Isotopes of radium

Isotopes of radium (₈₈Ra)
α	221Rn
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Radium (₈₈Ra) has no stable or nearly stable isotopes, and thus a standard atomic weight cannot be given. The longest lived, and most common, isotope of radium is ²²⁶Ra with a half-life of 1600 years. ²²⁶Ra occurs in the decay chain of ²³⁸U (often referred to as the radium series). Radium has 34 known isotopes from ²⁰¹Ra to ²³⁴Ra.

In the early history of the study of radioactivity, the different natural isotopes of radium were given different names, as it was not until Frederick Soddy's scientific career in the early 1900s that the concept of isotopes was realized.^[3] In this scheme, ²²³Ra was named actinium X (AcX), ²²⁴Ra thorium X (ThX), ²²⁶Ra radium (Ra), and ²²⁸Ra mesothorium 1 (MsTh₁).^[4] When it was realized that all of these are isotopes of the same element, many of these names fell out of use, and "radium" came to refer to all isotopes, not just ²²⁶Ra,^[5] though mesothorium 1 in particular was still used for some time, with a footnote explaining that it referred to ²²⁸Ra.^[6] Some of radium-226's decay products received historical names including "radium",^[7] ranging from radium A to radium G, with the letter indicating approximately how far they were down the chain from their parent ²²⁶Ra.^[a]

In 2013 it was discovered that the nucleus of radium-224 is pear-shaped.^[10] This was the first discovery of an asymmetrical nucleus.

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
Nuclide ^{[n 1]}	Historic name	Excitation energy^{[n 7]}			Half-life	Decay mode ^{[n 4]}	Daughter isotope ^{[n 5]}	Spin and parity ^{[n 6]}^{[n 7]}	Isotopic abundance
²⁰¹Ra^[11]		88	113		8+40 −4 ms	α	¹⁹⁷Rn	(3/2−)
^201mRa^[12]		260(30) keV			1.6+7.7 −0.7 ms	α	^197mRn	(13/2+)
²⁰²Ra		88	114	202.00989(7)	3.8+1.3 −0.8 ms^[11]	α	¹⁹⁸Rn	0+
²⁰³Ra		88	115	203.00927(9)	4(3) ms	α	¹⁹⁹Rn	(3/2−)
²⁰³Ra		88	115	203.00927(9)	4(3) ms	β⁺ (rare)	²⁰³Fr	(3/2−)
^203mRa		220(90) keV			41(17) ms	α	¹⁹⁹Rn	(13/2+)
^203mRa		220(90) keV			41(17) ms	β⁺ (rare)	²⁰³Fr	(13/2+)
²⁰⁴Ra		88	116	204.006500(17)	60(11) ms [59(+12−9) ms]	α (99.7%)	²⁰⁰Rn	0+
²⁰⁴Ra		88	116	204.006500(17)	60(11) ms [59(+12−9) ms]	β⁺ (.3%)	²⁰⁴Fr	0+
²⁰⁵Ra		88	117	205.00627(9)	220(40) ms [210(+60−40) ms]	α	²⁰¹Rn	(3/2−)
²⁰⁵Ra		88	117	205.00627(9)	220(40) ms [210(+60−40) ms]	β⁺ (rare)	²⁰⁵Fr	(3/2−)
^205mRa		310(110)# keV			180(50) ms [170(+60−40) ms]	α	²⁰¹Rn	(13/2+)
^205mRa		310(110)# keV			180(50) ms [170(+60−40) ms]	IT (rare)	²⁰⁵Ra	(13/2+)
²⁰⁶Ra		88	118	206.003827(19)	0.24(2) s	α	²⁰²Rn	0+
²⁰⁷Ra		88	119	207.00380(6)	1.3(2) s	α (90%)	²⁰³Rn	(5/2−, 3/2−)
²⁰⁷Ra		88	119	207.00380(6)	1.3(2) s	β⁺ (10%)	²⁰⁷Fr	(5/2−, 3/2−)
^207mRa		560(50) keV			57(8) ms	IT (85%)	²⁰⁷Ra	(13/2+)
						α (15%)	²⁰³Rn
						β⁺ (.55%)	²⁰⁷Fr
²⁰⁸Ra		88	120	208.001840(17)	1.3(2) s	α (95%)	²⁰⁴Rn	0+
²⁰⁸Ra		88	120	208.001840(17)	1.3(2) s	β⁺ (5%)	²⁰⁸Fr	0+
^208mRa		1800(200) keV			270 ns			(8+)
²⁰⁹Ra		88	121	209.00199(5)	4.6(2) s	α (90%)	²⁰⁵Rn	5/2−
²⁰⁹Ra		88	121	209.00199(5)	4.6(2) s	β⁺ (10%)	²⁰⁹Fr	5/2−
²¹⁰Ra		88	122	210.000495(16)	3.7(2) s	α (96%)	²⁰⁶Rn	0+
²¹⁰Ra		88	122	210.000495(16)	3.7(2) s	β⁺ (4%)	²¹⁰Fr	0+
^210mRa		1800(200) keV			2.24 μs			(8+)
²¹¹Ra		88	123	211.000898(28)	13(2) s	α (97%)	²⁰⁷Rn	5/2(−)
²¹¹Ra		88	123	211.000898(28)	13(2) s	β⁺ (3%)	²¹¹Fr	5/2(−)
²¹²Ra		88	124	211.999794(12)	13.0(2) s	α (85%)	²⁰⁸Rn	0+
²¹²Ra		88	124	211.999794(12)	13.0(2) s	β⁺ (15%)	²¹²Fr	0+
^212m1Ra		1958.4(5) keV			10.9(4) μs			(8)+
^212m2Ra		2613.4(5) keV			0.85(13) μs			(11)−
²¹³Ra		88	125	213.000384(22)	2.74(6) min	α (80%)	²⁰⁹Rn	1/2−
²¹³Ra		88	125	213.000384(22)	2.74(6) min	β⁺ (20%)	²¹³Fr	1/2−
^213mRa		1769(6) keV			2.1(1) ms	IT (99%)	²¹³Ra	17/2−#
^213mRa		1769(6) keV			2.1(1) ms	α (1%)	²⁰⁹Rn	17/2−#
²¹⁴Ra		88	126	214.000108(10)	2.46(3) s	α (99.94%)	²¹⁰Rn	0+
²¹⁴Ra		88	126	214.000108(10)	2.46(3) s	β⁺ (.06%)	²¹⁴Fr	0+
²¹⁵Ra		88	127	215.002720(8)	1.55(7) ms	α	²¹¹Rn	(9/2+)#
^215m1Ra		1877.8(5) keV			7.1(2) μs			(25/2+)
^215m2Ra		2246.9(5) keV			1.39(7) μs			(29/2−)
^215m3Ra		3756.6(6)+X keV			0.555(10) μs			(43/2−)
²¹⁶Ra		88	128	216.003533(9)	182(10) ns	α	²¹²Rn	0+
²¹⁶Ra		88	128	216.003533(9)	182(10) ns	EC (<1×10⁻⁸%)	²¹⁶Fr	0+
²¹⁷Ra		88	129	217.006320(9)	1.63(17) μs	α	²¹³Rn	(9/2+)
²¹⁸Ra		88	130	218.007140(12)	25.2(3) μs	α	²¹⁴Rn	0+
²¹⁹Ra		88	131	219.010085(9)	10(3) ms	α	²¹⁵Rn	(7/2)+
²²⁰Ra		88	132	220.011028(10)	17.9(14) ms	α	²¹⁶Rn	0+
²²¹Ra		88	133	221.013917(5)	28(2) s	α	²¹⁷Rn	5/2+	Trace^{[n 8]}
²²¹Ra		88	133	221.013917(5)	28(2) s	CD (1.2×10⁻¹⁰%)^{[n 9]}	²⁰⁷Pb ¹⁴C	5/2+	Trace^{[n 8]}
²²²Ra		88	134	222.015375(5)	38.0(5) s	α	²¹⁸Rn	0+
²²²Ra		88	134	222.015375(5)	38.0(5) s	CD (3×10⁻⁸%)	²⁰⁸Pb ¹⁴C	0+
²²³Ra^{[n 10]}	Actinium X	88	135	223.0185022(27)	11.43(5) d	α	²¹⁹Rn	3/2+	Trace^{[n 11]}
²²³Ra^{[n 10]}	Actinium X	88	135	223.0185022(27)	11.43(5) d	CD (6.4×10⁻⁸%)	²⁰⁹Pb ¹⁴C	3/2+	Trace^{[n 11]}
²²⁴Ra	Thorium X	88	136	224.0202118(24)	3.6319(23) d	α	²²⁰Rn	0+	Trace^{[n 12]}
²²⁴Ra	Thorium X	88	136	224.0202118(24)	3.6319(23) d	CD (4.3×10⁻⁹%)	²¹⁰Pb ¹⁴C	0+	Trace^{[n 12]}
²²⁵Ra		88	137	225.023612(3)	14.9(2) d	β⁻	²²⁵Ac	1/2+	Trace^{[n 13]}
²²⁵Ra		88	137	225.023612(3)	14.9(2) d	α (2.0×10⁻³%)^[2]	²²¹Rn	1/2+	Trace^{[n 13]}
²²⁶Ra	Radium^{[n 14]}	88	138	226.0254098(25)	1600(7) y	α^{[n 15]}	²²²Rn	0+	Trace^{[n 16]}
²²⁶Ra	Radium^{[n 14]}	88	138	226.0254098(25)	1600(7) y	CD (2.6×10⁻⁹%)	²¹²Pb ¹⁴C	0+	Trace^{[n 16]}
²²⁷Ra		88	139	227.0291778(25)	42.2(5) min	β⁻	²²⁷Ac	3/2+
²²⁸Ra	Mesothorium 1	88	140	228.0310703(26)	5.75(3) y	β⁻	²²⁸Ac	0+	Trace^{[n 12]}
²²⁹Ra		88	141	229.034958(20)	4.0(2) min	β⁻	²²⁹Ac	5/2(+)
²³⁰Ra		88	142	230.037056(13)	93(2) min	β⁻	²³⁰Ac	0+
²³¹Ra		88	143	231.04122(32)#	103(3) s	β⁻	²³¹Ac	(5/2+)
^231mRa		66.21(9) keV			~53 μs			(1/2+)
²³²Ra		88	144	232.04364(30)#	250(50) s	β⁻	²³²Ac	0+
²³³Ra		88	145	233.04806(50)#	30(5) s	β⁻	²³³Ac	1/2+#
²³⁴Ra		88	146	234.05070(53)#	30(10) s	β⁻	²³⁴Ac	0+
This table header & footer: view

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

CD: Cluster decay

EC: Electron capture

IT: Isomeric transition
^ Bold symbol as daughter – Daughter product is stable.
^ ( ) spin value – Indicates spin with weak assignment arguments.
^ ^a ^b # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
^ Intermediate decay product of ²³⁷Np
^ Lightest known nuclide to undergo cluster decay
^ Used for treating bone cancer
^ Intermediate decay product of ²³⁵U
^ ^a ^b Intermediate decay product of ²³²Th
^ Intermediate decay product of ²³⁷Np
^ Source of element's name
^ Theoretically capable of β^-β^- decay to ²²⁶Th^[1]
^ Intermediate decay product of ²³⁸U

Actinides vs fission products

Actinides and fission products by half-life v t e
Actinides^[13] by decay chain				Half-life range (a)	Fission products of ²³⁵U by yield^[14]
4n	4n + 1	4n + 2	4n + 3	Half-life range (a)	4.5–7%	0.04–1.25%	<0.001%
²²⁸Ra^№				4–6 a		¹⁵⁵Eu^þ
²⁴⁸Bk^[15]				> 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^[16]	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^[17]
²³²Th^№		²³⁸U^№	²³⁵U^ƒ№	0.7–14.1 Ga	... nor beyond 15.7 Ma^[17]
₡, 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)

References

^ ^a ^b 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.
^ ^a ^b Liang, C. F.; Paris, P.; Sheline, R. K. (2000-09-19). "α decay of ²²⁵Ra". Physical Review C. 62 (4). American Physical Society (APS): 047303. Bibcode:2000PhRvC..62d7303L. doi:10.1103/physrevc.62.047303. ISSN 0556-2813.
^ Nagel, Miriam C. (September 1982). "Frederick Soddy: From alchemy to isotopes". Journal of Chemical Education. 59 (9): 739. Bibcode:1982JChEd..59..739N. doi:10.1021/ed059p739. ISSN 0021-9584.
^ Kirby, H.W. & Salutsky, Murrell L. (December 1964). The Radiochemistry of Radium (Report). crediting UNT Libraries Government Documents Department. p. 3 – via University of North Texas, UNT Digital Library. Alternate source: https://sgp.fas.org/othergov/doe/lanl/lib-www/books/rc000041.pdf
^ Giunta, Carmen J. (2017). "ISOTOPES: IDENTIFYING THE BREAKTHROUGH PUBLICATION (1)" (PDF). Bull. Hist. Chem. 42 (2): 103–111.
^ Looney, William B. (1958). "Effects of Radium in Man". Science. 127 (3299): 630–633. Bibcode:1958Sci...127..630L. doi:10.1126/science.127.3299.630. ISSN 0036-8075. JSTOR 1755774. PMID 13529029.
^ Mitchell, S. A. "Is Radium in the Sun?". Popular Astronomy. 21: 321–331. Bibcode:1913PA.....21..321M.
^ Kuhn, W. (1929). "LXVIII. Scattering of thorium C" γ-radiation by radium G and ordinary lead". The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science. 8 (52): 628. doi:10.1080/14786441108564923. ISSN 1941-5982.
^ Kinsey, R. R. (December 18, 1997), "The radioactive series of radium-226" (PDF), The NUDAT/PCNUDAT Program for Nuclear Data – via CERN
^ Hills, Stephanie (8 May 2013). "First observations of short-lived pear-shaped atomic nuclei". CERN.
^ ^a ^b Kalaninová, Z.; Antalic, S.; Andreyev, A. N.; Heßberger, F. P.; Ackermann, D.; Andel, B.; Bianco, L.; Hofmann, S.; Huyse, M.; Kindler, B.; Lommel, B.; Mann, R.; Page, R. D.; Sapple, P. J.; Thomson, J.; Van Duppen, P.; Venhart, M. (12 May 2014). "Decay of ^201–203Ra and ^200–202Fr" (PDF). Physical Review C. 89 (5): 054312. Bibcode:2014PhRvC..89e4312K. doi:10.1103/PhysRevC.89.054312. ISSN 0556-2813. Retrieved 11 June 2023.
^ Uusitalo, J.; Leino, M.; Enqvist, T.; Eskola, K.; Grahn, T.; Greenlees, P. T.; Jones, P.; Julin, R.; Juutinen, S.; Keenan, A.; Kettunen, H.; Koivisto, H.; Kuusiniemi, P.; Leppänen, A.-P.; Nieminen, P.; Pakarinen, J.; Rahkila, P.; Scholey, C. (11 February 2005). "α decay studies of very neutron-deficient francium and radium isotopes". Physical Review C. 71 (2): 024306. Bibcode:2005PhRvC..71b4306U. doi:10.1103/PhysRevC.71.024306. ISSN 0556-2813.
^ 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.
^ Specifically from thermal neutron fission of uranium-235, e.g. in a typical nuclear reactor.
^ 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]."
^ This is the heaviest nuclide with a half-life of at least four years before the "sea of instability".
^ 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.

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.

Notes

^ Radium emanation = ²²²Rn, Ra A = ²¹⁸Po, Ra B = ²¹⁴Pb, Ra C = ²¹⁴Bi, Ra C₁ = ²¹⁴Po, Ra C₂ = ²¹⁰Tl, Ra D = ²¹⁰Pb, Ra E = ²¹⁰Bi, Ra F = ²¹⁰Po, and Ra G = ²⁰⁶Pb.^[8]^[9]

[12] Ra – Excited nuclear isomer.

[13] ( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.

[TMS-14] # – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).

[15] Modes of decay:

CD: Cluster decay

EC: Electron capture

IT: Isomeric transition

[16] Bold symbol as daughter – Daughter product is stable.

[17] ( ) spin value – Indicates spin with weak assignment arguments.

[TNN-18] # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).

[21] Intermediate decay product of ²³⁷Np

[22] Lightest known nuclide to undergo cluster decay

[23] Used for treating bone cancer

[24] Intermediate decay product of ²³⁵U

[ths-25] Intermediate decay product of ²³²Th

[26] Intermediate decay product of ²³⁷Np

[27] Source of element's name

[28] Theoretically capable of β^-β^- decay to ²²⁶Th^[1]

[29] Intermediate decay product of ²³⁸U

[NUBASE2020-1] 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.

[Ra-225-2] Liang, C. F.; Paris, P.; Sheline, R. K. (2000-09-19). "α decay of ²²⁵Ra". Physical Review C. 62 (4). American Physical Society (APS): 047303. Bibcode:2000PhRvC..62d7303L. doi:10.1103/physrevc.62.047303. ISSN 0556-2813.

[3] Nagel, Miriam C. (September 1982). "Frederick Soddy: From alchemy to isotopes". Journal of Chemical Education. 59 (9): 739. Bibcode:1982JChEd..59..739N. doi:10.1021/ed059p739. ISSN 0021-9584.

[4] Kirby, H.W. & Salutsky, Murrell L. (December 1964). The Radiochemistry of Radium (Report). crediting UNT Libraries Government Documents Department. p. 3 – via University of North Texas, UNT Digital Library. Alternate source: https://sgp.fas.org/othergov/doe/lanl/lib-www/books/rc000041.pdf

[5] Giunta, Carmen J. (2017). "ISOTOPES: IDENTIFYING THE BREAKTHROUGH PUBLICATION (1)" (PDF). Bull. Hist. Chem. 42 (2): 103–111.

[6] Looney, William B. (1958). "Effects of Radium in Man". Science. 127 (3299): 630–633. Bibcode:1958Sci...127..630L. doi:10.1126/science.127.3299.630. ISSN 0036-8075. JSTOR 1755774. PMID 13529029.

[7] Mitchell, S. A. "Is Radium in the Sun?". Popular Astronomy. 21: 321–331. Bibcode:1913PA.....21..321M.

[8] Kuhn, W. (1929). "LXVIII. Scattering of thorium C" γ-radiation by radium G and ordinary lead". The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science. 8 (52): 628. doi:10.1080/14786441108564923. ISSN 1941-5982.

[9] Kinsey, R. R. (December 18, 1997), "The radioactive series of radium-226" (PDF), The NUDAT/PCNUDAT Program for Nuclear Data – via CERN

[11] Hills, Stephanie (8 May 2013). "First observations of short-lived pear-shaped atomic nuclei". CERN.

[PRC89-19] Kalaninová, Z.; Antalic, S.; Andreyev, A. N.; Heßberger, F. P.; Ackermann, D.; Andel, B.; Bianco, L.; Hofmann, S.; Huyse, M.; Kindler, B.; Lommel, B.; Mann, R.; Page, R. D.; Sapple, P. J.; Thomson, J.; Van Duppen, P.; Venhart, M. (12 May 2014). "Decay of ^201–203Ra and ^200–202Fr" (PDF). Physical Review C. 89 (5): 054312. Bibcode:2014PhRvC..89e4312K. doi:10.1103/PhysRevC.89.054312. ISSN 0556-2813. Retrieved 11 June 2023.

[20] Uusitalo, J.; Leino, M.; Enqvist, T.; Eskola, K.; Grahn, T.; Greenlees, P. T.; Jones, P.; Julin, R.; Juutinen, S.; Keenan, A.; Kettunen, H.; Koivisto, H.; Kuusiniemi, P.; Leppänen, A.-P.; Nieminen, P.; Pakarinen, J.; Rahkila, P.; Scholey, C. (11 February 2005). "α decay studies of very neutron-deficient francium and radium isotopes". Physical Review C. 71 (2): 024306. Bibcode:2005PhRvC..71b4306U. doi:10.1103/PhysRevC.71.024306. ISSN 0556-2813.

[30] 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.

[31] Specifically from thermal neutron fission of uranium-235, e.g. in a typical nuclear reactor.

[32] 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]."

[33] This is the heaviest nuclide with a half-life of at least four years before the "sea of instability".

[34] 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.

[10] Radium emanation = ²²²Rn, Ra A = ²¹⁸Po, Ra B = ²¹⁴Pb, Ra C = ²¹⁴Bi, Ra C₁ = ²¹⁴Po, Ra C₂ = ²¹⁰Tl, Ra D = ²¹⁰Pb, Ra E = ²¹⁰Bi, Ra F = ²¹⁰Po, and Ra G = ²⁰⁶Pb.^[8]^[9]

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