Isotopes of seaborgium

Seaborgium (₁₀₆Sg) is a synthetic element, and thus a standard atomic weight cannot be given. Like all synthetic elements, it has no stable isotopes. The first isotope to be synthesized was ^263mSg in 1974. There are 12 known radioisotopes from ²⁵⁸Sg to ²⁷¹Sg and 2 known isomers (^261mSg and ^263mSg). The longest-lived isotope is ²⁶⁹Sg with a half-life of 3.1 minutes.

List of isotopes

1. ↑ Abbreviations:
   EC: Electron capture
   IC: Internal conversion
   SF: Spontaneous fission
2. ↑ Not directly synthesized, occurs in the decay chain of ²⁷⁰Ds
3. ↑ Not directly synthesized, occurs in the decay chain of ²⁷¹Ds
4. ↑ Not directly synthesized, occurs as decay product of ²⁷⁰Hs
5. ↑ Not directly synthesized, occurs as decay product of ²⁷¹Hs
6. ↑ Not directly synthesized, occurs in the decay chain of ²⁸⁵Fl
7. ↑ Not directly synthesized, occurs in the decay chain of ²⁸⁷Fl

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.

History of synthesis of isotopes by cold fusion

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.

²⁰⁸Pb(⁵⁴Cr,xn)^262−xSg (x=1,2,3)

The first attempt to synthesise seaborgium in cold fusion reactions was performed in September 1974 by a Soviet team led by G. N. Flerov at the Joint Institute for Nuclear Research at Dubna. They reported producing a 0.48 s spontaneous fission (SF) activity, which they assigned to the isotope ²⁵⁹Sg. Based on later evidence it was suggested that the team most likely measured the decay of ²⁶⁰Sg and its daughter ²⁵⁶Rf. The TWG concluded that, at the time, the results were insufficiently convincing.^[3]

The Dubna team revisited this problem in 1983–1984 and were able to detect a 5 ms SF activity assigned directly to ²⁶⁰Sg.^[3]

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) and ²⁶⁰Sg and measured a partial 1n neutron evaporation excitation function. ^[4]

In December 2000, the reaction was studied by a team at GANIL, France and were able to detect 10 atoms of ²⁶¹Sg and 2 atoms of ²⁶⁰Sg 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 resistant lead(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 isotope ²⁶¹Sg. 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 daughter rutherfordium element for the first time. They were also able to provide improved data for ²⁶⁰Sg, including the tentative observation of an isomeric level. The study was continued in September 2005 and March 2006. The accumulated work on ²⁶¹Sg was published in 2007.^[5] Work in September 2005 also aimed to begin spectroscopic studies on ²⁶⁰Sg.

The team at the LBNL recently restudied this reaction in an effort to look at the spectroscopy of the isotope ²⁶¹Sg. They were able to detect a new isomer,^261mSg, decaying by internal conversion into the ground state. In the same experiment, they were also able to confirm a K-isomer in the daughter²⁵⁷Rf, namely ^257m2Rf.^[6]

²⁰⁷Pb(⁵⁴Cr,xn)^261−xSg (x=1,2)

The team at Dubna also studied this reaction in 1974 with identical results as for their first experiments with a Pb-208 target. The SF activities were first assigned to ²⁵⁹Sg and later to ²⁶⁰Sg and/or ²⁵⁶Rf. Further work in 1983–1984 also detected a 5 ms SF activity assigned to the parent²⁶⁰Sg.^[3]

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.^[4]

The reaction was further used in March 2005 using PbS targets to begin a spectroscopic study of the even-even isotope ²⁶⁰Sg.

²⁰⁶Pb(⁵⁴Cr,xn)^260−xSg

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.^[3]

²⁰⁸Pb(⁵²Cr,xn)^260−xSg (x=1,2)

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.^[3] 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 identify ²⁵⁹Sg and ²⁵⁸Sg in their measurement of the 1n excitation function.^[7]

²⁰⁹Bi(⁵¹V,xn)^260−xSg (x=2)

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.^[3] In 1994, the synthesis of seaborgium was revisited using this reaction by the GSI team, in order to study the new even-even isotope ²⁵⁸Sg. Ten atoms of²⁵⁸Sg were detected and decayed by spontaneous fission.

History of synthesis of isotopes by hot fusion

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.

²³⁸U(³⁰Si,xn)^268−xSg (x=3,4,5,6)

This reaction was first studied by Japanese scientists at the Japan Atomic Energy Research Institute (JAERI) in 1998. They detected a spontaneous fission activity, which they tentatively assigned to the new isotope ²⁶⁴Sg or ²⁶³Db, formed by EC of ²⁶³Sg.^[8] 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.^[9][10][11] Both teams were able to identify the new isotope ²⁶⁴Sg, which decayed with a short lifetime by spontaneous fission.

²⁴⁸Cm(²²Ne,xn)^270−xSg (x=4?,5)

In 1993, at Dubna, Yuri Lazarev and his team announced the discovery of long-lived ²⁶⁶Sg and ²⁶⁵Sg 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 that ²⁶⁶Sg decayed by 8.57 MeV alpha-particle emission with a projected half-life of ~20 s, lending strong support to the stabilising effect of the Z=108,N=162 closed shells.^[12] This reaction was studied further in 1997 by a team at GSI and the yield, decay mode and half-lives for ²⁶⁶Sg and ²⁶⁵Sg have been confirmed, although there are still some discrepancies. In the recent synthesis of ²⁷⁰Hs (see hassium), 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 of ²⁶⁵Sg and ²⁶⁶Sg 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, whilst^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 of ²⁶⁹Hs and that ^265bSg decays into the shorter-lived^261gRf isotope. This means that the observation of ²⁶⁶Sg as a long-lived alpha emitter is retracted and that it does indeed undergo 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).

²⁴⁹Cf(¹⁸O,xn)^267−xSg (x=4)

The synthesis of seaborgium was first realized in 1974 by the LBNL/LLNL team.^[13] In their discovery experiment, they were able to apply the new method of correlation of genetic parent-daughter decays to identify the new isotope ²⁶³Sg. 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 for ²⁶³Sg. The original synthesis and discovery reaction was confirmed in 1994 by a different team at LBNL.^[14]

Synthesis of isotopes as decay products

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

Chronology of isotope discovery

Isotopes

There are 12 known isotopes of seaborgium (excluding meta-stable and K-spin isomers). The longest-lived is currently ²⁷¹Sg, which decays through alpha decay and spontaneous fission, with a half-life of around 2.4 minutes. The shortest-lived isotope is ²⁵⁸Sg, which also decays through alpha decay and spontaneous fission. It has a half-life of 2.9 ms.

Isomerism in seaborgium nuclides

²⁶⁶Sg

Initial work identified an 8.63 MeV alpha-decaying activity with a half-life of ~21 s and assigned to the ground state of ²⁶⁶Sg. 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 of ²⁷⁰Hs identified ²⁶⁶Sg decaying by SF with a short 360 ms half-life. The recent work on ²⁷⁷Cn and ²⁶⁹Hs has provided new information on the decay of ²⁶⁵Sg and ²⁶¹Rf. This work suggested that the initial 8.77 MeV activity should be reassigned to ²⁶⁵Sg. 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 with ²⁶⁵Sg and that ²⁶⁶Sg only undergoes fission.

²⁶⁵Sg

The recent direct synthesis of ²⁶⁵Sg 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 of ²⁶⁵Sg from the decay of ²⁷⁷Cn and ²⁶⁹Hs 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 of ²⁶⁶Sg and ²⁶⁵Sg 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.

²⁶³Sg

The discovery synthesis of ²⁶³Sg resulted in an alpha-line at 9.06 MeV.^[13] Observation of this nuclide by decay of ^271gDs,^271mDs and ²⁶⁷Hs 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 of ²⁶⁷Hs. In one such decay, ²⁶⁷Hs decayed to ²⁶³Sg, 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.

Spectroscopic decay schemes for seaborgium isotopes

²⁶¹Sg

This is the currently accepted decay scheme for ²⁶¹Sg from the study by Streicher et al. at GSI in 2003–2006

Retracted isotopes

²⁶⁹Sg

In the claimed synthesis of ²⁹³Og in 1999 the isotope ²⁶⁹Sg 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.^[18] This isotope was finally created in 2010.

Chemical yields of isotopes

Cold fusion

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.

Hot fusion

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.

References

• 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. 
  • 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. Archived from the original (PDF) on 2008-09-23. 
• 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. Archived from the original (PDF) on 2008-09-23. 
  • 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. 
  • GSI (2011). "Superheavy Element Research at GSI" (PDF). GSI. Retrieved August 2012.  Check date values in: |access-date= (help)