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 7 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[1] Decay abun­dance half-life (t1/2) mode pro­duct 225Ac trace 9.919 d α 221Fr CD 211Bi 226Ac synth 29.37 h β− 226Th ε 226Ra α 222Fr 227Ac trace 21.772 y β− 227Th α 223Fr

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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.^[2]

List of isotopes

Nuclide [n 1]	Historic name	Z	N	Isotopic mass (Da) [n 2][n 3]	Half-life	Decay mode [n 4]	Daughter isotope [n 5]	Spin and parity [n 6][n 7]	Isotopic abundance
		Excitation energy[n 7]
203Ac[3]		89	114		56+269 −26 μs	α	199Fr	(1/2+)
204Ac[4]		89	115		7.4+2.2 −1.4 ms	α	200Fr
205Ac[5]		89	116		7.7+2.7 −1.6 ms[4]	α	201Fr	9/2−?
206Ac		89	117	206.01450(8)	25(7) ms	α	202Fr	(3+)
206m1Ac		80(50) keV			15(6) ms	α	202Fr
206m2Ac		290(110)# keV			41(16) ms	α	202mFr	(10−)
207Ac		89	118	207.01195(6)	31(8) ms [27(+11−6) ms]	α	203Fr	9/2−#
208Ac		89	119	208.01155(6)	97(16) ms [95(+24−16) ms]	α (99%)	204Fr	(3+)
						β+ (1%)	208Ra
208mAc		506(26) keV			28(7) ms [25(+9−5) ms]	α (89%)	204Fr	(10−)
						IT (10%)	208Ac
						β+ (1%)	208Ra
209Ac		89	120	209.00949(5)	92(11) ms	α (99%)	205Fr	(9/2−)
						β+ (1%)	209Ra
210Ac		89	121	210.00944(6)	350(40) ms	α (96%)	206Fr	7+#
						β+ (4%)	210Ra
211Ac		89	122	211.00773(8)	213(25) ms	α (99.8%)	207Fr	9/2−#
						β+ (.2%)	211Ra
212Ac		89	123	212.00781(7)	920(50) ms	α (97%)	208Fr	6+#
						β+ (3%)	212Ra
213Ac		89	124	213.00661(6)	731(17) ms	α	209Fr	(9/2−)#
						β+ (rare)	213Ra
214Ac		89	125	214.006902(24)	8.2(2) s	α (89%)	210Fr	(5+)#
						β+ (11%)	214Ra
215Ac		89	126	215.006454(23)	0.17(1) s	α (99.91%)	211Fr	9/2−
						β+ (.09%)	215Ra
216Ac		89	127	216.008720(29)	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	217.009347(14)	69(4) ns	α	213Fr	9/2−
217mAc		2012(20) keV			740(40) ns			(29/2)+
218Ac		89	129	218.01164(5)	1.08(9) μs	α	214Fr	(1−)#
218mAc		607(86)# keV			103(11) ns	IT	218Ac	(11+)
219Ac		89	130	219.01242(5)	11.8(15) μs	α	215Fr	9/2−
220Ac		89	131	220.014763(16)	26.36(19) ms	α	216Fr	(3−)
221Ac		89	132	221.01559(5)	52(2) ms	α	217Fr	9/2−#
222Ac		89	133	222.017844(6)	5.0(5) s	α (99(1)%)	218Fr	1−
						β+ (1(1)%)[n 8]	222Ra
222mAc		78(21) keV			1.05(5) min	α (98.6%)	218Fr	5+#
						β+ (1.4%)	222Ra
						IT?	222Ac
223Ac		89	134	223.019137(8)	2.10(5) min	α (99%)	219Fr	(5/2−)
						EC (1%)	223Ra
						CD (3.2×10−9%)	209Bi 14C
224Ac		89	135	224.021723(4)	2.78(17) h	β+ (90.9%)	224Ra	0−
						α (9.1%)	220Fr
						β− (1.6%)	224Th
225Ac[n 9]		89	136	225.023230(5)	10.0(1) d	α	221Fr	(3/2−)	Trace[n 10]
						CD (6×10−10%)	211Bi 14C
226Ac		89	137	226.026098(4)	29.37(12) h	β− (83%)	226Th	(1)(−#)
						EC (17%)	226Ra
						α (.006%)	222Fr
227Ac	Actinium[n 11]	89	138	227.0277521(26)	21.772(3) y	β− (98.62%)	227Th	3/2−	Trace[n 12]
						α (1.38%)	223Fr
228Ac	Mesothorium 2	89	139	228.0310211(27)	6.13(2) h	β−	228Th	3+	Trace[n 13]
229Ac		89	140	229.03302(4)	62.7(5) min	β−	229Th	(3/2+)
230Ac		89	141	230.03629(32)	122(3) s	β−	230Th	(1+)
231Ac		89	142	231.03856(11)	7.5(1) min	β−	231Th	(1/2+)
232Ac		89	143	232.04203(11)	119(5) s	β−	232Th	(1+)
233Ac		89	144	233.04455(32)#	145(10) s	β−	233Th	(1/2+)
234Ac		89	145	234.04842(43)#	44(7) s	β−	234Th
235Ac		89	146	235.05123(38)#	60(4) s	β−	235Th	1/2+#
236Ac[6]		89	147	236.05530(54)#	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. This decay mode has been observed, but only an upper limit of branching ratio is experimentally known^[1]
9. Has medical uses
10. Intermediate decay product of ²³⁷Np
11. Source of element's name
12. Intermediate decay product of ²³⁵U
13. Intermediate decay product of ²³²Th

Actinides vs fission products

Actinides and fission products by half-life v t e
Actinides[7] by decay chain				Half-life range (a)	Fission products of 235U by yield[8]
4n	4n + 1	4n + 2	4n + 3		4.5–7%	0.04–1.25%	<0.001%
228Ra№				4–6 a		155Euþ
248Bk[9]				> 9 a
244Cmƒ	241Puƒ	250Cf	227Ac№	10–29 a	90Sr	85Kr	113mCdþ
232Uƒ		238Puƒ	243Cmƒ	29–97 a	137Cs	151Smþ	121mSn
	249Cfƒ	242mAmƒ		141–351 a	No fission products have a half-life in the range of 100 a–210 ka ...
	241Amƒ		251Cfƒ[10]	430–900 a
		226Ra№	247Bk	1.3–1.6 ka
240Pu	229Th	246Cmƒ	243Amƒ	4.7–7.4 ka
	245Cmƒ	250Cm		8.3–8.5 ka
			239Puƒ	24.1 ka
		230Th№	231Pa№	32–76 ka
236Npƒ	233Uƒ	234U№		150–250 ka	99Tc₡	126Sn
248Cm		242Pu		327–375 ka		79Se₡
				1.33 Ma	135Cs₡
	237Npƒ			1.61–6.5 Ma	93Zr	107Pd
236U			247Cmƒ	15–24 Ma		129I₡
244Pu				80 Ma	... nor beyond 15.7 Ma[11]
232Th№		238U№	235Uƒ№	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

Main article: 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.^[12][13][14] 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.^[15] Another benefit is that the decay chain of ²²⁵Ac ends in the nuclide ²⁰⁹Bi,^{[note 1]} which has a considerably shorter biological half-life than lead.^[16][17] 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.^[18]

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.^[1] There are researches on ²²⁶Ac to use it in SPECT.^[19][20]

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.^{[1] 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 ²²⁷Ac.^{[21][22] 227}Ac is prepared, in milligram amounts, by the neutron irradiation of ²²⁶Ra in a nuclear reactor.^[22][23]

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

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.^[24] 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:

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

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.^[25][26] Such probes are also used in well logging applications, in neutron radiography, tomography and other radiochemical investigations.^[27]

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.^[28][29]

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.

References

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2. G. D. Considine, ed. (2005). "Chemical Elements". Van Nostrand's Encyclopedia of Chemistry. Wiley-Interscience. p. 332. ISBN 978-0-471-61525-5.
3. Wang, J. G.; Gan, Z. G.; Zhang, Z. Y.; et al. (1 March 2024). "α-decay properties of new neutron-deficient isotope 203Ac". Physics Letters B. 850: 138503. doi:10.1016/j.physletb.2024.138503. ISSN 0370-2693.
4. Huang, M. H.; Gan, Z. G.; Zhang, Z. Y.; et al. (10 November 2022). "α decay of the new isotope ²⁰⁴Ac". Physics Letters B. 834: 137484. Bibcode:2022PhLB..83437484H. doi:10.1016/j.physletb.2022.137484. ISSN 0370-2693. S2CID 252730841.
5. Zhang, Z. Y.; Gan, Z. G.; Ma, L.; et al. (January 2014). "α decay of the new neutron-deficient isotope ²⁰⁵Ac". Physical Review C. 89 (1): 014308. Bibcode:2014PhRvC..89a4308Z. doi:10.1103/PhysRevC.89.014308.
6. Chen, L.; et al. (2010). "Discovery and investigation of heavy neutron-rich isotopes with time-resolved Schottky spectrometry in the element range from thallium to actinium" (PDF). Physics Letters B. 691 (5): 234–237. Bibcode:2010PhLB..691..234C. doi:10.1016/j.physletb.2010.05.078.
7. Plus radium (element 88). While actually a sub-actinide, it immediately precedes actinium (89) and follows a three-element gap of instability after polonium (84) where no nuclides have half-lives of at least four years (the longest-lived nuclide in the gap is radon-222 with a half life of less than four days). Radium's longest lived isotope, at 1,600 years, thus merits the element's inclusion here.
8. Specifically from thermal neutron fission of uranium-235, e.g. in a typical nuclear reactor.
9. Milsted, J.; Friedman, A. M.; Stevens, C. M. (1965). "The alpha half-life of berkelium-247; a new long-lived isomer of berkelium-248". Nuclear Physics. 71 (2): 299. Bibcode:1965NucPh..71..299M. doi:10.1016/0029-5582(65)90719-4.
   "The isotopic analyses disclosed a species of mass 248 in constant abundance in three samples analysed over a period of about 10 months. This was ascribed to an isomer of Bk²⁴⁸ with a half-life greater than 9 [years]. No growth of Cf²⁴⁸ was detected, and a lower limit for the β⁻ half-life can be set at about 10⁴ [years]. No alpha activity attributable to the new isomer has been detected; the alpha half-life is probably greater than 300 [years]."
10. This is the heaviest nuclide with a half-life of at least four years before the "sea of instability".
11. Excluding those "classically stable" nuclides with half-lives significantly in excess of ²³²Th; e.g., while ^113mCd has a half-life of only fourteen years, that of ¹¹³Cd is eight quadrillion years.
12. A. Scheinberg, David; R. McDevitt, Michael (1 October 2011). "Actinium-225 in Targeted Alpha-Particle Therapeutic Applications". Current Radiopharmaceuticals. 4 (4): 306–320. doi:10.2174/1874471011104040306. PMC 5565267. PMID 22202153.
13. Reissig, Falco; Bauer, David; Zarschler, Kristof; Novy, Zbynek; Bendova, Katerina; Ludik, Marie-Charlotte; Kopka, Klaus; Pietzsch, Hans-Jürgen; Petrik, Milos; Mamat, Constantin (20 April 2021). "Towards Targeted Alpha Therapy with Actinium-225: Chelators for Mild Condition Radiolabeling and Targeting PSMA—A Proof of Concept Study". Cancers. 13 (8): 1974. doi:10.3390/cancers13081974. PMC 8073976. PMID 33923965.
14. Bidkar, Anil P.; Zerefa, Luann; Yadav, Surekha; VanBrocklin, Henry F.; Flavell, Robert R. (2024). "Actinium-225 targeted alpha particle therapy for prostate cancer". Theranostics. 14 (7): 2969–2992. doi:10.7150/thno.96403.
15. Ahenkorah, Stephen; Cassells, Irwin; Deroose, Christophe M.; Cardinaels, Thomas; Burgoyne, Andrew R.; Bormans, Guy; Ooms, Maarten; Cleeren, Frederik (21 April 2021). "Bismuth-213 for Targeted Radionuclide Therapy: From Atom to Bedside". Pharmaceutics. 13 (5): 599. doi:10.3390/pharmaceutics13050599. PMC 8143329.
16. Handbook on the toxicology of metals. Volume 2: Specific metals (Fourth ed.). Amsterdam Boston Heidelberg London: Elsevier, Aademic Press. 2015. p. 655. ISBN 978-0-12-398293-3.
17. Wani, Ab Latif; Ara, Anjum; Usmani, Jawed Ahmad (1 June 2015). "Lead toxicity: a review". Interdisciplinary Toxicology. 8 (2): 55–64. doi:10.1515/intox-2015-0009. PMC 4961898.
18. Dhiman, Deeksha; Vatsa, Rakhee; Sood, Ashwani (September 2022). "Challenges and opportunities in developing Actinium-225 radiopharmaceuticals". Nuclear Medicine Communications. 43 (9): 970–977. doi:10.1097/MNM.0000000000001594. PMID 35950353.
19. Koniar, Helena; Rodríguez-Rodríguez, Cristina; Radchenko, Valery; Yang, Hua; Kunz, Peter; Rahmim, Arman; Uribe, Carlos; Schaffer, Paul (2022-09-12). "SPECT imaging of ²²⁶Ac as a theranostic isotope for ²²⁵Ac radiopharmaceutical development". Physics in Medicine and Biology. 67 (18). doi:10.1088/1361-6560/ac8b5f. ISSN 1361-6560. PMID 35985341.
20. Koniar, Helena; Wharton, Luke; Ingham, Aidan; Rodríguez-Rodríguez, Cristina; Kunz, Peter; Radchenko, Valery; Yang, Hua; Rahmim, Arman; Uribe, Carlos; Schaffer, Paul (2024-07-16). "In vivoquantitative SPECT imaging of actinium-226: feasibility and proof-of-concept". Physics in Medicine and Biology. 69 (15). doi:10.1088/1361-6560/ad5c37. ISSN 1361-6560. PMID 38925140.
21. Hagemann, French (1950). "The Isolation of Actinium". Journal of the American Chemical Society. 72 (2): 768–771. doi:10.1021/ja01158a033.
22. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 946. ISBN 978-0-08-037941-8.
23. Emeleus, H. J. (1987). Advances in inorganic chemistry and radiochemistry. Academic Press. pp. 16–. ISBN 978-0-12-023631-2.
24. Russell, Alan M. and Lee, Kok Loong (2005) Structure-property relations in nonferrous metals. Wiley. ISBN 0-471-64952-X, pp. 470–471
25. Majumdar, D. K. (2004) Irrigation Water Management: Principles and Practice. ISBN 81-203-1729-7 p. 108
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29. Geibert, W.; Rutgers Van Der Loeff, M. M.; Hanfland, C.; Dauelsberg, H.-J. (2002). "Actinium-227 as a deep-sea tracer: sources, distribution and applications". Earth and Planetary Science Letters. 198 (1–2): 147–165. Bibcode:2002E&PSL.198..147G. doi:10.1016/S0012-821X(02)00512-5.

• Isotope masses from:
  • Audi, Georges; Bersillon, Olivier; Blachot, Jean; Wapstra, Aaldert Hendrik (2003), "The NUBASE evaluation of nuclear and decay properties", Nuclear Physics A, 729: 3–128, Bibcode:2003NuPhA.729....3A, doi:10.1016/j.nuclphysa.2003.11.001
• Half-life, spin, and isomer data selected from the following sources.
  • Audi, Georges; Bersillon, Olivier; Blachot, Jean; Wapstra, Aaldert Hendrik (2003), "The NUBASE evaluation of nuclear and decay properties", Nuclear Physics A, 729: 3–128, Bibcode:2003NuPhA.729....3A, doi:10.1016/j.nuclphysa.2003.11.001
  • National Nuclear Data Center. "NuDat 2.x database". Brookhaven National Laboratory.
  • Holden, Norman E. (2004). "11. Table of the Isotopes". In Lide, David R. (ed.). CRC Handbook of Chemistry and Physics (85th ed.). Boca Raton, Florida: CRC Press. ISBN 978-0-8493-0485-9.