Isotopes of fermium

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Fermium (Fm) is an artificial element, and thus a standard atomic mass cannot be given. Like all artificial elements, it has no stable isotopes. The first isotope to be discovered (in fallout from nuclear testing) was 255Fm in 1952. 250Fm was independently synthesized shortly after the discovery of 255Fm. There are 20 known radioisotopes ranging in atomic mass from 241Fm to 260Fm (260Fm is unconfirmed), and 2 nuclear isomers, 250mFm and 251mFm. The longest-lived isotope is 257Fm with a half-life of 100.5 days, and the longest-lived isomer is 250mFm with a half-life of 1.8 seconds.

Table[edit]

nuclide
symbol
Z(p) N(n)  
isotopic mass (u)
 
half-life decay
mode(s)[1][n 1]
daughter
isotope(s)
nuclear
spin
excitation energy
241Fm 100 141 241.07421(32)# 730(60) µs SF(>78%) (various) 5/2#+
α (<14%) 237Cf
242Fm 100 142 242.07343(43)# 0.8(2) ms SF (various) 0+
α (rare) 238Cf
243Fm 100 143 243.07447(23)# 231(9) ms α (91%) 239Cf 7/2-#
SF (9%) (various)
β+ (rare) 243Es
244Fm 100 144 244.07404(22)# 3.12(8) ms SF (99%) (various) 0+
α (1%) 240Cf
245Fm 100 145 245.07535(21)# 4.2(13) s α (95.7%) 241Cf 1/2+#
β+ (4.2%) 245Es
SF (.13%) (various)
246Fm 100 146 246.075350(17) 1.54(4) s α (85%) 242Cf 0+
β+ (10%) 246Es
β+, SF (10%) (various)
SF (4.5%) (various)
247Fm 100 147 247.07695(12)# 31(1) s α (>50%) 243Cf (7/2+)
β+ (<50%) 247Es
248Fm 100 148 248.077186(9) 35.1(8) s α (93%) 244Cf 0+
β+ (7%) 248Es
SF (.10%) (various)
249Fm 100 149 249.078928(7) 1.6(1) min β+ (85%) 249Es (7/2+)#
α (15%) 245Cf
250Fm 100 150 250.079521(9) 30.4(15) min α (90%) 246Cf 0+
EC (10%) 250Es
SF (6.9×10−3%) (various)
250mFm 1199.2(10) keV 1.92(5) s IT 250Fm (8-)
251Fm 100 151 251.081540(16) 5.30(8) h β+ (98.2%) 251Es (9/2-)
α (1.8%) 247Cf
251mFm 200.09(11) keV 21.1(16) µs (5/2+)
252Fm 100 152 252.082467(6) 25.39(4) h α (99.99%) 248Cf 0+
SF (.0023%) (various)
β+β+ (rare) 252Cf
253Fm 100 153 253.085185(4) 3.00(12) d EC (88%) 253Es (1/2)+
α (12%) 249Cf
254Fm 100 154 254.0868544(30) 3.240(2) h α (99.94%) 250Cf 0+
SF (.0592%) (various)
255Fm 100 155 255.089964(5) 20.07(7) h α 251Cf 7/2+
SF (2.4×10−5%) (various)
256Fm 100 156 256.091774(8) 157.6(13) min SF (91.9%) (various) 0+
α (8.1%) 252Cf
257Fm[n 2] 100 157 257.095106(7) 100.5(2) d α (99.79%) 253Cf (9/2+)
SF (.21%) (various)
258Fm 100 158 258.09708(22)# 370(14) µs SF (various) 0+
259Fm 100 159 259.1006(3)# 1.5(3) s SF (various) 3/2+#
260Fm[n 3][n 4] 100 160 260.10281(55)# 4 ms 0+
  1. ^ Abbreviations:
    EC: Electron capture
    IT: Isomeric transition
    SF: Spontaneous fission
  2. ^ Heaviest nuclide produced via neutron capture
  3. ^ Discovery of this isotope is unconfirmed
  4. ^ Not directly synthesized, occurs as decay product of 260Md

Notes[edit]

  • 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.
  • The isotopes denoted by an isotope number with an "m" are metastable isomers.

Chronology of isotope discovery[edit]

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) [2]

260Fm? was not confirmed in 1997.

References[edit]

  1. ^ http://www.nucleonica.net/unc.aspx
  2. ^ see mendelevium


Isotopes of einsteinium Isotopes of fermium Isotopes of mendelevium
Table of nuclides