Georg Christoph Lichtenberg , the 18th-century philosopher scientist, said: “To see something new, you must build something new.” This adage certainly applies on the nuclear scale at CERN’s On-Line Isotope Mass Separator, ISOLDE, the pioneering rare-isotope factory. Measurements with the Penning-trap mass spectrometer ISOLTRAP, have determined new masses for several isotopes of the noble gases, xenon and radon, while discovering a new isotope of radon along the way.
ISOLDE is CERN’s longest-running facility and has always been at the forefront of development. Now the facility is the key player in the European sixth framework design study for EURISOL, a next-generation facility for isotope separation online (ISOL). At ISOLDE the short-lived nuclides are created using 1.4 GeV protons from CERN’s PS Booster. Once produced in the target, these rare species must be ionized efficiently to form secondary beams that can be accelerated and mass-separated for use in experiments. Thus, all ISOLDE targets have a built-in chemically selective ion source.
One of the tasks of EURISOL (in conjunction with the HighInt Marie-Curie Training programme) is the development of an efficient ion source that can accommodate the 50-fold increase in proton beam intensity that will become available at CERN through the upcoming Linac 4 and Superconducting Proton Linac upgrades. This has led to a prototype, the Versatile Arc Discharge Ion Source (VADIS). Its principle rests on the optimization of both the discharge-current densities within the ion-source geometry and the extracted ion-beam intensities (figure 1). The version designed for the selective ionization of noble gases increases ionization efficiency by over an order of magnitude.
VADIS was employed at ISOLDE in 2008 with spectacular results. The experiment in question involved another pioneering facility, ISOLTRAP. ISOLTRAP in effect weighs radioactive nuclides created by ISOLDE using the elegant technique of exciting the cyclotron motion of a single ion in a magnetic field. Knowledge of the mass gives access to the nuclear binding energy, which is not only a rich source of information for nuclear structure, size and shape, but also determines the amount of energy available for radioactive decay and for reactions of major importance for modelling nucleosynthesis, the cooking of elements in stars.
ISOLTRAP first weighed isotopes of xenon ionized by VADIS, determining masses for four more of them. The team then focused its efforts on the neutron-rich isotopes of radon, with impressive results. The experiment determined seven new masses, one for an isotope, 229Rn, that had never previously been observed in the laboratory. As there was no information to confirm this isotope’s identity, the experimenters needed to take particular care to make sure it was indeed what they thought it to be. As a result, they also determined the half live of this nuclide (figure 2), marking the first discovery of a nuclide by Penning-trap mass spectrometry (Neidherr et al.). To make things even more interesting, the new radon masses show a unique pattern that provides a link to a special type of nuclear octupole deformation, predicted to occur in this region of the nuclear chart.
Further reading
D Neidherr et al. 2009 accepted for publication in Phys. Rev. Letts.
