Discovery of the element with atomic number Z = 118 completing the 7^th row of the periodic table (IUPAC Technical Report)

1 Introduction

The working party of independent experts drawn from IUPAC and IUPAP has addressed recent results of experiments searching for new heavy elements. Laboratories involved in the studies were contacted requesting papers relevant to the discoveries’ consideration by the Joint Working Party on Discovery of Elements (JWP). The deadline for submission was 31 May 2012. Within the JWP, extensive review was conducted amongst members via electronic communications. This report is a continuation of the effort [1] that addressed elements 113, 115, and 117.

2 Criteria

The criteria that must be satisfied for the discovery of a new chemical element to be recognized were established by the IUPAP/IUPAC Transfermium Working Group (TWG) in 1991 [2]. Among those detailed criteria, sections particularly relevant to current deliberations follow. The numbers are the same as in the original work [2].

1. Discovery of a chemical element is the experimental demonstration, beyond reasonable doubt, of the existence of a nuclide with an atomic number Z not identified before, existing for at least 10^–14 s. (The exact Z need not be determined, only that it is different from all Z-values observed before.)

2. The TWG realizes that the term ‘reasonable doubt’ is necessarily somewhat vague. Confirmation demands reproducibility. In the case of the new elements the TWG attaches considerable importance to reproducibility and would indeed like to be able to suggest that no new element should be recognized officially until the data upon which the claim is based have been reproduced, preferably in another laboratory and preferably by a different technique. However, it would appear unreasonable to apply such a demand of demonstrated reproducibility in all rigidity. We do not believe that recognition of the discovery of a new element should always be held up until the experiment or its equivalent have been repeated, desirable in principle as this may be. However, we would waive this requirement only in cases where the data are of such a nature that no reasonable doubt is possible (for instance for data with a high degree of internal redundancy and of the highest quality), and under circumstances where a repetition of the experiment would imply an unreasonable burden.

3. Most assignment properties do not alone allow sufficient certainty for assigning a unique value to Z; a combination of them may.

4. The TWG has a strong preference for publication in regular journals of international standing. However, it does not wish to take up a rigid position on this matter and would not wish to exclude from admissibility any form of bona fide publication of wide general accessibility.

These have served effectively as guiding principles since then by subsequent JWPs [1, 3–6].

In our considerations, positive factors include low background events, cross-bombardments, excitation functions, internal reproducibility in productions and in decays, physicochemical behavior, spatial correlations of evaporation residues and subsequent decays, and separators distinguishing Z-values. When such favorable properties occur in combination, the case may be regarded as greatly strengthened. Observation of characteristic K- and /or L-X-rays would obviously be beneficial but are currently still not an established part of the data used for assignment. Factors that are troublesome to the JWP, but not necessarily invalidating, include missing anchors to known/familiar nuclei, irreproducibility, unpersuasive chemistry, and high background situations.

We would like to point out that for the newest superheavy elements cross-bombardment experiments have achieved increasing importance. Cross-bombardments were established as one of the criteria for discovery in 1991 by the TWG [2] and their growing influence has been extensively deliberated by the JWPs. The key to this importance of cross-bombardment lies in the fact that even in the case of missing anchors the Z of the superheavy can be reliably assigned as the sum of the Zs of the target and projectile, if different combinations of projectile and target are found to produce the same states. Such combinations essentially circumvent possible misidentifications of Z.

3 Discovery profiles

The JWP report follows discovery profiles. The concise profile begins with the pertinent content from earlier reports [1, 3–5]. An historical account of the relevant publications is given appended with the JWP’s consensus opinion(s) as to the value of the evidence on the basis of the criteria. Our resources for this report were articles submitted by 31 May 2012 by research groups and laboratories in response to formal solicitations by IUPAC. Additional relevant publications routinely available in research libraries or through modern electronic search techniques were also sought by the JWP.

Figure 1 is a convenient map of the superheavy nuclide region from the literature. The dashed N,Z grid coordinate at 184,114 indicates the composition of the hypothesized doubly magic stable nuclide ²⁹⁸Fl.

Fig. 1:

Map of the region of superheavy nuclei relevant to this report including the reactions used to synthesize the various nuclides. (Adapted from [7].) The colors indicate different decay modes for nuclide Z/A along with its half life in milliseconds (ms), seconds (s), minutes (m) or hours (h) following the convention and data of [7], q.v.

3.1 Discovery profile for Z=118

The 2006 collaboration of Oganessian et al. [8] observed three concordant events from the fusion of ⁴⁸Ca with ²⁴⁹Cf reported to produce ²⁹⁴118. The product underwent decay with average α-particle energy of 11.7 MeV in 1.3 ms to ²⁹⁰Lv which, in all events, decayed with average α-energy 10.8 MeV in 14 ms to ²⁸⁶Fl. Two chains of the latter isotope terminated by spontaneous fission, and the third event emitted a 10.16 MeV α-particle, all with an average lifetime of 0.23 s. The remaining event terminated by spontaneous fission of ²⁸²Cn in 3 ms. The alpha-particle energies in MeV and the lifetime in milliseconds are tabulated below (Table 1) for each chain. One of these chains was reported by Oganessian et al. in 2002 [9].

Table 1

Properties (E_α/MeV and lifetime in milliseconds) of the alpha-decay chains commencing with ²⁹⁴118 and terminating in spontaneous fission (SF).

Nuclide	Ref. [8] (2006)						Ref. [14] (2012)
	Eα/MeV	Lifetime/ms	Eα/MeV	Lifetime/ms	Eα/MeV	Lifetime/ms	Eα/MeV	Lifetime/ms
294118	11.65±0.06	2.5	11.65±0.10	0.5	11.8±0.5	0.8	11.68±0.06	0.14
290Lv	10.71±0.17	42	10.84±0.10	1	10.80±0.09	0.1	10.85±0.07	29
286Fl	SF	520	SF	11	10.16±0.09	150	SF	3.5
282Cn					SF	2.7

PRIOR JWP ASSESSMENT: The three events reported for the Z=118 isotope have very good internal redundancy but, with no anchor to known nuclei, do not satisfy the criteria for discovery.

A 2004 collaboration of Oganessian et al. [10] reported the observation of one complete alpha-decay chain assigned to the ²⁹⁰Lv isotope produced by the ²⁴⁵Cm(⁴⁸Ca, 3n) reaction. The ²⁹⁰Lv alpha energy of 10.88 MeV is in moderate agreement with the 2002 measurement of this intermediate [8]. Another Oganessian et al. collaboration in 2004 [11] employed the ²⁴²Pu(⁴⁸Ca, 3n) fusion to produce four chains with ²⁸⁶Fl with an average alpha-particle energy of 10.21 MeV. The third ²⁹⁴118 chain in [8] terminates with ²⁸⁶Fl decay in agreement with that value.

In the previously cited 2006 Oganessian collaboration [8], an additional study, of the ²⁴⁵Cm(⁴⁸Ca, 4n) fusion reaction, was used to produce nine chains following the path ²⁹⁰Lv(α) → ²⁸⁶Fl(α) → ²⁸²Cn(SF) of which five were complete. The average α-particle energies were 10.90 MeV and 10.15 MeV, respectively, and are in excellent internal agreement and also concordant with the 2004 results.

A (LBNL) Lawrence-Berkeley collaboration of Stavstra et al. in 2009 [12] measured one alpha-decay chain from the fusion of ²⁴²Pu with ⁴⁸Ca and observed ²⁸⁶Fl produced by the 4n evaporation channel followed by spontaneous fission of ²⁸²Cn. The ²⁸⁶Fl observed α-particle energy of 10.23 MeV is in excellent agreement with the Oganessian collaborations above.

A newer collaboration of Ellison et al. at Berkeley in 2010 [13] also determined one alpha-decay chain from the fusion of ²⁴²Pu with ⁴⁸Ca and observed ²⁸⁶Fl produced by the 4n evaporation channel followed by spontaneous fission of ²⁸²Cn. ²⁸⁶Fl’s α-particle energy of 10.31 MeV is in agreement with the earlier collaborations above.

In 2012, the Oganessian collaboration observed one 2 α-decay chain, starting with ²⁹⁴118 produced in the fusion of ⁴⁸Ca with ²⁴⁹Cf followed by SF [14]. The α-particle energies match very well indeed with the decay chain seen in their previous (2006) collaboration as shown in the right column of the above Table 1. The original goal of the experiment was the study of ²⁴⁹Bk+⁴⁸Ca, but the 330 d half-life of ²⁴⁹Bk resulted in up to 28% of the target as ²⁴⁹Cf. The new chain of ²⁹⁴118 was observed due to the target decay.

The case for discovery of Z=118 is enhanced by the several cross bombardments independently producing descendants ²⁹⁰Lv and ²⁸⁶Fl as displayed in the diagram, Fig. 2, showing the results of fusion reactions of ⁴⁸Ca with ²⁴⁵Cm or ²⁴²Pu discussed above. The two experiments [8, 14] are consistent in their connection to known isotopes of Lv, Fl and Cn. This provides solid evidence that the new atomic number has been observed.

Fig. 2:

Summary of the decay relevant work in the assignment of Z=118. The cross bombardments for the confirmation of identification of Z=118 daughters provide firm evidence that a new atomic number, distinct from previously observed atomic numbers, has been identified. Alpha particle energies in MeV and lifetimes in milliseconds (ms) or seconds (s) are shown.

JWP ASSESSMENT: The 2006 Dubna–Livermore collaboration of Oganessian et al. produced three concordant decay chains commencing with ²⁹⁴118. This result was confirmed in 2012. Three other independent heavy element fusion studies served to identify and confirm the existence and decay properties of ²⁹⁴118 descendants ²⁹⁰Lv and ²⁸⁶Fl serving to link atomic numbers through cross bombardments. The Dubna–Livermore 2006 collaboration [8] has satisfied the criteria for discovery and its claim is now acknowledged as validated.

4 Comments

Identification of Z,N becomes more and more difficult in the pursuit of superheavy elements with heavy ion fusions. Development of direct physical methods to determine Z, particularly X-ray measurements as often envisaged, is of growing importance as is the desirability of employing exotic radioactive nuclear beams to access nuclides that are currently synthesized via the hot fusion route.

5 Summary of JWP conclusions

The claim of the Dubna–Livermore 2006 collaboration for discovery of the element with atomic number Z=118 is acknowledged as validated.

6 Membership of sponsoring IUPAC body

Membership of the Inorganic Chemistry Division Committee for the period 2012–2015 was as follows:

2012–2013: President: Loss, Robert D (Australia); Vice President: Reedijk, Jan (Netherlands); Secretary: Leskelä, Markku (Finland); Titular members: Mathur, Sanjay (Germany); Drabik, Milan (Slovakia); Sakai, Ken (Japan); Holden, Norman E. (USA); Öhrström, Lars R. (Sweden); Karen, Pavel (Norway); Tshuva, Edit Y. (Israel); Associate members: Ding, Tiping (China/Beijing); Garcia-Martinez, Javier (Spain); Buchweishaija, Joseph (Tanzania); Rabinovich, Daniel (USA); Vannier, Rose-Noelle (France); Kiliç, Adem (Turkey); National representatives: Abdul Aziz, Farina (Malaysia); Trendafilova, Natasha (Bulgaria); Prugovečki, Biserka (Croatia); Chandrasekhar, V. (India); Youngme, Sujittra (Thailand); Toma, Henrique (Brasil).

2014–2015: President: Reedijk, Jan (Netherlands); Vice President: Öhrström, Lars (Sweden): Secretary: Leskelä, Markku (Finland); Past President: Loss, Robert D. (Australia): Titular Members: Ding, Tiping (China/Beijing); Drábik, Milan (Slovakia); Rabinovich, Daniel (USA); Tshuva, Edit Y. (Israel); Walczyk, Thomas R. (Switzerland); Wieser, Michael (Canada); Associate Members: Buchweishaija, Joseph (Tanzania); García-Martínez, Javier (Spain); Karen, Pavel (Norway); Kiliç, Adem (Turkey); Sakai, Ken (Japan); Vannier, Rose-Nöelle (France); National Representatives: Trendafilova, Natasha (Bulgaria); Prugovečki, Biserka (Croatia); Mathur, Sanjay (Germany); Chandrasekhar, V. (India); Armaleo, Lidia (Italy); Abdul Aziz, Yang F. (Malaysia); Badshah, Amin (Pakistan); Correira, João Galamba (Portugal); Kalmykov, Stepan N. (Russia); Meesuk, Ladda M. (Thailand).

This manuscript (PAC-REP-15-05-01) was prepared in the framework of IUPAC project 2012-047-1-200.