Ununhexium

Ununhexium (pronounced /uːnuːnˈhɛksiəm/ ( listen)[1] oon-oon-HEKS-ee-əm) is the temporary name of a synthetic superheavy element with the temporary symbol Uuh and atomic number 116.

It is placed as the heaviest member of group 16 (VIA) although a sufficiently stable isotope is not known at this time to allow chemical experiments to confirm its position as the heavier homologue to polonium.

It was first detected in 2000 and since the discovery about 30 atoms of ununhexium have been produced, either directly or as a decay product of ununoctium, and are associated with decays from the four neighbouring isotopes with masses 290–293. The most stable isotope to date is Uuh-293 with a half-life of ~60 ms.

History
De facto discovery

On July 19, 2000, scientists at Dubna (JLNR) detected a single decay from an atom of ununhexium following the irradiation of a Cm-248 target with Ca-48 ions. The results were published in December, 2000.[2] This 10.54 MeV alpha-emitting activity was originally assigned to 292Uuh due to the correlation of the daughter to previously assigned 288Uuq. However, that assignment was later altered to 289Uuq, and hence this activity was correspondingly changed to 293Uuh. Two further atoms were reported by the institute during their second experiment between April–May 2001.[3]

\( \,^{48}_{20}\mathrm{Ca} + \,^{248}_{96}\mathrm{Cm} \to \,^{296}_{116}\mathrm{Uuh} ^{*} \to \,^{293}_{116}\mathrm{Uuh} + 3\,^{1}_{0}\mathrm{n} \)

In the same experiment they also detected a decay chain which corresponded to the first observed decay of ununquadium and assigned to 289Uuq.[3] This activity has not been observed again in a repeat of the same reaction. However, its detection in this series of experiments indicates the possibility of the decay of an isomer of ununhexium, namely 293bUuh, or a rare decay branch of the already discovered isomer,293aUuh, in which the first alpha particle was missed. Further research is required to positively assign this activity.

The team repeated the experiment in April-May 2005 and detected 8 atoms of ununhexium. The measured decay data confirmed the assignment of the discovery isotope as 293Uuh. In this run, the team also observed 292Uuh in the 4n channel for the first time. [4]

In May 2009, the Joint Working Party reported on the discovery of copernicium and acknowledged the discovery of the isotope 283Cn.[5] This implies the de facto discovery of ununhexium, as 291Uuh (see below), from the acknowledgment of the data relating to the granddaughter 283Cn, although the actual discovery experiment may be determined as that above. An impending JWP report will discuss these issues further.

Naming

The element with atomic number 116 is historically known as eka-polonium. Ununhexium (Uuh) is a temporary IUPAC systematic element name. Research scientists usually refer to the element simply as element 116 (or E116).
Proposed names by claimants

According to IUPAC recommendations, the discoverer(s) of a new element has the right to suggest a name.[6] The JWP has not yet officially accepted the discovery of element 116 and so the naming process has not yet begun.
Future experiments

The team at Dubna have indicated plans to synthesize ununhexium using the reaction between plutonium-244 and titanium-50. This experiment will allow them to assess the feasibility of using projectiles with Z>20 required in the synthesis of superheavy elements with Z>118. Although initially scheduled for 2008, the reaction looking at the synthesis of evaporation residues has not been conducted to date.[7]

There are also plans to repeat the Cm-248 reaction at different projectile energies in order to probe the 2n channel, leading to the new isotope 294Uuh. In addition, they have future plans to complete the excitation function of the 4n channel product, 292Uuh, which will allow them to assess the stabilizing effect of the N=184 shell on the yield of evaporation residues.

The GSI also have plans to study the formation of 293,292Uuh in the 248Cm(48Ca,xn) reaction as a first step in their future program with a 248Cm target, aiming towards a synthesis of unbinilium.
Isotopes and nuclear properties
Nucleosynthesis
Target-projectile combinations leading to Z=116 compound nuclei

The below table contains various combinations of targets and projectiles which could be used to form compound nuclei with atomic number 116.

Target	Projectile	CN	Attempt result
²⁰⁸Pb	⁸²Se	²⁹⁰Uuh	Failure to date
²³²Th	⁵⁸Fe	²⁹⁰Uuh	Reaction yet to be attempted
²³⁸U	⁵⁴Cr	²⁹²Uuh	Failure to date
²⁴⁴Pu	⁵⁰Ti	²⁹⁴Uuh	Reaction yet to be attempted
²⁴⁸Cm	⁴⁸Ca	²⁹⁶Uuh	Successful reaction
²⁴⁶Cm	⁴⁸Ca	²⁹⁴Uuh	Reaction yet to be attempted
²⁴⁵Cm	⁴⁸Ca	²⁹³Uuh	Successful reaction
²⁴⁹Cf	⁴⁰Ar	²⁸⁹Uuh	Reaction yet to be attempted

Cold fusion
208Pb(82Se,xn)290−xUuh

In 1998, the team at GSI attempted the synthesis of 290Uuh as a radiative capture (x=0) product. No atoms were detected providing a cross section limit of 4.8 pb.

Hot fusion

This section deals with the synthesis of nuclei of ununhexium by so-called "hot" fusion reactions. These are processes which create compound nuclei at high excitation energy (~40-50 MeV, hence "hot"), leading to a reduced probability of survival from fission. The excited nucleus then decays to the ground state via the emission of 3-5 neutrons. Fusion reactions utilizing 48Ca nuclei usually produce compound nuclei with intermediate excitation energies (~30-35 MeV) and are sometimes referred to as "warm" fusion reactions. This leads, in part, to relatively high yields from these reactions.

238U(54Cr,xn)292−xUuh

There are sketchy indications that this reaction was attempted by the team at GSI in 2006. There are no published results on the outcome, presumably indicating that no atoms were detected. This is expected from a study of the systematics of cross sections for 238U targets.[8]

248Cm(48Ca,xn)296−xUuh (x=3,4)

The first attempt to synthesise ununhexium was performed in 1977 by Ken Hulet and his team at the Lawrence Livermore National Laboratory (LLNL). They were unable to detect any atoms of ununhexium.[9] Yuri Oganessian and his team at the Flerov Laboratory of Nuclear Reactions (FLNR) subsequently attempted the reaction in 1978 and were met by failure. In 1985, a joint experiment between Berkeley and Peter Armbruster's team at GSI, the result was again negative with a calculated cross-section limit of 10–100 pb.[10]

In 2000, Russian scientists at Dubna finally succeeded in detecting a single atom of ununhexium, assigned to the isotope 292Uuh.[2] In 2001, they repeated the reaction and formed a further 2 atoms in a confirmation of their discovery experiment. A third atom was tentatively assigned to 293Uuh on the basis of a missed parental alpha decay.[3] In April 2004, the team ran the experiment again at higher energy and were able to detect a new decay chain, assigned to 292Uuh. On this basis, the original data were reassigned to 293Uuh. The tentative chain is therefore possibly associated with a rare decay branch of this isotope. In this reaction, 2 further atoms of 293Uuh were detected.[4]

245Cm(48Ca,xn)293−x116 (x=2,3)

In order to assist in the assignment of isotope mass numbers for ununhexium, in March-May 2003 the Dubna team bombarded a 245Cm target with 48Ca ions. They were able to observe two new isotopes, assigned to 291Uuh and 290Uuh.[11] This experiment was successfully repeated in Feb-March 2005 where 10 atoms were created with identical decay data to those reported in the 2003 experiment.[12]

As a decay product

Ununhexium has also been observed in the decay of ununoctium. In October 2006 it was announced that 3 atoms of ununoctium had been detected by the bombardment of californium-249 with calcium-48 ions, which then rapidly decayed into ununhexium.[12]

The observation of 290Uuh allowed the assignment of the product to 294Uuo and proved the synthesis of ununoctium.

Fission of compound nuclei with Z=116

Several experiments have been performed between 2000-2006 at the Flerov laboratory of Nuclear Reactions in Dubna studying the fission characteristics of the compound nuclei 296,294,290Uuh. Four nuclear reactions have been used, namely 248Cm+48Ca, 246Ca+48Ca, 244Pu+50Ti and 232Th+58Fe. The results have revealed how nuclei such as this fission predominantly by expelling closed shell nuclei such as 132Sn (Z=50, N=82). It was also found that the yield for the fusion-fission pathway was similar between 48Ca and 58Fe projectiles, indicating a possible future use of 58Fe projectiles in superheavy element formation.In addition, in comparative experiments synthesizing 294Uuh using 48Ca and 50Ti projectiles, the yield from fusion-fission was ~3x less for 50Ti, also suggesting a future use in SHE production[13]

Retracted isotopes

289Uuh

In 1999, researchers at Lawrence Berkeley National Laboratory announced the synthesis of 293Uuo (see ununoctium), in a paper published in Physical Review Letters.[14] The claimed isotope 289Uuh decayed by 11.63 MeV alpha emission with a half-life of 0.64 ms. The following year, they published a retraction after other researchers were unable to duplicate the results.[15] 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 the principal author Victor Ninov. As such, this isotope of ununhexium is currently unknown.
Chronology of isotope discovery

Isotope	Year discovered	Discovery reaction
²⁹⁰Uuh	2002	²⁴⁹Cf(⁴⁸Ca,3n)^[16]
²⁹¹Uuh	2003	²⁴⁵Cm(⁴⁸Ca,2n)^[11]
²⁹²Uuh	2004	²⁴⁸Cm(⁴⁸Ca,4n)^[4]
²⁹³Uuh	2000	²⁴⁸Cm(⁴⁸Ca,3n)^[2]

Yields of isotopes
Hot fusion

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

Projectile	Target	CN	2n	3n	4n	5n
⁴⁸Ca	²⁴⁸Cm	²⁹⁶Uuh		1.1 pb, 38.9 MeV^[4]	3.3 pb, 38.9 MeV ^[4]
⁴⁸Ca	²⁴⁵Cm	²⁹³Uuh	0.9 pb, 33.0 MeV^[11]	3.7 pb, 37.9 MeV ^[11]

Theoretical calculations
Decay characteristics

Theoretical calculation in a quantum tunneling model supports the experimental data relating to the synthesis of 293,292Uuh.[17][18]
Evaporation residue 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; σ = cross section

Target	Projectile	CN	Channel (product)	σ_max	Model	Ref
²⁰⁸Pb	⁸²Se	²⁹⁰Uuh	1n (²⁸⁹Uuh)	0.1 pb	DNS	^[19]
²⁰⁸Pb	⁷⁹Se	²⁸⁷Uuh	1n (²⁸⁶Uuh)	0.5 pb	DNS	^[19]
²³⁸U	⁵⁴Cr	²⁹²Uuh	2n (²⁹⁰Uuh)	0.1 pb	DNS	^[20]
²⁵⁰Cm	⁴⁸Ca	²⁹⁸Uuh	4n (²⁹⁴Uuh)	5 pb	DNS	^[20]
²⁴⁸Cm	⁴⁸Ca	²⁹⁶Uuh	4n (²⁹²Uuh)	2 pb	DNS	^[20]
²⁴⁷Cm	⁴⁸Ca	²⁹⁵Uuh	3n (²⁹²Uuh)	3 pb	DNS	^[20]
²⁴⁵Cm	⁴⁸Ca	²⁹³Uuh	3n (²⁹⁰Uuh)	1.5 pb	DNS	^[20]

Chemical properties
Extrapolated chemical properties
Oxidation states

Ununhexium is projected to be the fourth member of the 7p series of non-metals and the heaviest member of group 16 (VIA) in the Periodic Table, below polonium. The group oxidation state of +VI is known for all the members apart from oxygen which lacks available d-orbitals for expansion and is limited to a maximum +II state, exhibited in the fluoride OF2. The +IV is known for sulfur, selenium, tellurium, and polonium, undergoing a shift in stability from reducing for S(IV) and Se(IV) to oxidizing in Po(IV). Tellurium(IV) is the most stable for this element. This suggests a decreasing stability for the higher oxidation states as the group is descended and ununhexium should portray an oxidizing +IV state and a more stable +II state. The lighter members are also known to form a −II state as oxide, sulfide, selenide, and telluride. Polonide formation is unconfirmed or only transient. The extrapolated electronegativity of ununhexium should eliminate this low oxidation state.

Chemistry

The possible chemistry of ununhexium can be extrapolated from that of polonium. It should therefore undergo oxidation to a dioxide, UuhO2, although a trioxide, UuhO3 is plausible, but unlikely. The stability of a +II state should manifest itself in the formation of a simple monoxide, UuhO. Fluorination will likely result in a tetrafluoride, UuhF4 and/or a difluoride, UuhF2. Chlorination and bromination may well stop at the corresponding dihalides, UuhCl2 and UuhBr2. Oxidation by iodine should certainly stop at UuhI2 and may even be inert to this element.[citation needed]
See also

* Island of stability

References

1. ^ J. Chatt (1979). "Recommendations for the Naming of Elements of Atomic Numbers Greater than 100". Pure Appl. Chem. 51: 381–384. doi:10.1351/pac197951020381.
2. ^ a b c Oganessian, Yu. Ts. (2000). "Observation of the decay of ^{292}116". Physical Review C 63: 011301. doi:10.1103/PhysRevC.63.011301.
3. ^ a b c "Confirmed results of the 248Cm(48Ca,4n)292116 experiment", Patin et al., LLNL report (2003). Retrieved 2008-03-03
4. ^ a b c d e Oganessian, Yu. Ts. (2004). "Measurements of cross sections and decay properties of the isotopes of elements 112, 114, and 116 produced in the fusion reactions ^{233,238}U, ^{242}Pu, and ^{248}Cm+^{48}Ca". Physical Review C 70: 064609. doi:10.1103/PhysRevC.70.064609.
5. ^ R.C.Barber; H.W.Gaeggeler;P.J.Karol;H. Nakahara; E.Verdaci; E. Vogt (2009). "Discovery of the element with atomic number 112" (IUPAC Technical Report). Pure Appl. Chem. 81: 1331. doi:10.1351/PAC-REP-08-03-05. http://media.iupac.org/publications/pac/asap/pdf/PAC-REP-08-03-05.pdf.
6. ^ Koppenol, W. H. (2002). "Naming of new elements(IUPAC Recommendations 2002)". Pure and Applied Chemistry 74: 787. doi:10.1351/pac200274050787. http://media.iupac.org/publications/pac/2002/pdf/7405x0787.pdf.
7. ^ Flerov Lab.
8. ^ "List of experiments 2000-2006"
9. ^ Hulet, E. K. (1977). "Search for Superheavy Elements in the Bombardment of 248Cm with 48Ca". Physical Review Letters 39: 385. doi:10.1103/PhysRevLett.39.385.
10. ^ Armbruster, P. (1985). "Attempts to Produce Superheavy Elements by Fusion of 48Ca with 248Cm in the Bombarding Energy Range of 4.5-5.2 MeV/u". Physical Review Letters 54: 406. doi:10.1103/PhysRevLett.54.406.
11. ^ a b c d Oganessian, Yu. Ts. (2004). "Measurements of cross sections for the fusion-evaporation reactions 244Pu(48Ca,xn)292−x114 and 245Cm(48Ca,xn)293−x116". Physical Review C 69: 054607. doi:10.1103/PhysRevC.69.054607.
12. ^ a b Synthesis of the isotopes of elements 118 and 116 in the 249Cf and 245Cm+48Ca fusion reactions.
13. ^ see Flerov lab annual reports 2000-2006
14. ^ Ninov, V.; et al. (1999). "Observation of Superheavy Nuclei Produced in the Reaction of 86Kr with 208Pb". Physical Review Letters 83: 1104. doi:10.1103/PhysRevLett.83.1104.
15. ^ Ninov, V. (2002). "Editorial Note: Observation of Superheavy Nuclei Produced in the Reaction of ^{86}Kr with ^{208}Pb [Phys. Rev. Lett. 83, 1104 (1999)]". Physical Review Letters 89: 039901. doi:10.1103/PhysRevLett.89.039901.
16. ^ see ununoctium
17. ^ P. Roy Chowdhury, C. Samanta, and D. N. Basu (2006). "α decay half-lives of new superheavy elements". Phys. Rev. C 73: 014612. doi:10.1103/PhysRevC.73.014612.
18. ^ C. Samanta, P. Roy Chowdhury and D.N. Basu (2007). "Predictions of alpha decay half lives of heavy and superheavy elements". Nucl. Phys. A 789: 142–154. doi:10.1016/j.nuclphysa.2007.04.001.
19. ^ a b Feng, Zhao-Qing (2007). "Formation of superheavy nuclei in cold fusion reactions". Physical Review C 76: 044606. doi:10.1103/PhysRevC.76.044606. http://arxiv.org/pdf/0707.2588.
20. ^ a b c d e Feng, Z (2009). "Production of heavy and superheavy nuclei in massive fusion reactions". Nuclear Physics A 816: 33. doi:10.1016/j.nuclphysa.2008.11.003. http://arxiv.org/pdf/0803.1117.