# Isotopes of actinium

Actinium ($_{89}$Ac) has no stable isotopes and no characteristic terrestrial isotopic composition, thus a standard atomic weight cannot be given. There are 34 known isotopes, from $^{203}$Ac to $^{236}$Ac, and 7 isomers. Three isotopes are found in nature, $^{225}$Ac, $^{227}$Ac and $^{228}$Ac, as intermediate decay products of, respectively, $^{237}$Np, $^{235}$U, and $^{232}$Th. $^{228}$Ac and $^{225}$Ac are extremely rare, so almost all natural actinium is $^{227}$Ac.
The most stable isotopes are $^{227}$Ac with a half-life of 21.772 years, $^{225}$Ac with a half-life of 10.0 days, and $^{226}$Ac with a half-life of 29.37 hours. All other isotopes have half-lives under 10 hours, and most under a minute. The shortest-lived known isotope is $^{217}$Ac with a half-life of 69 ns.
Purified $^{227}$Ac comes into equilibrium with its decay products ($^{227}$Th and $^{223}$Fr) after 185 days.

## List of isotopes

| Nuclide | Historic name | Z | N | Isotopic mass (Da) | Half-life | Decay mode | Daughter isotope | Spin and parity | Isotopic abundance |
| Isotopic abundance |||
| $^{203}$Ac | | 89 | 114 | | 56+269 −26 μs | α | $^{199}$Fr | (1/2+) | |
| $^{204}$Ac | | 89 | 115 | | 7.4+2.2 −1.4 ms | α | $^{200}$Fr | | |
| $^{205}$Ac | | 89 | 116 | | 7.7+2.7 −1.6 ms | α | $^{201}$Fr | 9/2−? | |
| $^{206}$Ac | | 89 | 117 | 206.01450(8) | 25(7) ms | α | $^{202}$Fr | (3+) | |
| $^{206m1}$Ac | | 80(50) keV | 80(50) keV | 80(50) keV | 15(6) ms | α | $^{202}$Fr | | |
| $^{206m2}$Ac | | 290(110)# keV | 290(110)# keV | 290(110)# keV | 41(16) ms | α | $^{202m}$Fr | (10−) | |
| $^{207}$Ac | | 89 | 118 | 207.01195(6) | 31(8) ms [27(+11−6) ms] | α | $^{203}$Fr | 9/2−# | |
| $^{208}$Ac | | 89 | 119 | 208.01155(6) | 97(16) ms [95(+24−16) ms] | α (99%) | $^{204}$Fr | (3+) | |
| $^{208}$Ac | | 89 | 119 | 208.01155(6) | 97(16) ms [95(+24−16) ms] | β$^+$ (1%) | $^{208}$Ra | (3+) | |
| $^{208m}$Ac | | 506(26) keV | 506(26) keV | 506(26) keV | 28(7) ms [25(+9−5) ms] | α (89%) | $^{204}$Fr | (10−) | |
| $^{208m}$Ac | | 506(26) keV | 506(26) keV | 506(26) keV | 28(7) ms [25(+9−5) ms] | IT (10%) | $^{208}$Ac | (10−) | |
| $^{208m}$Ac | | 506(26) keV | 506(26) keV | 506(26) keV | 28(7) ms [25(+9−5) ms] | β$^+$ (1%) | $^{208}$Ra | (10−) | |
| $^{209}$Ac | | 89 | 120 | 209.00949(5) | 92(11) ms | α (99%) | $^{205}$Fr | (9/2−) | |
| $^{209}$Ac | | 89 | 120 | 209.00949(5) | 92(11) ms | β$^+$ (1%) | $^{209}$Ra | (9/2−) | |
| $^{210}$Ac | | 89 | 121 | 210.00944(6) | 350(40) ms | α (96%) | $^{206}$Fr | 7+# | |
| $^{210}$Ac | | 89 | 121 | 210.00944(6) | 350(40) ms | β$^+$ (4%) | $^{210}$Ra | 7+# | |
| $^{211}$Ac | | 89 | 122 | 211.00773(8) | 213(25) ms | α (99.8%) | $^{207}$Fr | 9/2−# | |
| $^{211}$Ac | | 89 | 122 | 211.00773(8) | 213(25) ms | β$^+$ (.2%) | $^{211}$Ra | 9/2−# | |
| $^{212}$Ac | | 89 | 123 | 212.00781(7) | 920(50) ms | α (97%) | $^{208}$Fr | 6+# | |
| $^{212}$Ac | | 89 | 123 | 212.00781(7) | 920(50) ms | β$^+$ (3%) | $^{212}$Ra | 6+# | |
| $^{213}$Ac | | 89 | 124 | 213.00661(6) | 731(17) ms | α | $^{209}$Fr | (9/2−)# | |
| $^{213}$Ac | | 89 | 124 | 213.00661(6) | 731(17) ms | β$^+$ (rare) | $^{213}$Ra | (9/2−)# | |
| $^{214}$Ac | | 89 | 125 | 214.006902(24) | 8.2(2) s | α (89%) | $^{210}$Fr | (5+)# | |
| $^{214}$Ac | | 89 | 125 | 214.006902(24) | 8.2(2) s | β$^+$ (11%) | $^{214}$Ra | (5+)# | |
| $^{215}$Ac | | 89 | 126 | 215.006454(23) | 0.17(1) s | α (99.91%) | $^{211}$Fr | 9/2− | |
| $^{215}$Ac | | 89 | 126 | 215.006454(23) | 0.17(1) s | β$^+$ (.09%) | $^{215}$Ra | 9/2− | |
| $^{216}$Ac | | 89 | 127 | 216.008720(29) | 440(16) μs | α | $^{212}$Fr | (1−) | |
| $^{216m1}$Ac | | 38(5) keV | 38(5) keV | 38(5) keV | 441(7) μs | α | $^{212}$Fr | (9−) | |
| $^{216m2}$Ac | | 422#(100#) keV | 422#(100#) keV | 422#(100#) keV | ~300 ns | IT | $^{216}$Ac | | |
| $^{217}$Ac | | 89 | 128 | 217.009347(14) | 69(4) ns | α | $^{213}$Fr | 9/2− | |
| $^{217m}$Ac | | 2012(20) keV | 2012(20) keV | 2012(20) keV | 740(40) ns | | | (29/2)+ | |
| $^{218}$Ac | | 89 | 129 | 218.01164(5) | 1.08(9) μs | α | $^{214}$Fr | (1−)# | |
| $^{218m}$Ac | | 607(86)# keV | 607(86)# keV | 607(86)# keV | 103(11) ns | IT | $^{218}$Ac | (11+) | |
| $^{219}$Ac | | 89 | 130 | 219.01242(5) | 11.8(15) μs | α | $^{215}$Fr | 9/2− | |
| $^{220}$Ac | | 89 | 131 | 220.014763(16) | 26.36(19) ms | α | $^{216}$Fr | (3−) | |
| $^{221}$Ac | | 89 | 132 | 221.01559(5) | 52(2) ms | α | $^{217}$Fr | 9/2−# | |
| $^{222}$Ac | | 89 | 133 | 222.017844(6) | 5.0(5) s | α (99(1)%) | $^{218}$Fr | 1− | |
| $^{222}$Ac | | 89 | 133 | 222.017844(6) | 5.0(5) s | β$^+$ (1(1)%) | $^{222}$Ra | 1− | |
| $^{222m}$Ac | | 78(21) keV | 78(21) keV | 78(21) keV | 1.05(5) min | α (98.6%) | $^{218}$Fr | 5+# | |
| $^{222m}$Ac | | 78(21) keV | 78(21) keV | 78(21) keV | 1.05(5) min | β$^+$ (1.4%) | $^{222}$Ra | 5+# | |
| $^{222m}$Ac | | 78(21) keV | 78(21) keV | 78(21) keV | 1.05(5) min | IT? | $^{222}$Ac | 5+# | |
| $^{223}$Ac | | 89 | 134 | 223.019137(8) | 2.10(5) min | α (99%) | $^{219}$Fr | (5/2−) | |
| $^{223}$Ac | | 89 | 134 | 223.019137(8) | 2.10(5) min | EC (1%) | $^{223}$Ra | (5/2−) | |
| $^{223}$Ac | | 89 | 134 | 223.019137(8) | 2.10(5) min | CD (3.2×10$^{−9}$%) | $^{209}$Bi $^{14}$C | (5/2−) | |
| $^{224}$Ac | | 89 | 135 | 224.021723(4) | 2.78(17) h | β$^+$ (90.9%) | $^{224}$Ra | 0− | |
| $^{224}$Ac | | 89 | 135 | 224.021723(4) | 2.78(17) h | α (9.1%) | $^{220}$Fr | 0− | |
| $^{224}$Ac | | 89 | 135 | 224.021723(4) | 2.78(17) h | β$^−$ (1.6%) | $^{224}$Th | 0− | |
| $^{225}$Ac | | 89 | 136 | 225.023230(5) | 10.0(1) d | α | $^{221}$Fr | (3/2−) | Trace |
| $^{225}$Ac | | 89 | 136 | 225.023230(5) | 10.0(1) d | CD (6×10$^{−10}$%) | $^{211}$Bi $^{14}$C | (3/2−) | Trace |
| $^{226}$Ac | | 89 | 137 | 226.026098(4) | 29.37(12) h | β$^−$ (83%) | $^{226}$Th | (1)(−#) | |
| $^{226}$Ac | | 89 | 137 | 226.026098(4) | 29.37(12) h | EC (17%) | $^{226}$Ra | (1)(−#) | |
| $^{226}$Ac | | 89 | 137 | 226.026098(4) | 29.37(12) h | α (.006%) | $^{222}$Fr | (1)(−#) | |
| $^{227}$Ac | Actinium | 89 | 138 | 227.0277521(26) | 21.772(3) y | β$^−$ (98.62%) | $^{227}$Th | 3/2− | Trace |
| $^{227}$Ac | Actinium | 89 | 138 | 227.0277521(26) | 21.772(3) y | α (1.38%) | $^{223}$Fr | 3/2− | Trace |
| $^{228}$Ac | Mesothorium 2 | 89 | 139 | 228.0310211(27) | 6.13(2) h | β$^−$ | $^{228}$Th | 3+ | Trace |
| $^{229}$Ac | | 89 | 140 | 229.03302(4) | 62.7(5) min | β$^−$ | $^{229}$Th | (3/2+) | |
| $^{230}$Ac | | 89 | 141 | 230.03629(32) | 122(3) s | β$^−$ | $^{230}$Th | (1+) | |
| $^{231}$Ac | | 89 | 142 | 231.03856(11) | 7.5(1) min | β$^−$ | $^{231}$Th | (1/2+) | |
| $^{232}$Ac | | 89 | 143 | 232.04203(11) | 119(5) s | β$^−$ | $^{232}$Th | (1+) | |
| $^{233}$Ac | | 89 | 144 | 233.04455(32)# | 145(10) s | β$^−$ | $^{233}$Th | (1/2+) | |
| $^{234}$Ac | | 89 | 145 | 234.04842(43)# | 44(7) s | β$^−$ | $^{234}$Th | | |
| $^{235}$Ac | | 89 | 146 | 235.05123(38)# | 60(4) s | β$^−$ | $^{235}$Th | 1/2+# | |
| $^{236}$Ac | | 89 | 147 | 236.05530(54)# | 72+345 −33 s | β$^−$ | $^{236}$Th | | |
| This table header & footer: view ||||||||||
* ↑ $^m$Ac – 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).
* ↑ Modes of decay: CD:Cluster decay EC:Electron capture IT:Isomeric transition
* ↑ Bold italics symbol as daughter – Daughter product is nearly stable.
* ↑ ( ) spin value – Indicates spin with weak assignment arguments.
* 1 2 # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
* ↑ This decay mode has been observed, but only an upper limit of branching ratio is experimentally known
* ↑ Has medical uses
* ↑ Intermediate decay product of $^{237}$Np
* ↑ Source of element's name
* ↑ Intermediate decay product of $^{235}$U
* ↑ Intermediate decay product of $^{232}$Th

## Actinides vs fission products

Actinides and fission products by half-life vte :
| Actinides by decay chain | Actinides by decay chain | Actinides by decay chain | Actinides by decay chain | Half-life range (a) | Fission products of $^{235}$U by yield | Fission products of $^{235}$U by yield | Fission products of $^{235}$U by yield |
| 4n | 4n + 1 | 4n + 2 | 4n + 3 | Half-life range (a) | 4.5–7% | 0.04–1.25% | <0.001% |
| Ra$^№$ | | | | 4–6 a | | Eu$^þ$ | |
| Bk | | | | > 9 a | | | |
| Cm$^ƒ$ | Pu$^ƒ$ | Cf | Ac$^№$ | 10–29 a | Sr | Kr | Cd$^þ$ |
| U$^ƒ$ | | Pu$^ƒ$ | Cm$^ƒ$ | 29–97 a | Cs | Sm$^þ$ | Sn |
| | Cf$^ƒ$ | Am$^ƒ$ | | 141–351 a | No fission products have a half-life in the range of 100 a–210 ka ... | No fission products have a half-life in the range of 100 a–210 ka ... | No fission products have a half-life in the range of 100 a–210 ka ... |
| | Am$^ƒ$ | | Cf$^ƒ$ | 430–900 a | No fission products have a half-life in the range of 100 a–210 ka ... | No fission products have a half-life in the range of 100 a–210 ka ... | No fission products have a half-life in the range of 100 a–210 ka ... |
| | | Ra$^№$ | Bk | 1.3–1.6 ka | No fission products have a half-life in the range of 100 a–210 ka ... | No fission products have a half-life in the range of 100 a–210 ka ... | No fission products have a half-life in the range of 100 a–210 ka ... |
| Pu | Th | Cm$^ƒ$ | Am$^ƒ$ | 4.7–7.4 ka | No fission products have a half-life in the range of 100 a–210 ka ... | No fission products have a half-life in the range of 100 a–210 ka ... | No fission products have a half-life in the range of 100 a–210 ka ... |
| | Cm$^ƒ$ | Cm | | 8.3–8.5 ka | No fission products have a half-life in the range of 100 a–210 ka ... | No fission products have a half-life in the range of 100 a–210 ka ... | No fission products have a half-life in the range of 100 a–210 ka ... |
| | | | Pu$^ƒ$ | 24.1 ka | No fission products have a half-life in the range of 100 a–210 ka ... | No fission products have a half-life in the range of 100 a–210 ka ... | No fission products have a half-life in the range of 100 a–210 ka ... |
| | | Th$^№$ | Pa$^№$ | 32–76 ka | No fission products have a half-life in the range of 100 a–210 ka ... | No fission products have a half-life in the range of 100 a–210 ka ... | No fission products have a half-life in the range of 100 a–210 ka ... |
| Np$^ƒ$ | U$^ƒ$ | U$^№$ | | 150–250 ka | Tc$^₡$ | Sn |
| Cm | | Pu | | 327–375 ka | | Se$^₡$ |
| | | | | 1.33 Ma | Cs$^₡$ |
| | Np$^ƒ$ | | | 1.61–6.5 Ma | Zr | Pd |
| U | | | Cm$^ƒ$ | 15–24 Ma | | I$^₡$ |
| Pu | | | | 80 Ma | ... nor beyond 15.7 Ma | ... nor beyond 15.7 Ma | ... nor beyond 15.7 Ma |
| Th$^№$ | | U$^№$ | U$^{ƒ№}$ | 0.7–14.1 Ga | ... nor beyond 15.7 Ma | ... nor beyond 15.7 Ma | ... nor beyond 15.7 Ma |
| ₡, has thermal neutron capture cross section in the range of 8–50 barnsƒ, fissile№, primarily a naturally occurring radioactive material (NORM)þ, neutron poison (thermal neutron capture cross section greater than 3k barns) |||||||||

## Notable isotopes
### Actinium-225

Actinium-225 is a highly radioactive isotope with 136 neutrons. It is an alpha emitter and has a half-life of 9.919 days. As of 2024, it is being researched as a possible alpha source in targeted alpha therapy. Actinium-225 undergoes a series of three alpha decays – via the short-lived francium-221 and astatine-217 – to $^{213}$Bi, which itself is used as an alpha source. Another benefit is that the decay chain of $^{225}$Ac ends in the nuclide $^{209}$Bi, which has a considerably shorter biological half-life than lead. However, a major factor limiting its usage is the difficulty in producing the short-lived isotope, as it is most commonly isolated from aging parent nuclides (such as $^{233}$U); it may also be produced in cyclotrons, linear accelerators, or fast breeder reactors.

### Actinium-226

Actinium-226 is an isotope of actinium with a half-life of 29.37 hours. It mainly (83%) undergos beta decay, sometimes (17%) undergo electron capture, and rarely (0.006%) undergo alpha decay. There are researches on $^{226}$Ac to use it in SPECT.

### Actinium-227

Actinium-227 is the most stable isotope of actinium, with a half-life of 21.772 years. It mainly (98.62%) undergos beta decay, but sometimes (1.38%) it will undergo alpha decay instead. $^{227}$Ac is a member of the actinium series. It is found only in traces in uranium ores – one tonne of uranium in ore contains about 0.2 milligrams of $^{227}$Ac. $^{227}$Ac is prepared, in milligram amounts, by the neutron irradiation of $^{226}$Ra in a nuclear reactor.
    ${\ce {^{226}_{88}Ra + ^{1}_{0}n -> ^{227}_{88}Ra ->[\beta^-][42.2 \ {\ce {min}}] ^{227}_{89}Ac}}$
$^{227}$Ac is highly radioactive and was therefore studied for use as an active element of radioisotope thermoelectric generators, for example in spacecraft. The oxide of $^{227}$Ac pressed with beryllium is also an efficient neutron source with the activity exceeding that of the standard americium-beryllium and radium-beryllium pairs. In all those applications, $^{227}$Ac (a beta source) is merely a progenitor which generates alpha-emitting isotopes upon its decay. Beryllium captures alpha particles and emits neutrons owing to its large cross-section for the (α,n) nuclear reaction:
    ${\ce {^{9}_{4}Be + ^{4}_{2}He -> ^{12}_{6}C + ^{1}_{0}n + \gamma}}$
The $^{227}$AcBe neutron sources can be applied in a neutron probe – a standard device for measuring the quantity of water present in soil, as well as moisture/density for quality control in highway construction. Such probes are also used in well logging applications, in neutron radiography, tomography and other radiochemical investigations.
The medium half-life of $^{227}$Ac makes it a very convenient radioactive isotope in modeling the slow vertical mixing of oceanic waters. The associated processes cannot be studied with the required accuracy by direct measurements of current velocities (of the order 50 meters per year). However, evaluation of the concentration depth-profiles for different isotopes allows estimating the mixing rates. The physics behind this method is as follows: oceanic waters contain homogeneously dispersed $^{235}$U. Its decay product, $^{231}$Pa, gradually precipitates to the bottom, so that its concentration first increases with depth and then stays nearly constant. $^{231}$Pa decays to $^{227}$Ac; however, the concentration of the latter isotope does not follow the $^{231}$Pa depth profile, but instead increases toward the sea bottom. This occurs because of the mixing processes which raise some additional $^{227}$Ac from the sea bottom. Thus analysis of both $^{231}$Pa and $^{227}$Ac depth profiles allows researchers to model the mixing behavior.