Isotopes of fermium

Fermium (₁₀₀Fm) is a synthetic element, and thus a standard atomic weight cannot be given. Like all artificial elements, it has no stable isotopes. The first isotope to be discovered (in nuclear fallout from the Ivy Mike H-bomb test) was ²⁵⁵Fm in 1952. ²⁵⁰Fm was independently synthesized (to establish priority if the former result had to remain classified) shortly after the discovery of ²⁵⁵Fm.

Isotopes of fermium (₁₀₀Fm)

Main isotopes[1] Decay abun­dance half-life (t1/2) mode pro­duct 252Fm synth 25.39 h α 248Cf SF – 253Fm synth 3 d ε 253Es α 249Cf 255Fm synth 20.07 h α 251Cf SF – 257Fm synth 100.5 d α 253Cf SF –

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There are 20 known radioisotopes ranging in atomic mass from ²⁴¹Fm to ²⁶⁰Fm (²⁶⁰Fm is unconfirmed), and 5 nuclear isomers. The longest-lived isotope is ²⁵⁷Fm with a half-life of 100.5 days, and the longest-lived isomer is ^247mFm with a half-life of 5.1 seconds.

List of isotopes

Nuclide [n 1]	Z	N	Isotopic mass (Da)[2] [n 2][n 3]	Half-life[1]	Decay mode[1] [n 4]	Daughter isotope	Spin and parity[1] [n 5][n 6]
	Excitation energy
241Fm	100	141	241.07431(32)#	730(60) μs	SF	(various)	5/2+#
					α (<14%)	237Cf
					β+ (<12%)	241Es
242Fm	100	142	242.07343(43)#	0.8(2) ms	SF	(various)	0+
243Fm	100	143	243.07441(14)#	231(9) ms	α (91%)	239Cf	7/2−#
					SF (9%)	(various)
244Fm	100	144	244.07404(22)#	3.12(8) ms	SF (>97%)	(various)	0+
					β+ (<2%)	244Es
					α (<1%)	240Cf
245Fm	100	145	245.07535(21)#	4.2(13) s	α (88.5%)	241Cf	1/2+#
					β+ (11.5%)[3]	245Es
					SF (<0.3%)	(various)
246Fm	100	146	246.075353(15)	1.54(4) s	α (93.2%)	242Cf	0+
					SF (6.8%)	(various)
					EC (<1.3%)	246Es
247Fm	100	147	247.07694(19)#	31(1) s	α (~64%)	243Cf	(7/2+)
					β+? (~36%)	247Es
247mFm	49(8) keV			5.1(2) s	α (88%)	243Cf	(1/2+)
					IT? (12%)	247Fm
248Fm	100	148	248.077185(9)	34.5(12) s	α (99.9%)	244Cf	0+
					SF (0.1%)	(various)
248mFm	1200(100)# keV			10.1(6) ms	IT?	248Fm	6+#
					α?	244Cf
					β+?	248Es
249Fm	100	149	249.078926(7)	1.6(1) min	β+? (67%)	249Es	7/2+
					α (33%)	245Cf
250Fm	100	150	250.079520(8)	31.0(11) min	α (99.99%)	246Cf	0+
					SF (6.9×10−3%)	(various)
250mFm	1199.2(10) keV			1.92(5) s	IT	250Fm	(8−)
251Fm	100	151	251.081545(15)	5.30(8) h	β+ (98.20%)	251Es	9/2−
					α (1.80%)	247Cf
251mFm	200.0(1) keV			21.8(8) μs	IT	251Fm	5/2+
252Fm	100	152	252.082466(6)	25.39(4) h	α (99.99%)[n 7]	248Cf	0+
					SF (0.0023%)	(various)
253Fm	100	153	253.0851809(17)	3.00(12) d	EC (88%)	253Es	1/2+
					α (12%)	249Cf
253mFm	351(6) keV			0.56(6) μs	IT	253Fm	11/2−#
254Fm	100	154	254.0868524(20)	3.240(2) h	α (99.94%)	250Cf	0+
					SF (0.0592%)	(various)
255Fm	100	155	255.089963(4)	20.07(7) h	α	251Cf	7/2+
					SF (2.4×10−5%)	(various)
256Fm	100	156	256.091772(3)	157.1(13) min	SF (91.9%)	(various)	0+
					α (8.1%)	252Cf
257Fm[n 8]	100	157	257.095105(5)	100.5(2) d	α (99.79%)	253Cf	9/2+
					SF (0.210%)	(various)
258Fm	100	158	258.09708(22)#	370(14) μs	SF	(various)	0+
259Fm	100	159	259.10060(30)#	1.5(2) s	SF	(various)
260Fm[n 9][n 10]	100	160	260.10281(47)#	1# min	SF	(various)	0+
This table header & footer: view

1. ^mFm – 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
   IT:	Isomeric transition
   SF:	Spontaneous fission

5. ( ) spin value – Indicates spin with weak assignment arguments.
6. # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
7. Theoretically capable of β⁺β⁺ decay to ²⁵²Cf
8. Heaviest nuclide produced via neutron capture
9. Discovery of this isotope is unconfirmed
10. Not directly synthesized, occurs as decay product of ²⁶⁰Md

Chronology of isotope discovery

Isotope	Discovered	Reaction
241Fm	2008	204Pb(40Ar,3n)
242Fm	1975	204Pb(40Ar,2n), 206Pb(40Ar,4n)
243Fm	1981	206Pb(40Ar,3n)
244Fm	1967	233U(16O,5n)
245Fm	1967	233U(16O,4n)
246Fm	1966	235U(16O,5n)
247Fm	1967	239Pu(12C,4n)
248Fm	1958	240Pu(12C,4n)
249Fm	1960	238U(16O,5n)
250Fm	1954	238U(16O,4n)
251Fm	1957	249Cf(α,2n)
252Fm	1956	249Cf(α,n)
253Fm	1957	252Cf(α,3n)
254Fm	1954	Neutron capture
255Fm	1954	Neutron capture
256Fm	1955	Neutron capture
257Fm	1964	Neutron capture
258Fm	1971	257Fm(d,p)
259Fm	1980	257Fm(t,p)
260Fm?	1992?	254Es+18O, 22Ne — transfer (EC of 260Md)

²⁶⁰Fm was not confirmed in 1997.