In 1996 an international research team led by Peter Armbruster and Sigurd Hofmann at the Institute for Heavy Ion Research at Darmstadt, Germany bombarded lead-208 atoms with high-energy zinc-70 ions. In a two-week experiment, one of the resultant atoms was unambiguously identified as an isotope of element 112 with mass number 277 and a half-life of 280 microseconds. Element 112 has been called ununbium, from the Latin roots un for one and bi for two, under a convention for neutral temporary names proposed by the International Union of Pure and Applied Chemistry (IUPAC) in 1980; in 2009 the name copernicium (Cp), for Copernicus, was proposed for the element by its discoverers.
Ununbium was reportedly first created on February 9, 1996 at the Gesellschaft für Schwerionenforschung (GSI) in Darmstadt, Germany 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.
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. In a quantum tuneling model the alpha-decay chains from 277112 have been studied and the calculated results furnish corroborating evidence for the experimental findings at RIKEN and GSI.
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 a metastable isomer, namely 261mRf.
The JWP has completed the re-assessment of the claim of discovery by the various teams and has written a draft report which should be published as a technical report in Pure and Applied Chemistry in early 2008.
Ununbium is element 112 in the Periodic Table. The two forms of the projected electronic structure are:
Bohr model: 2, 8, 18, 32, 32, 18, 2
Quantum mechanical model: 1s22s22p63s23p64s23d10 4p65s24d105p66s24f145d10 6p67s25f146d10
The first experiments were conducted using the 238U(48Ca,3n)283112 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 283112 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)287114. In this experiment, two atoms of 283112 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.
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 283112, 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 283112 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 283112 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 283112, 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 a meta-stable isomer, namely 283mUub, whose yield is obviously dependent upon the exact production methods.
|Evaporation Residue||Observed Uub isotope|
|293116 , 289114||285112|
|292116 , 288114||284112|
|291116 , 287114||283112|
|294118 , 290116 , 286114||282112|
It was found that the final nucleus undergoes spontaneous fission.
|Isotope||Year discovered||discovery reaction|
|70Zn||208Pb||278Uub||0.5 pb , 10.0;12.0 MeV|
|68Zn||208Pb||276Uub||< 1.2 pb , 11.3;12.8 MeV|
|48Ca||238U||286Uub||2.5 pb , 35.0 MeV||0.6 pb|
|48Ca||233U||281Uub||< 0.6 pb , 34.9 MeV|