 1/2+ | | |
| $^{195m}$Tl | | 482.63(17) keV | 482.63(17) keV | 482.63(17) keV | 3.6(4) s | IT | $^{195}$Tl | 9/2− | | |
| $^{196}$Tl | | 81 | 115 | 195.970481(13) | 1.84(3) h | β$^+$ | $^{196}$Hg | 2− | | |
| $^{196m}$Tl | | 394.2(5) keV | 394.2(5) keV | 394.2(5) keV | 1.41(2) h | β$^+$ (95.5%) | $^{196}$Hg | (7+) | | |
| $^{196m}$Tl | | 394.2(5) keV | 394.2(5) keV | 394.2(5) keV | 1.41(2) h | IT (4.5%) | $^{196}$Tl | (7+) | | |
| $^{197}$Tl | | 81 | 116 | 196.969575(18) | 2.84(4) h | β$^+$ | $^{197}$Hg | 1/2+ | | |
| $^{197m}$Tl | | 608.22(8) keV | 608.22(8) keV | 608.22(8) keV | 540(10) ms | IT | $^{197}$Tl | 9/2− | | |
| $^{198}$Tl | | 81 | 117 | 197.97048(9) | 5.3(5) h | β$^+$ | $^{198}$Hg | 2− | | |
| $^{198m1}$Tl | | 543.5(4) keV | 543.5(4) keV | 543.5(4) keV | 1.87(3) h | β$^+$ (54%) | $^{198}$Hg | 7+ | | |
| $^{198m1}$Tl | | 543.5(4) keV | 543.5(4) keV | 543.5(4) keV | 1.87(3) h | IT (46%) | $^{198}$Tl | 7+ | | |
| $^{198m2}$Tl | | 687.2(5) keV | 687.2(5) keV | 687.2(5) keV | 150(40) ns | | | (5+) | | |
| $^{198m3}$Tl | | 742.3(4) keV | 742.3(4) keV | 742.3(4) keV | 32.1(10) ms | | | (10−)# | | |
| $^{199}$Tl | | 81 | 118 | 198.96988(3) | 7.42(8) h | β$^+$ | $^{199}$Hg | 1/2+ | | |
| $^{199m}$Tl | | 749.7(3) keV | 749.7(3) keV | 749.7(3) keV | 28.4(2) ms | IT | $^{199}$Tl | 9/2− | | |
| $^{200}$Tl | | 81 | 119 | 199.970963(6) | 26.1(1) h | β$^+$ | $^{200}$Hg | 2− | | |
| $^{200m1}$Tl | | 753.6(2) keV | 753.6(2) keV | 753.6(2) keV | 34.3(10) ms | IT | $^{200}$Tl | 7+ | | |
| $^{200m2}$Tl | | 762.0(2) keV | 762.0(2) keV | 762.0(2) keV | 0.33(5) μs | | | 5+ | | |
| $^{201}$Tl | | 81 | 120 | 200.970819(16) | 72.912(17) h | EC | $^{201}$Hg | 1/2+ | | |
| $^{201m}$Tl | | 919.50(9) keV | 919.50(9) keV | 919.50(9) keV | 2.035(7) ms | IT | $^{201}$Tl | (9/2−) | | |
| $^{202}$Tl | | 81 | 121 | 201.972106(16) | 12.23(2) d | β$^+$ | $^{202}$Hg | 2− | | |
| $^{202m}$Tl | | 950.19(10) keV | 950.19(10) keV | 950.19(10) keV | 572(7) μs | | | 7+ | | |
| $^{203}$Tl | | 81 | 122 | 202.9723442(14) | Observationally Stable | Observationally Stable | Observationally Stable | 1/2+ | 0.2952(1) | 0.29494–0.29528 |
| $^{203m}$Tl | | 3400(300) keV | 3400(300) keV | 3400(300) keV | 7.7(5) μs | | | (25/2+) | | |
| $^{204}$Tl | | 81 | 123 | 203.9738635(13) | 3.78(2) y | β$^−$ (97.1%) | $^{204}$Pb | 2− | | |
| $^{204}$Tl | | 81 | 123 | 203.9738635(13) | 3.78(2) y | EC (2.9%) | $^{204}$Hg | 2− | | |
| $^{204m1}$Tl | | 1104.0(4) keV | 1104.0(4) keV | 1104.0(4) keV | 63(2) μs | | | (7)+ | | |
| $^{204m2}$Tl | | 2500(500) keV | 2500(500) keV | 2500(500) keV | 2.6(2) μs | | | (12−) | | |
| $^{204m3}$Tl | | 3500(500) keV | 3500(500) keV | 3500(500) keV | 1.6(2) μs | | | (20+) | | |
| $^{205}$Tl | | 81 | 124 | 204.9744275(14) | Observationally Stable | Observationally Stable | Observationally Stable | 1/2+ | 0.7048(1) | 0.70472–0.70506 |
| $^{205m1}$Tl | | 3290.63(17) keV | 3290.63(17) keV | 3290.63(17) keV | 2.6(2) μs | | | 25/2+ | | |
| $^{205m2}$Tl | | 4835.6(15) keV | 4835.6(15) keV | 4835.6(15) keV | 235(10) ns | | | (35/2–) | | |
| $^{206}$Tl | Radium E | 81 | 125 | 205.9761103(15) | 4.200(17) min | β$^−$ | $^{206}$Pb | 0− | Trace | |
| $^{206m}$Tl | | 2643.11(19) keV | 2643.11(19) keV | 2643.11(19) keV | 3.74(3) min | IT | $^{206}$Tl | (12–) | | |
| $^{207}$Tl | Actinium C | 81 | 126 | 206.977419(6) | 4.77(2) min | β$^−$ | $^{207}$Pb | 1/2+ | Trace | |
| $^{207m}$Tl | | 1348.1(3) keV | 1348.1(3) keV | 1348.1(3) keV | 1.33(11) s | IT (99.9%) | $^{207}$Tl | 11/2– | | |
| $^{207m}$Tl | | 1348.1(3) keV | 1348.1(3) keV | 1348.1(3) keV | 1.33(11) s | β$^−$ (.1%) | $^{207}$Pb | 11/2– | | |
| $^{208}$Tl | Thorium C" | 81 | 127 | 207.9820187(21) | 3.053(4) min | β$^−$ | $^{208}$Pb | 5+ | Trace | |
| $^{209}$Tl | | 81 | 128 | 208.985359(8) | 2.161(7) min | β$^−$ | $^{209}$Pb | 1/2+ | Trace | |
| $^{210}$Tl | Radium C″ | 81 | 129 | 209.990074(12) | 1.30(3) min | β$^−$ (99.991%) | $^{210}$Pb | (5+)# | Trace | |
| $^{210}$Tl | Radium C″ | 81 | 129 | 209.990074(12) | 1.30(3) min | β$^−$, n (.009%) | $^{209}$Pb | (5+)# | Trace | |
| $^{211}$Tl | | 81 | 130 | 210.993480(50) | 80(16) s | β$^−$ (97.8%) | $^{211}$Pb | 1/2+ | | |
| $^{211}$Tl | | 81 | 130 | 210.993480(50) | 80(16) s | β$^−$, n (2.2%) | $^{210}$Pb | 1/2+ | | |
| $^{212}$Tl | | 81 | 131 | 211.998340(220)# | 31(8) s | β$^−$ (98.2%) | $^{212}$Pb | (5+) | | |
| $^{212}$Tl | | 81 | 131 | 211.998340(220)# | 31(8) s | β$^−$, n (1.8%) | $^{211}$Pb | (5+) | | |
| $^{213}$Tl | | 81 | 132 | 213.001915(29) | 24(4) s | β$^−$ (92.4%) | $^{213}$Pb | 1/2+ | | |
| $^{213}$Tl | | 81 | 132 | 213.001915(29) | 24(4) s | β$^−$, n (7.6%) | $^{212}$Pb | 1/2+ | | |
| $^{214}$Tl | | 81 | 133 | 214.006940(210)# | 11(2) s | β$^−$ (66%) | $^{214}$Pb | 5+# | | |
| $^{214}$Tl | | 81 | 133 | 214.006940(210)# | 11(2) s | β$^−$, n (34%) | $^{213}$Pb | 5+# | | |
| $^{215}$Tl | | 81 | 134 | 215.010640(320)# | 10(4) s | β$^−$ (95.4%) | $^{215}$Pb | 1/2+# | | |
| $^{215}$Tl | | 81 | 134 | 215.010640(320)# | 10(4) s | β$^−$, n (4.6%) | $^{214}$Pb | 1/2+# | | |
| $^{216}$Tl | | 81 | 135 | 216.015800(320)# | 6(3) s | β$^−$ | $^{216}$Pb | 5+# | | |
| $^{216}$Tl | | 81 | 135 | 216.015800(320)# | 6(3) s | β$^−$, n (<11.5%) | $^{215}$Pb | 5+# | | |
| This table header & footer: view |||||||||||
* ↑ $^m$Tl – Excited nuclear isomer.
* ↑ ( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
* ↑ # – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).
* 1 2 3 # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
* ↑ Modes of decay: EC:Electron capture IT:Isomeric transition n:Neutron emission p:Proton emission
* ↑ Bold symbol as daughter – Daughter product is stable.
* ↑ ( ) spin value – Indicates spin with weak assignment arguments.
* ↑ Main isotope used in scintigraphy
* ↑ Believed to undergo α decay to $^{199}$Au
* ↑ Final decay product of 4n+1 decay chain (the Neptunium series)
* ↑ Believed to undergo α decay to $^{201}$Au
* ↑ Can undergo bound-state β$^−$ decay to $^{205}$Pb$^{81+}$ with a half-life of 291+33 −27 days when fully ionized
* 1 2 Intermediate decay product of $^{238}$U
* ↑ Intermediate decay product of $^{235}$U
* ↑ Intermediate decay product of $^{232}$Th
* ↑ Intermediate decay product of $^{237}$Np

## Thallium-201

Thallium-201 ($^{201}$Tl) is a synthetic radioisotope of thallium. It has a half-life of 73 hours and decays by electron capture, emitting X-rays (~70–80 keV), and photons of 135 and 167 keV in 10% total abundance. Thallium-201 is synthesized by the neutron activation of stable thallium in a nuclear reactor, or by the $^{203}$Tl(p, 3n)$^{201}$Pb nuclear reaction in cyclotrons, as $^{201}$Pb naturally decays to $^{201}$Tl afterwards. It is a radiopharmaceutical, as it has good imaging characteristics without excessive patient radiation dose. It is the most popular isotope used for thallium nuclear cardiac stress tests.