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Ununbium is a synthetic radioactive chemical element with the temporary symbol Uub and atomic number 112. "Ununbium" ( ) is a IUPAC systematic element name, used until the element receives an accepted name.

Ununbium was first created by the in 1996, who have now proposed the permanent name copernicium ( ) and the symbol Cn. This name is expected to be officially endorsed by IUPAC in January 2010, after six months for discussion. Element 112 is currently the highest-numbered element to be officially recognised by IUPAC.

The most stable isotope discovered to date is 285Uub with a half-life of ~30 s. In total, about 75 atoms of ununbium have been detected using various nuclear reactionssee references in this article relating to 277112, 282112 and 283112, as well as references in ununquadium, ununhexium and ununoctium regarding observed daughter nuclei. An unconfirmed isotope, 285bUub, has a possible half-life of ~9 minutes, and would be one of the longest-lived superheavy isotopes known to date .

Recent experiments strongly suggest that ununbium behaves as a typical member of group 12, demonstrating properties consistent with a volatile metal.

History

Official discovery

Ununbium was first created on February 9, 1996 at the Gesellschaft für Schwerionenforschungmarker (GSI) in Darmstadtmarker, Germanymarker by Sigurd Hofmann, Victor Ninov et al. This element was created by firing accelerated zinc-70 nuclei at a target made of lead-208 nuclei in a heavy ion accelerator. A single atom (the second has subsequently been dismissed) of ununbium was produced with a mass number of 277.

Pb + Zn → Uub → Uub + n


In May 2000, the GSI successfully repeated the experiment to synthesise a further atom of Uub-277.This reaction was repeated at RIKEN using the GARIS set-up in 2004 to synthesise two further atoms and confirm the decay data reported by the GSI team.

The IUPAC/IUPAP Joint Working Party (JWP) assessed the claim of discovery by the GSI team in 2001 and 2003. In both cases, they found that there was insufficient evidence to support their claim. This was primarily related to the contradicting decay data for the known isotope 261Rf. However, between 2001-2005, the GSI team studied the reaction 248Cm(26Mg,5n)269Hs, and were able to confirm the decay data for 269Hs and 261Rf. It was found that the existing data on 261Rf was for an isomer,now designated 261a Rf.

In May 2009, the JWP reported on the claims of discovery of element 112 again and officially recognised the GSI team as the discoverers of element 112. This decision was based on recent confirmation of the decay properties of daughter nuclei as well as the confirmatory experiments at RIKEN.

Naming

The element with Z=112 is historically known as eka-mercury. Ununbium is a temporary IUPAC systematic element name. Research scientists usually refer to the element simply as element 112 (or just E112).

After acknowledging their discovery, the IUPAC has asked the discovery team at GSI to suggest a permanent name for ununbium. On 14 July 2009, they proposed copernicium with the element symbol Cp, after Nicolaus Copernicus "to honor an outstanding scientist, who changed our view of the world." IUPAC has not yet officially recognized this name, pending the results of a six month discussion period among the scientific community.

Some news articles have referred to the suggested name as "copernicum" in error. However, the IUPAC allows only the suffix -ium for new elements. Alternative spellings have been suggested to Hofmann, namely "copernicum", "copernium", and "kopernikium" (Kp), and Hofmann has said that the team had discussed the possibility of "copernicum" or "kopernikum", but that they had agreed on "copernicium" in order to comply with current IUPAC rules.

However, it has been pointed out that the symbol Cp was previously associated with the name cassiopeium (cassiopium), now known as lutetium (Lu). Furthermore, the symbol Cp is also used in inorganic chemistry to denote the ligand cyclopentadiene. For this reason, the IUPAC has disallowed the use of Cp as a future symbol and the GSI team has put forward the symbol Cn as an alternative proposal.

Isotopes and nuclear properties

Nucleosynthesis

Target-projectile combinations leading to Z=112 compound nuclei

The below table contains various combinations of targets and projectiles which could be used to form compound nuclei with Z=112.

Target Projectile CN Attempt result
208Pb 70Zn 278112
232Th 50Ti 282112
238U 48Ca 286112
244Pu 40Ar 284112
248Cm 36S 284112
249Cf 30Si 279112


Cold fusion

This section deals with the synthesis of nuclei of ununbium by so-called "cold" fusion reactions. These are processes which 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(70Zn,xn)278-xUub (x=1)
The team at GSI first studied this reaction in 1996 and detected two decay chains of 277Uub.In a review of the data in 2000, the first decay chain was retracted. In a repeat of the reaction in 2000 they were able to synthesise a further atom. They attempted to measure the 1n excitation function in 2002 but suffered from a failure of the Zn-70 beam.The unofficial discovery of 277Uub was confirmed in 2004 at RIKEN who detected a further 2 atoms of the isotope and were able to confirm the decay data for the entire chain.

208Pb(68Zn,xn)276-xUub
Following the successful synthesis of 277Uub, the GSI team performed a reaction using a 68Zn projectile in 1997 in an effort to study the effect of isospin (neutron richness) on the chemical yield. The experiment was initiated following the discovery of a yield enhancement during the synthesis of darmstadtium isotopes using 62Ni and 64Ni ions. No decay chains of 275Uub were detected leading to a cross section limit of 1.2 pb. However, the revision of the yield for the 70Zn reaction to 0.5 pb does not rule out a similar yield for this reaction.

184W(88Sr,xn)272-xUub
In 1990, after some early indications for the formation of isotopes of element 112 in the irradiation of a tungsten target with multi-GeV protons, a collaboration between GSI and the University of Jerusalem studied the above reaction. They were able to detect some spontaneous fission activity and a 12.5 MeV alpha decay, both of which they tentatively assigned to the radiative capture product 272Uub or the 1n evaporation residue 271Uub. Both the TWG and JWP have concluded that a lot more research is required to confirm these conclusions.

Hot fusion

This section deals with the synthesis of nuclei of ununbium 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 and quasi-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(48Ca,xn)286-xUub (x=3,4)
In 1998, the team at the Flerov Laboratory of Nuclear Research began a research program using Ca-48 nuclei in "warm" fusion reactions leading to superheavy elements (SHE's).In March 1998, they claimed to have synthesised the element (2 atoms) in this reaction.The product, 283Uub, had a claimed half-life of 5 min, decaying by spontaneous fission (SF).

The long lifetime of the product initiated first chemical experiments on the gas phase atomic chemistry of element 112. In 2000, Yuri Yukashev at Dubna repeated the experiment but was unable to observe any spontaneous fission from 5 min activities. The experiment was repeated in 2001 and an accumulation of 8 SF fragments were found in the low temperature section, indicating that ununbium had radon-like properties. However, there is now some serious doubt about the origin of these results.

In order to confirm the synthesis, the reaction was successfully repeated by the same team in Jan 2003, confirming the decay mode and half life. They were also able to calculate an estimate of the mass of the SF activity to ~285 lending support to the assignment.

The team at LBNL entered the debate and performed the reaction in 2002. They were unable to detect any SF activities and calculated a cross section limit of 1.6 pb for the detection of a single event.

The reaction was repeated in 2003-2004 by the team at Dubna using a slightly different set-up, the Dubna Gas Filled Recoil Separator (DGFRS). This time, 283Uub was found to decay by emission of a 9.53 MeV alpha-particle with a half-life of 4 seconds. 282Uub was also observed in the 4n channel.

In 2003, the team at GSI entered the debate and performed a search for the 5 minute SF activity in chemical experiments. Like the Dubna team, they were able to detect 7 SF fragments in the low temperature section. However, these SF events were uncorrelated, suggesting they were not from actual direct SF of element 112 nuclei and raised doubts about the original indications for radon-like properties. After the announcement from Dubna of different decay properties for 283Uub, the GSI team repeated the experiment in September 2004. They were unable to detect any SF events and calculated a cross section limit of ~ 1.6 pb for the detection of one event, not in contradiction with the reported 2.5 pb yield by Dubna.

In May 2005, the GSI performed a physical experiment and identified a single atom of 283Uub decaying by SF with a short lifetime suggesting a previously unknown SF branch.However, initial work by Dubna had detected several direct SF events but had assumed that the parent alpha decay had been missed. These results indicated that this was not the case.

In 2006, the new decay data on 283Uub was confirmed by a joint PSI-FLNR experiment aimed at probing the chemical properties of ununbium. Two atoms of 283Uub were observed in the decay of the parent 287Uuq nuclei. The experiment indicated that contrary to previous experiments, element 112 behaves as a typical member of group 12, demonstrating properties of a volatile metal.

Finally, the team at GSI successfully repeated their physical experiment in Jan 2007 and detected 3 atoms of 283Uub, confirming both the alpha and SF decay modes.

As such, the 5 min SF activity is still unconfirmed and unidentified. It is possible that it refers to an isomer, namely 283bUub, whose yield is obviously dependent upon the exact production methods.

233U(48Ca,xn)281-xUub
The team at FLNR studied this reaction in 2004. They were unable to detect any atoms of element 112 and calculated a cross section limit of 600 fb. The team concluded that this indicated that the neutron mass number for the compound nucleus had an effect on the yield of evaporation residues.

As a decay product

Element 112 has also been observed as decay products of elements 114, 116, and 118 (see ununoctium).

Evaporation Residue Observed Uub isotope
293116, 289114 285112
292116, 288114 284112
291116, 287114 283112
294118, 290116 , 286114 282112


As an example, in May 2006, the Dubna team (JINRmarker) identified 282Uub as a final product in the decay of ununoctium via the alpha decay sequence:

→ → →


It was found that the final nucleus undergoes spontaneous fission.

Retracted isotopes

281Uub
In the claimed synthesis of 293Uuo in 1999 (see ununoctium) the isotope 281Uub was identified as decaying by emission of a 10.68 MeV alpha particle with half-life 0.90 ms. The claim was retracted in 2001 and hence this ununbium isotope is currently unknown or unconfirmed.

Chronology of isotope discovery

Isotope Year discovered discovery reaction
277Uub 1996 208Pb(70Zn,n)
278Uub unknown
279Uub unknown
280Uub unknown
281Uub unknown
282Uub 2004 238U(48Ca,4n)
283Uub 2002 244Pu(48Ca,5n)
283bUub ?? 1998 238U(48Ca,3n)
284Uub 2002 244Pu(48Ca,4n)
285Uub 1999 244Pu(48Ca,3n)
285bUub ? 1999 244Pu(48Ca,3n)


Nuclear isomerism

285a,bUub

In the synthesis of 289Uuq and 293Uuh, a 8.63 MeV alpha-decaying activity has been detected with a half-life of 8.9 minutes. Although unconfirmed in recent experiments, it is highly possible that this is associated with an isomer, namely 285bUub.

283a,bUub

First experiments on the synthesis of 283Uub produced a SF activity with half-life ~5 min. This activity was also observed from the alpha decay of 287Uuq. The decay mode and half-life were also confirmed in a repeat of the first experiment. However, more recently,283Uub has been observed to undergo 9.52 MeV alpha decay and SF with a half-life of 3.9 s. These results suggest the assignment of the two activities to two different isomeric levels in 283Uub, creating 283aUub and 283bUub. Further research is required to address these discrepancies.

Chemical yields of isotopes

Cold fusion

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

Projectile Target CN 1n 2n 3n
70Zn 208Pb}>278Uub 0.5 pb, 10.0, 12.0 MeV
68Zn 208Pb 276Uub <1.2 pb, 11.3, 12.8 MeV


Hot fusion

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

Projectile Target CN 3n 4n 5n
48Ca 238U 286Uub 2.5 pb, 35.0 MeV 0.6 pb
48Ca 233U 281Uub <0.6 pb, 34.9 MeV


Fission of compound nuclei with Z=112

Several experiments have been performed between 2001-2004 at the Flerov Laboratory of Nuclear Reactions in Dubna studying the fission characteristics of the compound nucleus 286Uub. The nuclear reaction used is 238U+48Ca. 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.

Theoretical calculations

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) σmax Model Ref
208Pb 70Zn 278Uub 1n (277Uub) 1.5 pb DNS
208Pb 67Zn 275Uub 1n (274Uub) 2 pb DNS
238U 48Ca 286Uub 4n (282Uub) 0.2 pb DNS
244Pu 40Ar 284Uub 4n (280Uub) 0.1 pb DNS
250Cm 36S 286Uub 4n (282Uub) 5 pb DNS
252Cf 30Si 282Uub 3n (279Uub) 10 pb DNS


Chemical properties

Extrapolated chemical properties

Oxidation states

Element 112 is projected to be the last member of the 6d series of transition metals and the heaviest member of group 12 (IIB) in the Periodic Table, below zinc, cadmium and mercury. Each of the members of this group show a stable +2 oxidation state. In addition, mercury(I), , is also well known. Element 112 is therefore expected to form a stable +2 state.

Chemistry

The known members of group 12 all react with oxygen and sulfur directly to form the oxides and sulfides, MO and MS, respectively. Mercury(II) oxide, HgO, can be decomposed by heat to the liquid metal. Mercury also has a well known affinity for sulfur. Therefore, element 112 should form an analogous oxide UubO and sulfide UubS.In their halogen chemistry, all the metals form the ionic difluoride MF2 upon reaction with fluorine. The other halides are known but for mercury, the soft nature of the Hg(II) ion leads to a high degree of covalency and HgCl2, HgBr2 and HgI2 are low-melting, volatile solids. Therefore, element 112 is expected to form an ionic fluoride, UubF2, but volatile halides, UubCl2, UubBr2 and UubI2.In addition, mercury is well known for its alloying properties, with the concomitant formation of amalgams, especially with gold and silver. It is also a volatile metal and is monatomic in the vapour phase. Element 112 is therefore also predicted to be a volatile metal which readily combines with gold to form a Au-Uub metal-metal bond.

Experimental chemistry

Atomic gas phase

Ununbium is expected to have the ground state electron configuration [Rn]5f14 6d10 7s2 and thus belong to group 12 of the Periodic Table. As such, it should behave as the heavier homologue of mercury (Hg) and form strong binary compounds with noble metals like gold. Experiments probing the reactivity of ununbium have focused on the adsorption of atoms of element 112 onto a gold surface held at varying temperatures, in order to calculate an adsorption enthalpy. Due to possible relativistic stabilisation of the 7s electrons, leading to radon-like properties, experiments were performed with the simultaneous formation of mercury and radon radioisotopes, allowing a comparison of adsorption characteristics.

The first experiments were conducted using the 238U(48Ca,3n)283Uub reaction. Detection was by spontaneous fission of the claimed 5 min parent isotope. Analysis of the data indicated that ununbium was more volatile than mercury and had noble-gas properties. However, the confusion regarding the synthesis of 283Uub has cast some doubt on these experimental results.

Given this uncertainty, between April-May 2006 at the JINR, a FLNR-PSI team conducted experiments probing the synthesis of this isotope as a daughter in the nuclear reaction 242Pu(48Ca,3n)287Uuq. In this experiment, two atoms of 283Uub were unambiguously identified and the adsorption properties indicated that ununbium is a more volatile homologue of mercury, due to formation of a weak metal-metal bond with gold, placing it firmly in group 12.

In April 2007 this experiment was repeated and a further 3 atoms of 283112 were positively identified. The adsorption property was confirmed and indicated that element 112 has adsorption properties completely in agreement with being the heaviest member of group 12.

See also



References

External links




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