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.

Isotopes of actinium (₈₉Ac)

Main isotopes Decay Isotope abun­dance half-life (t1/2) mode pro­duct ^225Ac trace 9.919 d α ^221Fr ^226Ac synth 29.37 h β^− ^226Th ε ^226Ra α ^222Fr ^227Ac trace 21.772 y β^− ^227Th α ^223Fr ^228Ac trace 6.15 h β^− ^228Th

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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

Nuclide	Historic name	Z	N	Isotopic mass (Da)	Half-life	Decay mode	Daughter isotope	Spin and parity	Isotopic abundance
		Excitation energy
^203Ac		89	114		56+269 −26 μs	α	^199Fr	(1/2+)
^204Ac		89	115		7.4+2.2 −1.4 ms	α	^200Fr
^205Ac		89	116	205.01514(6)	7.7+2.7 −1.6 ms	α	^201Fr	9/2−
^206Ac		89	117	206.01448(7)	25(7) ms	α	^202Fr	(3+)
^206mAc		200(70) keV			41(16) ms	α	^202mFr	(10−)
^207Ac		89	118	207.01197(6)	31(8) ms	α	^203Fr	9/2−#
^208Ac		89	119	208.01155(7)	97(15) ms	α	^204Fr	(3+)
^208mAc		500(60) keV			28(7) ms	α	^204Fr	(10−)
						IT ?	^208Ac
^209Ac		89	120	209.00950(6)	94(10) ms	α	^205Fr	(9/2−)
^210Ac		89	121	210.00941(7)	350(40) ms	α	^206Fr	7+#
^211Ac		89	122	211.00767(6)	213(25) ms	α	^207Fr	9/2−
^212Ac		89	123	212.007836(23)	895(28) ms	α	^208Fr	7+
^213Ac		89	124	213.006593(13)	738(16) ms	α	^209Fr	9/2−
^214Ac		89	125	214.006906(15)	8.2(2) s	α (93%)	^210Fr	5+
						β^+ (7%)	^214Ra
^215Ac		89	126	215.006474(13)	171(10) ms	α (99.91%)	^211Fr	9/2−
						β^+ (0.09%)	^215Ra
^215m1Ac		1796.0(9) keV			185(30) ns	IT	^215Ac	(21/2−)
^215m2Ac		2488(50)# keV			335(10) ns	IT	^215Ac	(29/2+)
^216Ac		89	127	216.008749(10)	440(16) μs	α	^212Fr	(1−)
^216m1Ac		38(5) keV			441(7) μs	α	^212Fr	(9−)
^216m2Ac		422(100)# keV			~300 ns	IT	^216Ac
^217Ac		89	128	217.009342(12)	69(4) ns	α	^213Fr	9/2−
^217mAc		2012(20) keV			740(40) ns			29/2+
^218Ac		89	129	218.01165(6)	1.00(4) μs	α	^214Fr	(1−)
^218mAc		607(86)# keV			103(11) ns	IT	^218Ac	(11+)
^219Ac		89	130	219.01242(6)	9.4(10) μs	α	^215Fr	9/2−
^220Ac		89	131	220.014755(7)	26.36(19) ms	α	^216Fr	(3−)
^221Ac		89	132	221.01560(6)	52(2) ms	α	^217Fr	9/2−#
^222Ac		89	133	222.017844(5)	5.0(5) s	α	^218Fr	1−
						β^+ (<2%)	^222Ra
^222mAc		78(21) keV			1.05(5) min	α (98.6%)	^218Fr	5+#
						β^+ (1.4%)	^222Ra
						IT?	^222Ac
^223Ac		89	134	223.019136(7)	2.10(5) min	α	^219Fr	(5/2−)
						EC ?	^223Ra
						CD (3.2×10^−9%)	^209Bi ^14C
^224Ac		89	135	224.021722(4)	2.78(16) h	β^+ (90.5%)	^224Ra	(0−)
						α (9.5%)	^220Fr
						β^− ?	^224Th
^225Ac		89	136	225.023229(5)	9.9190(21) d	α	^221Fr	3/2−	Trace
						CD (5.3×10^−10%)	^211Bi ^14C
^226Ac		89	137	226.026097(3)	29.37(12) h	β^− (83%)	^226Th	(1−)
						EC (17%)	^226Ra
						α (0.006%)	^222Fr
^227Ac	Actinium	89	138	227.0277506(21)	21.772(3) y	β^− (98.62%)	^227Th	3/2−	Trace
						α (1.38%)	^223Fr
^228Ac	Mesothorium 2	89	139	228.0310197(22)	6.15(2) h	β^−	^228Th	3+	Trace
^229Ac		89	140	229.032947(13)	62.7(5) min	β^−	^229Th	3/2+
^230Ac		89	141	230.036327(17)	122(3) s	β^−	^230Th	(1+)
^231Ac		89	142	231.038393(14)	7.5(1) min	β^−	^231Th	1/2+
^232Ac		89	143	232.042034(14)	1.98(8) min	β^−	^232Th	(1+)
^233Ac		89	144	233.044346(14)	143(10) s	β^−	^233Th	(1/2+)
^234Ac		89	145	234.048139(15)	45(2) s	β^−	^234Th
^235Ac		89	146	235.050840(15)	62(4) s	β^−	^235Th	1/2+#
^236Ac		89	147	236.05499(4)	72+345 −33 s	β^−	^236Th
This table header & footer: view

1. ^mAc – Excited nuclear isomer.
2. ( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
3. # – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).
4. Modes of decay:

   EC:	Electron capture
   CD:	Cluster decay
   IT:	Isomeric transition

5. Bold italics symbol as daughter – Daughter product is nearly stable.
6. ( ) spin value – Indicates spin with weak assignment arguments.
7. # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
8. Has medical uses
9. Intermediate decay product of ²³⁷Np
10. Source of element's name
11. Intermediate decay product of ²³⁵U
12. Intermediate decay product of ²³²Th

Actinides vs fission products

Actinides and fission products by half-life v t e
Actinides by decay chain				Half-life range (a)	Fission products of ^235U by yield
4n (Thorium)	4n + 1 (Neptunium)	4n + 2 (Radium)	4n + 3 (Actinium)		4.5–7%	0.04–1.25%	<0.001%
^228Ra^№				4–6 a		^155Eu^þ
^248Bk				> 9 a
^244Cm^ƒ	^241Pu^ƒ	^250Cf	^227Ac^№	10–29 a	^90Sr	^85Kr	^113mCd^þ
^232U^ƒ		^238Pu^ƒ	^243Cm^ƒ	29–97 a	^137Cs	^151Sm^þ	^121mSn
	^249Cf^ƒ	^242mAm^ƒ		141–351 a	No fission products have a half-life in the range of 100 a–210 ka ...
	^241Am^ƒ		^251Cf^ƒ	430–900 a
		^226Ra^№	^247Bk	1.3–1.6 ka
^240Pu	^229Th	^246Cm^ƒ	^243Am^ƒ	4.7–7.4 ka
	^245Cm^ƒ	^250Cm		8.3–8.5 ka
			^239Pu^ƒ	24.1 ka
		^230Th^№	^231Pa^№	32–76 ka
^236Np^ƒ	^233U^ƒ	^234U^№		150–250 ka	^99Tc^₡	^126Sn
^248Cm		^242Pu		327–375 ka		^79Se^₡
				1.33 Ma	^135Cs^₡
	^237Np^ƒ			1.61–6.5 Ma	^93Zr	^107Pd
^236U			^247Cm^ƒ	15–24 Ma		^129I^₡
^244Pu				80 Ma	... nor beyond 15.7 Ma
^232Th^№		^238U^№	^235U^ƒ№	0.7–14.1 Ga
₡,  has thermal neutron capture cross section in the range of 8–50 barns ƒ,  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 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%) undergos 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 – one tonne of uranium in ore contains about 0.2 milligrams of ²²⁷Ac. ²²⁷Ac is prepared, in milligram amounts, by the neutron irradiation of ²²⁶Ra in a nuclear reactor.

${\displaystyle {\ce {^{226}_{88}Ra + ^{1}_{0}n -> ^{227}_{88}Ra ->[\beta^-][42.2 \ {\ce {min}}] ^{227}_{89}Ac}}}$

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:

${\displaystyle {\ce {^{9}_{4}Be + ^{4}_{2}He -> ^{12}_{6}C + ^{1}_{0}n + \gamma}}}$

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.

See also

• Actinium series
• Actinide

Notes

1. Bismuth-209 decays into thallium-205 with a half-life exceeding 10¹⁹ years, but this half-life is so long that for practical purposes bismuth-209 can be considered stable.

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