Seaborgium (₁₀₆Sg) is asynthetic element and so has nostable isotopes. Astandard atomic weight cannot be given. The firstisotope to be synthesized was²⁶³Sg in 1974. There are thirteen knownradioisotopes from²⁵⁸Sg to²⁷¹Sg and five knownisomers (^259mSg,^261mSg,^263mSg,^265mSg, and^267mSg). The longest-lived isotopes are²⁶⁷Sg with ahalf-life of 9.8 minutes and²⁶⁹Sg with a half-life of 5 minutes. Due to a low number of measurements, and the consequent overlapping measurement uncertainties at the confidence level corresponding to onestandard deviation, a definite assignment of the most stable isotope cannot be made.

This section deals with the synthesis of nuclei of seaborgium by so-called "cold" fusion reactions. These are processes that create compound nuclei at low excitation energy (~10–20 MeV, hence "cold"), leading to a higher probability of survival from fission. The excited nucleus then decays to the ground state via the emission of one or two neutrons only.

The first attempt to synthesise seaborgium in cold fusion reactions was performed in September 1974 by aSoviet team led byG. N. Flerov at theJoint Institute for Nuclear Research atDubna. They reported producing a 0.48 s spontaneous fission (SF) activity, which they assigned to the isotope²⁵⁹Sg. Based on later evidence it was suggested that the team most likely measured the decay of²⁶⁰Sg and its daughter²⁵⁶Rf. The TWG concluded that, at the time, the results were insufficiently convincing.^[9]

The Dubna team revisited this problem in 1983–1984 and were able to detect a 5 ms SF activity assigned directly to²⁶⁰Sg.^[9]

The team at GSI studied this reaction for the first time in 1985 using the improved method of correlation of genetic parent-daughter decays. They were able to detect²⁶¹Sg (x=1) and²⁶⁰Sg and measured a partial 1n neutron evaporation excitation function.^[10]

In December 2000, the reaction was studied by a team atGANIL, France; they were able to detect 10 atoms of²⁶¹Sg and 2 atoms of²⁶⁰Sg to add to previous data on the reaction.

After a facility upgrade, the GSI team measured the 1n excitation function in 2003 using a metallic lead target. Of significance, in May 2003, the team successfully replaced the lead-208 target with more resistantlead(II) sulfide targets (PbS), which will allow more intense beams to be used in the future. They were able to measure the 1n,2n and 3n excitation functions and performed the first detailed alpha-gamma spectroscopy on the isotope²⁶¹Sg. They detected ~1600 atoms of the isotope and identified new alpha lines as well as measuring a more accurate half-life and new EC and SF branchings. Furthermore, they were able to detect the K X-rays from the daughterrutherfordium isotope for the first time. They were also able to provide improved data for²⁶⁰Sg, including the tentative observation of an isomeric level. The study was continued in September 2005 and March 2006. The accumulated work on²⁶¹Sg was published in 2007.^[11] Work in September 2005 also aimed to begin spectroscopic studies on²⁶⁰Sg.

The team at the LBNL recently restudied this reaction in an effort to look at the spectroscopy of the isotope²⁶¹Sg. They were able to detect a new isomer,^261mSg, decaying by internal conversion into theground state. In the same experiment, they were also able to confirm a K-isomer in the daughter²⁵⁷Rf, namely^257m2Rf.^[12]

The team at Dubna also studied this reaction in 1974 with identical results as for their first experiments with a lead-208 target. The SF activities were first assigned to²⁵⁹Sg and later to²⁶⁰Sg and/or²⁵⁶Rf. Further work in 1983–1984 also detected a 5 ms SF activity assigned to the parent²⁶⁰Sg.^[9]

The GSI team studied this reaction for the first time in 1985 using the method of correlation of genetic parent-daughter decays. They were able to positively identify²⁵⁹Sg as a product from the 2n neutron evaporation channel.^[10]

The reaction was further used in March 2005 using PbS targets to begin a spectroscopic study of the even-even isotope²⁶⁰Sg.

This reaction was studied in 1974 by the team at Dubna. It was used to assist them in their assignment of the observed SF activities in reactions using Pb-207 and Pb-208 targets. They were unable to detect any SF, indicating the formation of isotopes decaying primarily by alpha decay.^[9]

The team at Dubna also studied this reaction in their series of cold fusion reactions performed in 1974. Once again they were unable to detect any SF activities.^[9] The reaction was revisited in 2006 by the team at LBNL as part of their studies on the effect of the isospin of the projectile and hence the mass number of the compound nucleus on the yield of evaporation residues. They were able to identify²⁵⁹Sg and²⁵⁸Sg in their measurement of the 1n excitation function.^[13]

The team at Dubna also studied this reaction in their series of cold fusion reactions performed in 1974. Once again they were unable to detect any SF activities.^[9]In 1994, the synthesis of seaborgium was revisited using this reaction by the GSI team, in order to study the new even-even isotope²⁵⁸Sg. Ten atoms of²⁵⁸Sg were detected and decayed by spontaneous fission.

This section deals with the synthesis of nuclei of seaborgium by so-called "hot" fusion reactions. These are processes that create compound nuclei at high excitation energy (~40–50 MeV, hence "hot"), leading to a reduced probability of survival from fission and quasi-fission. The excited nucleus then decays to the ground state via the emission of 3–5 neutrons.

This reaction was first studied by Japanese scientists at the Japan Atomic Energy Research Institute (JAERI) in 1998. They detected aspontaneous fission activity, which they tentatively assigned to the new isotope²⁶⁴Sg or²⁶³Db, formed by EC of²⁶³Sg.^[14]In 2006, the teams at GSI and LBNL both studied this reaction using the method of correlation of genetic parent-daughter decays. The LBNL team measured an excitation function for the 4n,5n and 6n channels, whilst the GSI team were able to observe an additional 3n activity.^[15][16][17] Both teams were able to identify the new isotope²⁶⁴Sg, which decayed with a short lifetime byspontaneous fission.

In 1993, at Dubna, Yuri Lazarev and his team announced the discovery of long-lived²⁶⁶Sg and²⁶⁵Sg produced in the 4n and 5n channels of this nuclear reaction following the search for seaborgium isotopes suitable for a first chemical study.It was announced that²⁶⁶Sg decayed by 8.57 MeV alpha-particle emission with a projected half-life of ~20 s, lending strong support to the stabilising effect of theZ = 108,N = 162 closed shells.^[18]This reaction was studied further in 1997 by a team at GSI and the yield, decay mode and half-lives for²⁶⁶Sg and²⁶⁵Sg have been confirmed, although there are still some discrepancies. In the synthesis of²⁷⁰Hs (seehassium),²⁶⁶Sg was found to undergo exclusively SF with a short half-life (T_SF = 360 ms). It is possible that this is the ground state, (^266gSg) and that the other activity, produced directly, belongs to a high spin K-isomer,^266mSg, but further results are required to confirm this.

A recent re-evaluation of the decay characteristics of²⁶⁵Sg and²⁶⁶Sg has suggested that all decays to date in this reaction were in fact from²⁶⁵Sg, which exists in two isomeric forms. The first,^265aSg has a principal alpha-line at 8.85 MeV and a calculated half-life of 8.9 s, while^265bSg has a decay energy of 8.70 MeV and a half-life of 16.2 s. Both isomeric levels are populated when produced directly. Data from the decay of²⁶⁹Hs indicates that^265bSg is produced during the decay of²⁶⁹Hs and that^265bSg decays into the shorter-lived^261gRf isotope. This contradicts the assignment of the long-lived alpha activity to²⁶⁶Sg, instead suggesting that²⁶⁶Sg undergoes fission in a short time.

Regardless of these assignments, the reaction has been successfully used in the recent attempts to study the chemistry of seaborgium (see below).

The synthesis of seaborgium was first realized in 1974 by the LBNL/LLNL team.^[19] In their discovery experiment, they were able to apply the new method of correlation of genetic parent-daughter decays to identify the new isotope²⁶³Sg. In 1975, the team at Oak Ridge were able to confirm the decay data but were unable to identify coincident X-rays in order to prove that seaborgium was produced. In 1979, the team at Dubna studied the reaction by detection of SF activities. In comparison with data from Berkeley, they calculated a 70% SF branching for²⁶³Sg. The original synthesis and discovery reaction was confirmed in 1994 by a different team at LBNL.^[20]

Isotopes of seaborgium have also been observed in the decay of heavier elements. Observations to date are summarised in the table below:

Initial work identified an 8.63 MeV alpha-decaying activity with a half-life of ~21 s and assigned to the ground state of²⁶⁶Sg. Later work identified a nuclide decaying by 8.52 and 8.77 MeV alpha emission with a half-life of ~21 s, which is unusual for an even-even nuclide. Recent work on the synthesis of²⁷⁰Hs identified²⁶⁶Sg decaying by SF with a short 360 ms half-life. The recent work on²⁷⁷Cn and²⁶⁹Hs has provided new information on the decay of²⁶⁵Sg and²⁶¹Rf. This work suggested that the initial 8.77 MeV activity should be reassigned to²⁶⁵Sg. Therefore, the current information suggests that the SF activity is the ground state and the 8.52 MeV activity is a high spin K-isomer. Further work is required to confirm these assignments. A recent re-evaluation of the data has suggested that the 8.52 MeV activity should be associated with²⁶⁵Sg and that²⁶⁶Sg only undergoes fission.

The recent direct synthesis of²⁶⁵Sg resulted in four alpha-lines at 8.94, 8.84, 8.76 and 8.69 MeV with a half-life of 7.4 seconds. The observation of the decay of²⁶⁵Sg from the decay of²⁷⁷Cn and²⁶⁹Hs indicated that the 8.69 MeV line may be associated with an isomeric level with an associated half-life of ~ 20 s. It is plausible that this level is causing confusion between assignments of²⁶⁶Sg and²⁶⁵Sg since both can decay to fissioning rutherfordium isotopes.

A recent re-evaluation of the data has indicated that there are indeed two isomers, one with a principal decay energy of 8.85 MeV with a half-life of 8.9 s, and a second isomer that decays with energy 8.70 MeV with a half-life of 16.2 s.

The discovery synthesis of²⁶³Sg resulted in an alpha-line at 9.06 MeV.^[19] Observation of this nuclide by decay of^271gDs,^271mDs and²⁶⁷Hs has confirmed an isomer decaying by 9.25 MeV alpha emission. The 9.06 MeV decay was also confirmed. The 9.06 MeV activity has been assigned to the ground state isomer with an associated half-life of 0.3 s. The 9.25 MeV activity has been assigned to an isomeric level decaying with a half-life of 0.9 s.

Recent work on the synthesis of^271g,mDs was resulted in some confusing data regarding the decay of²⁶⁷Hs. In one such decay,²⁶⁷Hs decayed to²⁶³Sg, which decayed by alpha emission with a half-life of ~ 6 s. This activity has not yet been positively assigned to an isomer and further research is required.

In the claimed synthesis of²⁹³Og in 1999 the isotope²⁶⁹Sg was identified as a daughter product. It decayed by 8.74 MeV alpha emission with a half-life of 22 s. The claim was retracted in 2001. This isotope was finally created in 2010.

The table below provides cross-sections and excitation energies for cold fusion reactions producing seaborgium isotopes directly. Data in bold represents maxima derived from excitation function measurements. + represents an observed exit channel.

The table below provides cross-sections and excitation energies for hot fusion reactions producing seaborgium isotopes directly. Data in bold represents maxima derived from excitation function measurements. + represents an observed exit channel.

• Isotope masses from:
  • M. Wang; G. Audi; A. H. Wapstra; F. G. Kondev; M. MacCormick; X. Xu; et al. (2012)."The AME2012 atomic mass evaluation (II). Tables, graphs and references"(PDF).Chinese Physics C.36 (12):1603–2014.Bibcode:2012ChPhC..36....3M.doi:10.1088/1674-1137/36/12/003.hdl:11858/00-001M-0000-0010-23E8-5.S2CID 250839471.
  • 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.
  • GSI (2011)."Superheavy Element Research at GSI"(PDF). GSI. Retrieved24 August 2012.

• Isotopes of seaborgium
• Seaborgium
• Lists of isotopes by element

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