Isotopes of curium

Actinides and fission products by half-life
Actinides[1] by decay chain				Half-life range (y)	Fission products of ^235U by yield[2]
4n	4n+1	4n+2	4n+3
						4.5–7%	0.04–1.25%	<0.001%
^228Ra^№				4–6	†		^155Eu^þ
^244Cm^ƒ	^241Pu^ƒ	^250Cf	^227Ac^№	10–29		^90Sr	^85Kr	^113mCd^þ
^232U^ƒ		^238Pu^ƒ№	^243Cm^ƒ	29–97		^137Cs	^151Sm^þ	^121mSn
^248Bk[3]	^249Cf^ƒ	^242mAm^ƒ		141–351	No fission products have a half-life in the range of 100–210 k years ...
	^241Am^ƒ		^251Cf^ƒ[4]	430–900
		^226Ra^№	^247Bk	1.3 k – 1.6 k
^240Pu^ƒ№	^229Th^№	^246Cm^ƒ	^243Am^ƒ	4.7 k – 7.4 k
	^245Cm^ƒ	^250Cm		8.3 k – 8.5 k
			^239Pu^ƒ№	24.1 k
		^230Th^№	^231Pa^№	32 k – 76 k
^236Np^ƒ	^233U^ƒ№	^234U^№		150 k – 250 k	‡	^99Tc^₡	^126Sn
^248Cm		^242Pu^ƒ		327 k – 375 k			^79Se^₡
				1.53 M		^93Zr
	^237Np^ƒ№			2.1 M – 6.5 M		^135Cs^₡	^107Pd
^236U^№			^247Cm^ƒ	15 M – 24 M			^129I^₡
^244Pu^№				80 M	... nor beyond 15.7 M years[5]
^232Th^№		^238U^№	^235U^ƒ№	0.7 G – 14.1 G
Legend for superscript symbols ₡  has thermal neutron capture cross section in the range of 8–50 barns ƒ  fissile m  metastable isomer №  naturally occurring radioactive material (NORM) þ  neutron poison (thermal neutron capture cross section greater than 3k barns) †  range 4–97 y: Medium-lived fission product ‡  over 200,000 y: Long-lived fission product

Curium (Cm) is an artificial element with an atomic number of 96. Because it is an artificial element, a standard atomic mass cannot be given, and it has no stable isotopes. The first isotope synthesized was ²⁴²Cm in 1944, which has 146 neutrons. All of the isotopes are radioactive.

There are 21 known radioisotopes with atomic masses ranging from ²³²Cm to ²⁵²Cm. There are also four known nuclear isomers (^243mCm, ^244mCm, ^245mCm, and ^249mCm). The longest-lived isotope is ²⁴⁷Cm, with a half-life of 15.6 million years – several orders of magnitude longer than the half-life of all known nuclei of elements beyond curium in the periodic table. The shortest-lived isomer is ^244mCm with a half-life of 34 milliseconds.

Table

nuclide symbol	Z(p)	N(n)	isotopic mass (u)	half-life	decay mode(s)[6][n 1]	daughter isotope(s)[n 2]	nuclear spin
	excitation energy
^232Cm	96	136		1? min			0+
^233Cm	96	137	233.05077(8)	1# min	β^+	^233Am	3/2+#
					α	^229Pu
^234Cm	96	138	234.05016(2)	51(12) s	β^+	^234Am	0+
					α	^230Pu
^235Cm	96	139	235.05143(22)#	5# min	β^+	^235Am	5/2+#
					α	^231Pu
^236Cm	96	140	236.05141(22)#	10# min	β^+	^236Am	0+
					α	^232Pu
^237Cm	96	141	237.05290(22)#	20# min	β^+	^237Am	5/2+#
					α	^233Pu
^238Cm	96	142	238.05303(4)	2.4(1) h	EC (90%)	^238Am	0+
					α (10%)	^234Pu
^239Cm	96	143	239.05496(11)#	~2.9 h	β^+ (99.9%)	^239Am	(7/2−)
					α (.1%)	^235Pu
^240Cm	96	144	240.0555295(25)	27(1) d	α (99.5%)	^236Pu	0+
					EC (.5%)	^240Am
					SF (3.9×10^−6%)	(various)
^241Cm	96	145	241.0576530(23)	32.8(2) d	EC (99%)	^241Am	1/2+
					α (1%)	^237Pu
^242Cm[n 3]	96	146	242.0588358(20)	162.8(2) d	α	^238Pu	0+
					SF (6.33×10^−6%)	(various)
					CD (10^−14%)[n 4]	^208Pb ^34Si
					β^+β^+ (rare)	^242Pu
^243Cm	96	147	243.0613891(22)	29.1(1) y	α (99.71%)	^239Pu	5/2+
					EC (.29%)	^243Am
					SF (5.3×10^−9%)	(various)
^243mCm	87.4(1) keV			1.08(3) µs			1/2+
^244Cm[n 3]	96	148	244.0627526(20)	18.10(2) y	α	^240Pu	0+
					SF (1.34×10^−4%)	(various)
^244mCm	1040.188(12) keV			34(2) ms	IT	^244Cm	6+
^245Cm	96	149	245.0654912(22)	8.5(1)×10^3 y	α	^241Pu	7/2+
					SF (6.1×10^−7%)	(various)
^245mCm	355.90(10) keV			290(20) ns			1/2+
^246Cm	96	150	246.0672237(22)	4.76(4)×10^3 y	α (99.97%)	^242Pu	0+
					SF (.0261%)	(various)
^247Cm	96	151	247.070354(5)	1.56(5)×10^7 y	α	^243Pu	9/2−
^248Cm	96	152	248.072349(5)	3.48(6)×10^5 y	α (91.74%)	^244Pu	0+
					SF (8.26%)	(various)
					β^−β^− (rare)	^248Cf
^249Cm	96	153	249.075953(5)	64.15(3) min	β^−	^249Bk	1/2(+)
^249mCm	48.758(17) keV			23 µs			(7/2+)
^250Cm	96	154	250.078357(12)	8,300# y	SF (80%)[n 5]	(various)	0+
					α (11%)	^246Pu
					β^− (9%)	^250Bk
^251Cm	96	155	251.082285(24)	16.8(2) min	β^−	^251Bk	(1/2+)
^252Cm	96	156	252.08487(32)#	<1 d	β^−	^252Bk	0+

1. Abbreviations:
   CD: Cluster decay
   EC: Electron capture
   IT: Isomeric transition
   SF: Spontaneous fission
2. Bold for stable isotopes
3. Most common isotopes
4. Heaviest known nuclide to undergo cluster decay
5. The nuclide with the lowest atomic number known to undergo spontaneous fission as the main decay mode

Notes

• Values marked # are not purely derived from experimental data, but at least partly from systematic trends. Spins with weak assignment arguments are enclosed in parentheses.
• Uncertainties are given in concise form in parentheses after the corresponding last digits. Uncertainty values denote one standard deviation, except isotopic composition and standard atomic mass from IUPAC, which use expanded uncertainties.

References

1. 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 isotopes have half-lives of at least four years (the longest-lived isotope 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.
2. Specifically from thermal neutron fission of U-235, e.g. in a typical nuclear reactor.
3. 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. 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 y. No growth of Cf²⁴⁸ was detected, and a lower limit for the β⁻ half-life can be set at about 10⁴ y. No alpha activity attributable to the new isomer has been detected; the alpha half-life is probably greater than 300 y."
4. This is the heaviest isotope with a half-life of at least four years before the "Sea of Instability".
5. Excluding those "classically stable" isotopes with half-lives significantly in excess of ²³²Th; e.g., while ^113mCd has a half-life of only fourteen years, that of ¹¹³Cd is nearly eight quadrillion years.
6. "Universal Nuclide Chart". nucleonica. (registration required (help)).

• Isotope masses from:
  • G. Audi; A. H. Wapstra; C. Thibault; J. Blachot; O. Bersillon (2003). "The NUBASE evaluation of nuclear and decay properties" (PDF). Nuclear Physics A. 729: 3–128. Bibcode:2003NuPhA.729....3A. doi:10.1016/j.nuclphysa.2003.11.001.
• Isotopic compositions and standard atomic masses from:
  • J. R. de Laeter; J. K. Böhlke; P. De Bièvre; H. Hidaka; H. S. Peiser; K. J. R. Rosman; P. D. P. Taylor (2003). "Atomic weights of the elements. Review 2000 (IUPAC Technical Report)". Pure and Applied Chemistry. 75 (6): 683–800. doi:10.1351/pac200375060683.
  • M. E. Wieser (2006). "Atomic weights of the elements 2005 (IUPAC Technical Report)". Pure and Applied Chemistry. 78 (11): 2051–2066. doi:10.1351/pac200678112051. Lay summary.
• Half-life, spin, and isomer data selected from the following sources. See editing notes on this article's talk page.
  • G. Audi; A. H. Wapstra; C. Thibault; J. Blachot; O. Bersillon (2003). "The NUBASE evaluation of nuclear and decay properties" (PDF). Nuclear Physics A. 729: 3–128. Bibcode:2003NuPhA.729....3A. doi:10.1016/j.nuclphysa.2003.11.001.
  • National Nuclear Data Center. "NuDat 2.1 database". Brookhaven National Laboratory. Retrieved September 2005. Check date values in: |access-date= (help)
  • N. E. Holden (2004). "Table of the Isotopes". In D. R. Lide. CRC Handbook of Chemistry and Physics (85th ed.). CRC Press. Section 11. ISBN 978-0-8493-0485-9.