Image credit: G Otto/GSI.
An international team of researchers at GSI Darmstadt has successfully contained atoms of nobelium (atomic number 102) in an ion trap. This is the first time that a superheavy element has been trapped. It allowed the team to measure the mass of three isotopes of the element with unprecedented accuracy.
The measurements took place in the SHIPTRAP facility at GSI, which combines an ion trap with the Separator for Heavy Ion reaction Products (SHIP) – a velocity filter that has already been used in the discovery of six superheavy elements at GSI. SHIPTRAP consists of a stopping cell, an RFQ buncher and a double Penning trap system inside a 7 T superconducting magnet. The cell of high-purity helium stops and thermalizes radioactive nuclei, which SHIP delivers at energies of a few 100 keV/u. The stopped ions are extracted into the RFQ structure where they are cooled, accumulated, and bunched. The ions then enter the first Penning trap, where they are selected according to mass by a buffer-gas cooling technique with resolving power of about 50,000. Finally, a purified sample of ions is injected into the second Penning trap where their mass is determined precisely via their cyclotron frequency.
The nobelium ions were produced in fusion reactions of a beam of 60Ca and a target of lead foil (206–208Pb). They were then separated out from the beam in the SHIP velocity filter, passing at a rate of less than one ion per second (in the case of 252No) into the stopping cell. The decelerated ions were extracted into the RFQ within a few milliseconds and then injected in pulses into SHIPTRAP’s double Penning trap system.
By directly comparing the cyclotron frequency of the nobelium ions in SHIPTRAP with the frequency of precisely known reference ions, the research team was able to determine the masses of the nobelium isotopes 252–254No to uncertainties of about 10 keV/c2 – a relative precision of 0.05 ppm (Block et al. 2010). 254No is now the heaviest radionuclide to have its mass measured directly and 252No is the lowest-production-rate radionuclide whose mass has been measured with a Penning trap.
These mass values provide new, accurate reference points in the region of superheavy elements. The technique also holds promise for identifying elements on the way to the predicted “island of stability”. One of the next goals of SHIPTRAP is to extend these accurate mass measurements to the transactinide region, starting with long-lived rutherfordium isotopes that terminate decay chains originating from Z = 116.
• Element 112, first observed at GSI in 1996, now officially carries the name copernicium and the chemical symbol Cn, after approval by the International Union of Pure and Applied Chemistry (IUPAC). The name honours scientist and astronomer Nicolaus Copernicus. The discoverers had suggested Cp as the symbol, but as this abbreviation has other scientific meanings, they agreed with IUPAC on Cn. Copernicium is the heaviest element officially recognized by IUPAC.
Further reading
M Block et al. 2010 Nature 463 785.
