Isotopes of oganesson

Oganesson (₁₁₈Og) is a synthetic element created in particle accelerators, and thus a standard atomic weight cannot be given. Like all synthetic elements, it has no stable isotopes. The first (and so far only) isotope to be synthesized was ²⁹⁴Og in 2002 and 2005; it has a half-life of 700 microseconds. An unconfirmed isotope, ²⁹⁵Og, may have been observed in 2011 with a longer half-life of 181 milliseconds.

Main isotopes of oganesson (₁₁₈Og)

Iso­tope Decay abun­dance half-life (t1/2) mode pro­duct ^294Og[1] syn 700 μs α ^290Lv SF ^295Og[2] syn 181 ms? α ^291Lv

List of isotopes

Nuclide	Z	N	Isotopic mass (Da) [n 1][n 2]	Half-life	Decay mode [n 3]	Daughter isotope	Spin and parity
^294Og	118	176	294.21392(71)#	700 μs	α	^290Lv	0+
					SF	(various)
^295Og[n 4]	118	177	295.21624(69)#	181 ms[2]	α	^291Lv

1. ( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
2. # – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).
3. Modes of decay:

   SF:	Spontaneous fission

4. Not directly synthesized, occurs in decay chain of ²⁹⁹Ubn; unconfirmed

Nucleosynthesis

Target-projectile combinations leading to Z=118 compound nuclei

The below table contains various combinations of targets and projectiles that could be used to form compound nuclei with Z=118.

Target	Projectile	CN	Attempt result
^208Pb	^86Kr	^294Og	Failure to date
^238U	^58Fe	^296Og	Reaction yet to be attempted
^248Cm	^50Ti	^298Og	Failure to date
^250Cm	^50Ti	^300Og	Reaction yet to be attempted
^249Cf	^48Ca	^297Og	Successful reaction
^250Cf	^48Ca	^298Og	Failure to date
^251Cf	^48Ca	^299Og	Failure to date
^252Cf	^48Ca	^300Og	Reaction yet to be attempted

²⁰⁸Pb(⁸⁶Kr,xn)^294-xOg

In 1999, a team led by Victor Ninov at the Lawrence Berkeley National Laboratory performed this experiment, as a 1998 calculation by Robert Smolańczuk suggested a promising outcome. After eleven days of irradiation, three events of ²⁹³Og and its alpha decay products were reported in this reaction; this was the first reported discovery of element 118 and then-unknown element 116.[3]

The following year, they published a retraction after researchers at other laboratories were unable to duplicate the results and the Berkeley lab could not duplicate them either.[4] In June 2002, the director of the lab announced that the original claim of the discovery of these two elements had been based on data fabricated by principal author Victor Ninov.[5][6] Newer experimental results and theoretical predictions have confirmed the exponential decrease in cross-sections with lead and bismuth targets as the atomic number of the resulting nuclide increases.[7]

²⁴⁹Cf(⁴⁸Ca,xn)^297-xOg (x=3)

Following successful experiments utilizing calcium-48 projectiles and actinide targets to generate elements 114 and 116,[8] the search for element 118 was first performed at the Joint Institute for Nuclear Research (JINR) in 2002. One or two atoms of ²⁹⁴Og were produced in the 2002 experiment, and two more atoms were produced in a 2005 confirmation run. The discovery of element 118 was announced in 2006.[1]

Because of the very small fusion reaction probability (the fusion cross section is ~0.3–0.6 pb), the experiment took four months and involved a beam dose of 2.5×10¹⁹ calcium ions that had to be shot at the californium target to produce the first recorded event believed to be the synthesis of oganesson.[9] Nevertheless, researchers were highly confident that the results were not a false positive; the chance that they were random events was estimated to be less than one part in 100,000.[10]

In a 2012 experiment aimed at the confirmation of tennessine, one alpha decay chain was attributed to ²⁹⁴Og. This synthesis event resulted from the population of ²⁴⁹Cf in the target as the decay product of the ²⁴⁹Bk target (half-life 330 days); the cross section and decays were consistent with previously reported observations of ²⁹⁴Og.[8]

From October 1, 2015 until April 6, 2016, the team at the JINR conducted a search for new isotopes of oganesson using a ⁴⁸Ca beam and a target comprising a mixture of ²⁴⁹Cf (50.7%), ²⁵⁰Cf (12.9%), and ²⁵¹Cf (36.4%). The experiment was performed at 252 MeV and 258 MeV beam energies. One event of ²⁹⁴Og was found at the lower beam energy, while no decays of oganesson isotopes were found at the higher beam energy; a cross section of 0.9 pb for the ²⁴⁹Cf(⁴⁸Ca,3n) was estimated.[11]

^250,251Cf(⁴⁸Ca,xn)^298,299-xOg

In the same experiment, these reactions were performed in a search for ²⁹⁵Og and ²⁹⁶Og. No events attributable to a reaction with the ²⁵⁰Cf or ²⁵¹Cf portions of the target were found. A repeat of this experiment was planned for 2017–2018.[11]

²⁴⁸Cm(⁵⁰Ti,xn)^298-xOg

This reaction was originally planned to be tested at the JINR and RIKEN in 2017–2018, as it uses the same ⁵⁰Ti projectile as planned experiments leading to elements 119 and 120.[12] A search beginning in summer 2016 at RIKEN for ²⁹⁵Og in the 3n channel of this reaction was unsuccessful, though the study is planned to resume; a detailed analysis and cross section limit were not provided.[13][14]

Theoretical calculations

Theoretical calculations done on the synthetic pathways for, and the half-life of, other isotopes have shown that some could be slightly more stable than the synthesized isotope ²⁹⁴Og, most likely ²⁹³Og, ²⁹⁵Og, ²⁹⁶Og, ²⁹⁷Og, ²⁹⁸Og, ³⁰⁰Og and ³⁰²Og.[15][16][17] Of these, ²⁹⁷Og might provide the best chances for obtaining longer-lived nuclei,[15][17] and thus might become the focus of future work with this element. Some isotopes with many more neutrons, such as some located around ³¹³Og, could also provide longer-lived nuclei.[18]

Theoretical calculations on evaporation cross sections

The below table contains various targets-projectile combinations for which calculations have provided estimates for cross section yields from various neutron evaporation channels. The channel with the highest expected yield is given.

DNS = Di-nuclear system; 2S = Two-step; σ = cross section

Target	Projectile	CN	Channel (product)	σ max	Model	Ref
^208Pb	^86Kr	^294Og	1n (^293Og)	0.1 pb	DNS	[19]
^208Pb	^85Kr	^293Og	1n (^292Og)	0.18 pb	DNS	[19]
^246Cm	^50Ti	^296Og	3n (^293Og)	40 fb	2S	[20]
^244Cm	^50Ti	^294Og	2n (^292Og)	53 fb	2S	[20]
^252Cf	^48Ca	^300Og	3n (^297Og)	1.2 pb	DNS	[21]
^251Cf	^48Ca	^299Og	3n (^296Og)	1.2 pb	DNS	[21]
^249Cf	^48Ca	^297Og	3n (^294Og)	0.3 pb	DNS	[21]

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.
  • 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

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