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

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) 0.440(16) ms α 212Fr (1−)
β+ (7×10−5%) 216Ra
216mAc 44(7) keV 443(7) μs α 212Fr (9−)
217Ac 89 128 217.009347(14) 69(4) ns α 213Fr 9/2−
β+ (6.9×10−9%) 217Ra
217mAc 2012(20) keV 740(40) ns (29/2)+
218Ac 89 129 218.01164(5) 1.08(9) μs α 214Fr (1−)#
218mAc 584(50)# keV 103(11) ns (11+)
219Ac 89 130 219.01242(5) 11.8(15) μs α 215Fr 9/2−
β+ (10−6%) 219Ra
220Ac 89 131 220.014763(16) 26.36(19) ms α 216Fr (3−)
β+ (5×10−4%) 220Ra
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%) 218Fr 1−
β+ (1%) 222Ra
222mAc 200(150)# keV 1.05(7) min α (88.6%) 218Fr high
IT (10%) 222Ac
β+ (1.4%) 222Ra
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 8] 89 136 225.023230(5) 10.0(1) d α 221Fr (3/2−) Trace[n 9]
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 10] 89 138 227.0277521(26) 21.772(3) y β− (98.62%) 227Th 3/2− Trace[n 11]
α (1.38%) 223Fr
228Ac Mesothorium 2 89 139 228.0310211(27) 6.13(2) h β− 228Th 3+ Trace[n 12]
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:
  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:
    CD:Cluster decay
    EC:Electron capture
    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
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
244Cmƒ 241Puƒ 250Cf 227Ac№ 10–29 a 90Sr 85Kr 113mCdþ
232Uƒ 238Puƒ 243Cmƒ 29–97 a 137Cs 151Smþ 121mSn
248Bk[9] 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₡
1.53 Ma 93Zr
237Npƒ 2.1–6.5 Ma 135Cs₡ 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)

See also

References
  1. Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S.; Audi, G. (2021). "The NUBASE2020 evaluation of nuclear properties" (PDF). Chinese Physics C. 45 (3): 030001. doi:10.1088/1674-1137/abddae.
  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.
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