Isotopes of actinium

Actinium (₈₉Ac) has no stable isotopes and no characteristic terrestrial isotopic composition, thus a standard atomic weight cannot be given. There are 34 known isotopes, from ²⁰³Ac to ²³⁶Ac, and 9 isomers. Three isotopes are found in nature, ²²⁵Ac, ²²⁷Ac and ²²⁸Ac, as intermediate decay products of, respectively, ²³⁷Np, ²³⁵U, and ²³²Th. ²²⁸Ac and ²²⁵Ac are extremely rare, so almost all natural actinium is ²²⁷Ac.

The most stable isotopes are ²²⁷Ac with a half-life of 21.772 years, ²²⁵Ac with a half-life of 9.919 days, and ²²⁶Ac with a half-life of 29.37 hours. All other isotopes have half-lives under seven hours, and most under a minute. The shortest-lived known isotope is ²¹⁷Ac with a half-life of 69 ns.

Purified ²²⁷Ac comes into equilibrium with its decay products (mainly ²²⁷Th and ²²³Ra) after 185 days.

List of isotopes

|-id=Actinium-203 | ²⁰³Ac | | style="text-align:right" | 89 | style="text-align:right" | 114 | | | α | ¹⁹⁹Fr | (1/2+) | |-id=Actinium-204 |²⁰⁴Ac | | style="text-align:right" | 89 | style="text-align:right" | 115 | | | α | ²⁰⁰Fr | | |-id=Actinium-205 |²⁰⁵Ac | | style="text-align:right" | 89 | style="text-align:right" | 116 | 205.01514(6) | | α | ²⁰¹Fr | 9/2− | |-id=Actinium-206 | ²⁰⁶Ac | | style="text-align:right" | 89 | style="text-align:right" | 117 | 206.01448(7) | 25(7)&nbsp;ms | α | ²⁰²Fr | (3+) | |-id=Actinium-206m | style="text-indent:1em" | ^206mAc | | colspan="3" style="text-indent:2em" | 200(70) keV | 41(16)&nbsp;ms | α | ^202mFr | (10−) | |-id=Actinium-207 | ²⁰⁷Ac | | style="text-align:right" | 89 | style="text-align:right" | 118 | 207.01197(6) | 31(8)&nbsp;ms | α | ²⁰³Fr | 9/2−# | |-id=Actinium-208 | ²⁰⁸Ac | | style="text-align:right" | 89 | style="text-align:right" | 119 | 208.01155(7) | 97(15)&nbsp;ms | α | ²⁰⁴Fr | (3+) | |-id=Actinium-208m | rowspan=2 style="text-indent:1em" | ^208mAc | rowspan=2| | rowspan=2 colspan="3" style="text-indent:2em" | 500(60) keV | rowspan=2|28(7)&nbsp;ms | α | ²⁰⁴Fr | rowspan=2|(10−) | rowspan=2| |- | IT ? | ²⁰⁸Ac |-id=Actinium-209 | ²⁰⁹Ac | | style="text-align:right" | 89 | style="text-align:right" | 120 | 209.00950(6) | 94(10)&nbsp;ms | α | ²⁰⁵Fr | (9/2−) | |-id=Actinium-210 | ²¹⁰Ac | | style="text-align:right" | 89 | style="text-align:right" | 121 | 210.00941(7) | 350(40)&nbsp;ms | α | ²⁰⁶Fr | 7+# | |-id=Actinium-211 | ²¹¹Ac | | style="text-align:right" | 89 | style="text-align:right" | 122 | 211.00767(6) | 213(25)&nbsp;ms | α | ²⁰⁷Fr | 9/2− | |-id=Actinium-212 | ²¹²Ac | | style="text-align:right" | 89 | style="text-align:right" | 123 | 212.007836(23) | 895(28)&nbsp;ms | α | ²⁰⁸Fr | 7+ | |-id=Actinium-213 |²¹³Ac | | style="text-align:right" | 89 | style="text-align:right" | 124 | 213.006593(13) | 738(16)&nbsp;ms | α | ²⁰⁹Fr | 9/2− | |-id=Actinium-214 | rowspan=2|²¹⁴Ac | rowspan=2| | rowspan=2 style="text-align:right" | 89 | rowspan=2 style="text-align:right" | 125 | rowspan=2|214.006906(15) | rowspan=2|8.2(2)&nbsp;s | α (93%) | ²¹⁰Fr | rowspan=2|5+ | rowspan=2| |- | β⁺ (7%) | ²¹⁴Ra |-id=Actinium-215 | rowspan=2|²¹⁵Ac | rowspan=2| | rowspan=2 style="text-align:right" | 89 | rowspan=2 style="text-align:right" | 126 | rowspan=2|215.006474(13) | rowspan=2|171(10)&nbsp;ms | α (99.91%) | ²¹¹Fr | rowspan=2|9/2− | rowspan=2| |- | β⁺ (0.09%) | ²¹⁵Ra |-id=Actinium-215m1 | style="text-indent:1em" | ^215m1Ac | | colspan="3" style="text-indent:2em" | 1796.0(9) keV | 185(30)&nbsp;ns | IT | ²¹⁵Ac | (21/2−) | |-id=Actinium-215m2 | style="text-indent:1em" | ^215m2Ac | | colspan="3" style="text-indent:2em" | 2488(50)# keV | 335(10)&nbsp;ns | IT | ²¹⁵Ac | (29/2+) | |-id=Actinium-216 | ²¹⁶Ac | | style="text-align:right" | 89 | style="text-align:right" | 127 | 216.008749(10) | 440(16)&nbsp;μs | α | ²¹²Fr | (1−) | |-id=Actinium-216m1 | style="text-indent:1em" | ^216m1Ac | | colspan="3" style="text-indent:2em" | 38(5) keV | 441(7)&nbsp;μs | α | ²¹²Fr | (9−) | |-id=Actinium-216m2 | style="text-indent:1em" | ^216m2Ac | | colspan="3" style="text-indent:2em" | 422(100)# keV | ~300&nbsp;ns | IT | ²¹⁶Ac | | |-id=Actinium-217 | ²¹⁷Ac | | style="text-align:right" | 89 | style="text-align:right" | 128 | 217.009342(12) | 69(4)&nbsp;ns | α | ²¹³Fr | 9/2− | |-id=Actinium-217m | style="text-indent:1em" | ^217mAc | | colspan="3" style="text-indent:2em" | 2012(20) keV | 740(40)&nbsp;ns | | | 29/2+ | |-id=Actinium-218 | ²¹⁸Ac | | style="text-align:right" | 89 | style="text-align:right" | 129 | 218.01165(6) | 1.00(4)&nbsp;μs | α | ²¹⁴Fr | (1−) | |-id=Actinium-218m | style="text-indent:1em" | ^218mAc | | colspan="3" style="text-indent:2em" | 607(86)# keV | 103(11)&nbsp;ns | IT | ²¹⁸Ac | (11+) | |-id=Actinium-219 | ²¹⁹Ac | | style="text-align:right" | 89 | style="text-align:right" | 130 | 219.01242(6) | 9.4(10)&nbsp;μs | α | ²¹⁵Fr | 9/2− | |-id=Actinium-220 | ²²⁰Ac | | style="text-align:right" | 89 | style="text-align:right" | 131 | 220.014755(7) | 26.36(19)&nbsp;ms | α | ²¹⁶Fr | (3−) | |-id=Actinium-221 | ²²¹Ac | | style="text-align:right" | 89 | style="text-align:right" | 132 | 221.01560(6) | 52(2)&nbsp;ms | α | ²¹⁷Fr | 9/2−# | |-id=Actinium-222 | rowspan=2|²²²Ac | rowspan=2| | rowspan=2 style="text-align:right" | 89 | rowspan=2 style="text-align:right" | 133 | rowspan=2|222.017844(5) | rowspan=2|5.0(5)&nbsp;s | α | ²¹⁸Fr | rowspan=2|1− | rowspan=2| |- | β⁺ (<2%) | ²²²Ra |-id=Actinium-222m | rowspan=3 style="text-indent:1em" | ^222mAc | rowspan=3| | rowspan=3 colspan="3" style="text-indent:2em" | 78(21) keV | rowspan=3|1.05(5)&nbsp;min | α (98.6%) | ²¹⁸Fr | rowspan=3|5+# | rowspan=3| |- | β⁺ (1.4%) | ²²²Ra |- | IT? | ²²²Ac |-id=Actinium-223 | rowspan=3|²²³Ac | rowspan=3| | rowspan=3 style="text-align:right" | 89 | rowspan=3 style="text-align:right" | 134 | rowspan=3|223.019136(7) | rowspan=3|2.10(5)&nbsp;min | α | ²¹⁹Fr | rowspan=3|(5/2−) | rowspan=3| |- | EC ? | ²²³Ra |- | CD (3.2×10^{−9}%) | ²⁰⁹Bi<br/>¹⁴C |-id=Actinium-224 | rowspan=3|²²⁴Ac | rowspan=3| | rowspan=3 style="text-align:right" | 89 | rowspan=3 style="text-align:right" | 135 | rowspan=3|224.021722(4) | rowspan=3|2.78(16)&nbsp;h | β⁺ (90.5%) | ²²⁴Ra | rowspan=3|(0−) | rowspan=3| |- | α (9.5%) | ²²⁰Fr |- | β^− ? | ²²⁴Th |- | rowspan=2|²²⁵Ac | rowspan=2| | rowspan=2 style="text-align:right" | 89 | rowspan=2 style="text-align:right" | 136 | rowspan=2|225.023229(5) | rowspan=2|9.9190(21)&nbsp;d | α | ²²¹Fr | rowspan=2|3/2− | rowspan=2|Trace |- | CD (5.3×10^{−10}%) | ²¹¹Bi<br/>¹⁴C |- | rowspan=3|²²⁶Ac | rowspan=3| | rowspan=3 style="text-align:right" | 89 | rowspan=3 style="text-align:right" | 137 | rowspan=3|226.026097(3) | rowspan=3|29.37(12)&nbsp;h | β^− (83%) | ²²⁶Th | rowspan=3|(1−) | rowspan=3| |- | EC (17%) | ²²⁶Ra |- | α (0.006%) | ²²²Fr |- | rowspan=2|²²⁷Ac | rowspan=2|Actinium | rowspan=2 style="text-align:right" | 89 | rowspan=2 style="text-align:right" | 138 | rowspan=2|227.0277506(21) | rowspan=2|21.772(3)&nbsp;y | β^− (98.62%) | ²²⁷Th | rowspan=2|3/2− | rowspan=2|Trace |- | α (1.38%) | ²²³Fr |-id=Actinium-228 | ²²⁸Ac | Mesothorium 2 | style="text-align:right" | 89 | style="text-align:right" | 139 | 228.0310197(22) | 6.15(2)&nbsp;h | β^− | ²²⁸Th | 3+ | Trace |-id=Actinium-229 | ²²⁹Ac | | style="text-align:right" | 89 | style="text-align:right" | 140 | 229.032947(13) | 62.7(5)&nbsp;min | β^− | ²²⁹Th | 3/2+ | |-id=Actinium-230 | ²³⁰Ac | | style="text-align:right" | 89 | style="text-align:right" | 141 | 230.036327(17) | 122(3)&nbsp;s | β^− | ²³⁰Th | (1+) | |-id=Actinium-231 | ²³¹Ac | | style="text-align:right" | 89 | style="text-align:right" | 142 | 231.038393(14) | 7.5(1)&nbsp;min | β^− | ²³¹Th | 1/2+ | |-id=Actinium-232 | ²³²Ac | | style="text-align:right" | 89 | style="text-align:right" | 143 | 232.042034(14) | 1.98(8)&nbsp;min | β^− | ²³²Th | (1+) | |-id=Actinium-233 | ²³³Ac | | style="text-align:right" | 89 | style="text-align:right" | 144 | 233.044346(14) | 143(10)&nbsp;s | β^− | ²³³Th | (1/2+) | |-id=Actinium-234 | ²³⁴Ac | | style="text-align:right" | 89 | style="text-align:right" | 145 | 234.048139(15) | 45(2)&nbsp;s | β^− | ²³⁴Th | | |-id=Actinium-235 | ²³⁵Ac | | style="text-align:right" | 89 | style="text-align:right" | 146 | 235.050840(15) | 62(4)&nbsp;s | β^− | ²³⁵Th | 1/2+# | |-id=Actinium-236 | ²³⁶Ac | | style="text-align:right" | 89 | style="text-align:right" | 147 | 236.05499(4) | | β^− | ²³⁶Th | |

Actinides vs fission products

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 ²¹³Bi, which itself is used as an alpha source. Another benefit is that the decay chain of ²²⁵Ac ends in the nuclide ²⁰⁹Bi, 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 ²³³U); 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%) undergoes beta decay, sometimes (17%) undergo electron capture, and rarely (0.006%) undergo alpha decay. There are researches on ²²⁶Ac 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%) undergoes beta decay, but sometimes (1.38%) it will undergo alpha decay instead. ²²⁷Ac is a member of the actinium series. It is found only in traces in uranium ores&nbsp;– one tonne of uranium in ore contains about 0.2 milligrams of ²²⁷Ac. ²²⁷Ac is prepared, in milligram amounts, by the neutron irradiation of in a nuclear reactor.

²²⁷Ac is highly radioactive and was therefore studied for use as an active element of radioisotope thermoelectric generators, for example in spacecraft. The oxide of ²²⁷Ac pressed with beryllium is also an efficient neutron source with the activity exceeding that of the standard americium-beryllium and radium-beryllium pairs. In these applications, ²²⁷Ac (a weak beta source emitting few alphas of its own) is essentially 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:

The ²²⁷AcBe neutron sources can be applied in a neutron probe&nbsp;– 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 ²²⁷Ac 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 ²³⁵U. Its decay product, ²³¹Pa, gradually precipitates to the bottom, so that its concentration first increases with depth and then stays nearly constant. ²³¹Pa decays to ²²⁷Ac; however, the concentration of the latter isotope does not follow the ²³¹Pa depth profile, but instead increases toward the sea bottom. This occurs because of the mixing processes which raise some additional ²²⁷Ac from the sea bottom. Thus analysis of both ²³¹Pa and ²²⁷Ac depth profiles allows researchers to model the mixing behavior.

See also

• Actinium series
• Actinide

Notes

References