Actinium Actinium is a chemical element; it has chemical symbol, symbol Ac and atomic number 89. It was discovered by Friedrich Oskar Giesel in 1902, who gave it the name ''emanium''; the element got its name by being wrongly identified with a substa ...

(₈₉Ac) has no stable

isotope Isotopes are distinct nuclear species (or ''nuclides'') of the same chemical element. They have the same atomic number (number of protons in their Atomic nucleus, nuclei) and position in the periodic table (and hence belong to the same chemica ...

s and no characteristic terrestrial isotopic composition, thus a

standard atomic weight The standard atomic weight of a chemical element (symbol ''A''r°(E) for element "E") is the weighted arithmetic mean of the relative isotopic masses of all isotopes of that element weighted by each isotope's abundance on Earth. For example, ...

cannot be given. There are 34 known isotopes, from ²⁰³Ac to ²³⁶Ac, and 7

isomers In chemistry, isomers are molecules or polyatomic ions with identical molecular formula – that is, the same number of atoms of each element – but distinct arrangements of atoms in space. ''Isomerism'' refers to the existence or possibili ...

. 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 10.0 days, and ²²⁶Ac 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 ²¹⁷Ac with a half-life of 69 ns. Purified ²²⁷Ac comes into equilibrium with its decay products (²²⁷Th and ²²³Fr) 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 , , , α , ²⁰¹Fr , 9/2−? , , -id=Actinium-206 , ²⁰⁶Ac , , style="text-align:right" , 89 , style="text-align:right" , 117 , 206.01450(8) , 25(7) ms , α , ²⁰²Fr , (3+) , , -id=Actinium-206m1 , style="text-indent:1em" , ^206m1Ac , , colspan="3" style="text-indent:2em" , 80(50) keV , 15(6) ms , α , ²⁰²Fr , , , -id=Actinium-206m2 , style="text-indent:1em" , ^206m2Ac , , colspan="3" style="text-indent:2em" , 290(110)# keV , 41(16) ms , α , ^202mFr , (10−) , , -id=Actinium-207 , ²⁰⁷Ac , , style="text-align:right" , 89 , style="text-align:right" , 118 , 207.01195(6) , 31(8) ms
7(+11−6) ms, α , ²⁰³Fr , 9/2−# , , -id=Actinium-208 , rowspan=2, ²⁰⁸Ac , rowspan=2, , rowspan=2 style="text-align:right" , 89 , rowspan=2 style="text-align:right" , 119 , rowspan=2, 208.01155(6) , rowspan=2, 97(16) ms
5(+24−16) ms, α (99%) , ²⁰⁴Fr , rowspan=2, (3+) , rowspan=2, , - , β⁺ (1%) , ²⁰⁸Ra , -id=Actinium-208m , rowspan=3 style="text-indent:1em" , ^208mAc , rowspan=3, , rowspan=3 colspan="3" style="text-indent:2em" , 506(26) keV , rowspan=3, 28(7) ms
5(+9−5) ms, α (89%) , ²⁰⁴Fr , rowspan=3, (10−) , rowspan=3, , - , IT (10%) , ²⁰⁸Ac , - , β⁺ (1%) , ²⁰⁸Ra , -id=Actinium-209 , rowspan=2, ²⁰⁹Ac , rowspan=2, , rowspan=2 style="text-align:right" , 89 , rowspan=2 style="text-align:right" , 120 , rowspan=2, 209.00949(5) , rowspan=2, 92(11) ms , α (99%) , ²⁰⁵Fr , rowspan=2, (9/2−) , rowspan=2, , - , β⁺ (1%) , ²⁰⁹Ra , -id=Actinium-210 , rowspan=2, ²¹⁰Ac , rowspan=2, , rowspan=2 style="text-align:right" , 89 , rowspan=2 style="text-align:right" , 121 , rowspan=2, 210.00944(6) , rowspan=2, 350(40) ms , α (96%) , ²⁰⁶Fr , rowspan=2, 7+# , rowspan=2, , - , β⁺ (4%) , ²¹⁰Ra , -id=Actinium-211 , rowspan=2, ²¹¹Ac , rowspan=2, , rowspan=2 style="text-align:right" , 89 , rowspan=2 style="text-align:right" , 122 , rowspan=2, 211.00773(8) , rowspan=2, 213(25) ms , α (99.8%) , ²⁰⁷Fr , rowspan=2, 9/2−# , rowspan=2, , - , β⁺ (.2%) , ²¹¹Ra , -id=Actinium-212 , rowspan=2, ²¹²Ac , rowspan=2, , rowspan=2 style="text-align:right" , 89 , rowspan=2 style="text-align:right" , 123 , rowspan=2, 212.00781(7) , rowspan=2, 920(50) ms , α (97%) , ²⁰⁸Fr , rowspan=2, 6+# , rowspan=2, , - , β⁺ (3%) , ²¹²Ra , -id=Actinium-213 , rowspan=2, ²¹³Ac , rowspan=2, , rowspan=2 style="text-align:right" , 89 , rowspan=2 style="text-align:right" , 124 , rowspan=2, 213.00661(6) , rowspan=2, 731(17) ms , α , ²⁰⁹Fr , rowspan=2, (9/2−)# , rowspan=2, , - , β⁺ (rare) , ²¹³Ra , -id=Actinium-214 , rowspan=2, ²¹⁴Ac , rowspan=2, , rowspan=2 style="text-align:right" , 89 , rowspan=2 style="text-align:right" , 125 , rowspan=2, 214.006902(24) , rowspan=2, 8.2(2) s , α (89%) , ²¹⁰Fr , rowspan=2, (5+)# , rowspan=2, , - , β⁺ (11%) , 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, 215.006454(23) , rowspan=2, 0.17(1) s , α (99.91%) , ²¹¹Fr , rowspan=2, 9/2− , rowspan=2, , - , β⁺ (.09%) , ²¹⁵Ra , -id=Actinium-216 , ²¹⁶Ac , , style="text-align:right" , 89 , style="text-align:right" , 127 , 216.008720(29) , 440(16) μs , α , ²¹²Fr , (1−) , , -id=Actinium-216m1 , style="text-indent:1em" , ^216m1Ac , , colspan="3" style="text-indent:2em" , 38(5) keV , 441(7) μs , α , ²¹²Fr , (9−) , , -id=Actinium-216m2 , style="text-indent:1em" , ^216m2Ac , , colspan="3" style="text-indent:2em" , 422#(100#) keV , ~300 ns , IT , ²¹⁶Ac , , , -id=Actinium-217 , ²¹⁷Ac , , style="text-align:right" , 89 , style="text-align:right" , 128 , 217.009347(14) , 69(4) ns , α , ²¹³Fr , 9/2− , , -id=Actinium-217m , style="text-indent:1em" , ^217mAc , , colspan="3" style="text-indent:2em" , 2012(20) keV , 740(40) ns , , , (29/2)+ , , -id=Actinium-218 , ²¹⁸Ac , , style="text-align:right" , 89 , style="text-align:right" , 129 , 218.01164(5) , 1.08(9) μs , α , ²¹⁴Fr , (1−)# , , -id=Actinium-218m , style="text-indent:1em" , ^218mAc , , colspan="3" style="text-indent:2em" , 607(86)# keV , 103(11) ns , IT , ²¹⁸Ac , (11+) , , -id=Actinium-219 , ²¹⁹Ac , , style="text-align:right" , 89 , style="text-align:right" , 130 , 219.01242(5) , 11.8(15) μs , α , ²¹⁵Fr , 9/2− , , -id=Actinium-220 , ²²⁰Ac , , style="text-align:right" , 89 , style="text-align:right" , 131 , 220.014763(16) , 26.36(19) ms , α , ²¹⁶Fr , (3−) , , -id=Actinium-221 , ²²¹Ac , , style="text-align:right" , 89 , style="text-align:right" , 132 , 221.01559(5) , 52(2) 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(6) , rowspan=2, 5.0(5) s , α (99(1)%) , ²¹⁸Fr , rowspan=2, 1− , rowspan=2, , - , β⁺ (1(1)%) , 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) 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, 223.019137(8) , rowspan=3, 2.10(5) min , α (99%) , ²¹⁹Fr , rowspan=3, (5/2−) , rowspan=3, , - , EC (1%) , ²²³Ra , - , CD (3.2×10⁻⁹%) , ''²⁰⁹Bi''
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, 224.021723(4) , rowspan=3, 2.78(17) h , β⁺ (90.9%) , ²²⁴Ra , rowspan=3, 0− , rowspan=3, , - , α (9.1%) , ²²⁰Fr , - , β⁻ (<1.6%) , ²²⁴Th , - , rowspan=2, ²²⁵AcHas medical uses , rowspan=2, , rowspan=2 style="text-align:right" , 89 , rowspan=2 style="text-align:right" , 136 , rowspan=2, 225.023230(5) , rowspan=2, 10.0(1) d , α , ²²¹Fr , rowspan=2, (3/2−) , rowspan=2, TraceIntermediate decay product of ²³⁷Np , - , CD (6×10⁻¹⁰%) , ²¹¹Bi
¹⁴C , - , rowspan=3, ²²⁶Ac , rowspan=3, , rowspan=3 style="text-align:right" , 89 , rowspan=3 style="text-align:right" , 137 , rowspan=3, 226.026098(4) , rowspan=3, 29.37(12) h , β⁻ (83%) , ²²⁶Th , rowspan=3, (1)(−#) , rowspan=3, , - , EC (17%) , ²²⁶Ra , - , α (.006%) , ²²²Fr , - , rowspan=2, ²²⁷Ac , rowspan=2, ActiniumSource of element's name , rowspan=2 style="text-align:right" , 89 , rowspan=2 style="text-align:right" , 138 , rowspan=2, 227.0277521(26) , rowspan=2, 21.772(3) y , β⁻ (98.62%) , ²²⁷Th , rowspan=2, 3/2− , rowspan=2, TraceIntermediate

decay product In nuclear physics, a decay product (also known as a daughter product, daughter isotope, radio-daughter, or daughter nuclide) is the remaining nuclide left over from radioactive decay. Radioactive decay often proceeds via a sequence of steps ( d ...

of ²³⁵U , - , α (1.38%) , ²²³Fr , -id=Actinium-228 , ²²⁸Ac , Mesothorium 2 , style="text-align:right" , 89 , style="text-align:right" , 139 , 228.0310211(27) , 6.13(2) h , β⁻ , ²²⁸Th , 3+ , TraceIntermediate decay product of ²³²Th , -id=Actinium-229 , ²²⁹Ac , , style="text-align:right" , 89 , style="text-align:right" , 140 , 229.03302(4) , 62.7(5) min , β⁻ , ²²⁹Th , (3/2+) , , -id=Actinium-230 , ²³⁰Ac , , style="text-align:right" , 89 , style="text-align:right" , 141 , 230.03629(32) , 122(3) s , β⁻ , ²³⁰Th , (1+) , , -id=Actinium-231 , ²³¹Ac , , style="text-align:right" , 89 , style="text-align:right" , 142 , 231.03856(11) , 7.5(1) min , β⁻ , ²³¹Th , (1/2+) , , -id=Actinium-232 , ²³²Ac , , style="text-align:right" , 89 , style="text-align:right" , 143 , 232.04203(11) , 119(5) s , β⁻ , ''²³²Th'' , (1+) , , -id=Actinium-233 , ²³³Ac , , style="text-align:right" , 89 , style="text-align:right" , 144 , 233.04455(32)# , 145(10) s , β⁻ , ²³³Th , (1/2+) , , -id=Actinium-234 , ²³⁴Ac , , style="text-align:right" , 89 , style="text-align:right" , 145 , 234.04842(43)# , 44(7) s , β⁻ , ²³⁴Th , , , -id=Actinium-235 , ²³⁵Ac , , style="text-align:right" , 89 , style="text-align:right" , 146 , 235.05123(38)# , 60(4) s , β⁻ , ²³⁵Th , 1/2+# , , -id=Actinium-236 , ²³⁶Ac , , style="text-align:right" , 89 , style="text-align:right" , 147 , 236.05530(54)# , , 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 A breeder reactor is a nuclear reactor that generates more fissile material than it consumes. These reactors can be fueled with more-commonly available isotopes of uranium and thorium, such as uranium-238 and thorium-232, as opposed to the ...

Actinium-226

Actinium-226 is an isotope of actinium with a half-life of 29.37 hours. It mainly (83%) undergos

beta decay In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which an atomic nucleus emits a beta particle (fast energetic electron or positron), transforming into an isobar of that nuclide. For example, beta decay of a neutron ...

, sometimes (17%) undergo

electron capture Electron capture (K-electron capture, also K-capture, or L-electron capture, L-capture) is a process in which the proton-rich nucleus of an electrically neutral atom absorbs an inner atomic electron, usually from the K or L electron shells. Th ...

, and rarely (0.006%) undergo

alpha decay Alpha decay or α-decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle (helium nucleus). The parent nucleus transforms or "decays" into a daughter product, with a mass number that is reduced by four and an a ...

. There are researches on ²²⁶Ac to use it in

SPECT Single-photon emission computed tomography (SPECT, or less commonly, SPET) is a nuclear medicine tomographic imaging technique using gamma rays. It is very similar to conventional nuclear medicine planar imaging using a gamma camera (that is, ...

Actinium-227

Actinium-227 is the most stable isotope of actinium, with a half-life of 21.772 years. It mainly (98.62%) undergos

, but sometimes (1.38%) it will undergo

instead. ²²⁷Ac is a member of the

actinium series In nuclear science a decay chain refers to the predictable series of radioactive disintegrations undergone by the nuclei of certain unstable chemical elements. Radioactive isotopes do not usually decay directly to stable isotopes, but rather ...

. It is found only in traces in

uranium Uranium is a chemical element; it has chemical symbol, symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Ura ...

ores – 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 A nuclear reactor is a device used to initiate and control a Nuclear fission, fission nuclear chain reaction. They are used for Nuclear power, commercial electricity, nuclear marine propulsion, marine propulsion, Weapons-grade plutonium, weapons ...

. :^_Ra + ^_n -> ^_Ra -> beta^-42.2 \ \ce] ^_Ac ²²⁷Ac is highly radioactive and was therefore studied for use as an active element of

radioisotope thermoelectric generator A radioisotope thermoelectric generator (RTG, RITEG), or radioisotope power system (RPS), is a type of nuclear battery that uses an array of thermocouples to convert the Decay heat, heat released by the decay of a suitable radioactive material i ...

s, for example in spacecraft. The oxide of ²²⁷Ac pressed with

beryllium Beryllium is a chemical element; it has Symbol (chemistry), symbol Be and atomic number 4. It is a steel-gray, hard, strong, lightweight and brittle alkaline earth metal. It is a divalent element that occurs naturally only in combination with ...

is also an efficient

neutron source A neutron source is any device that emits neutrons, irrespective of the mechanism used to produce the neutrons. Neutron sources are used in physics, engineering, medicine, nuclear weapons, petroleum exploration, biology, chemistry, and nuclear p ...

with the activity exceeding that of the standard americium-beryllium and radium-beryllium pairs.Russell, Alan M. and Lee, Kok Loong (2005
''Structure-property relations in nonferrous metals''
Wiley. , pp. 470–471 In all those applications, ²²⁷Ac (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: : ^_Be + ^_He -> ^_C + ^_n + \gamma The ²²⁷AcBe 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 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.

Notes

References

* Isotope masses from: ** * Half-life, spin, and isomer data selected from the following sources. ** ** ** {{Authority control Actinium