Experiments with superheavy elements: The SHE (SuperHeavy Elements) section
Above a certain size, atomic nuclei become increasingly unstable. The heaviest element occurring in nature is uranium with the atomic number 92. Even heavier elements can be produced artificially – partly in nuclear reactors, partly with accelerators. At the Helmholtz Institute Mainz, such superheavy elements are the focus of the SHE (SuperHeavy Elements) section. The analysis of these elements enables both the models of nuclear physics and the theories of chemistry to be tested and improved.
The HIM researchers produce the superheavy nuclei at accelerator facilities such as those of GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt. There, an accelerator fires medium-heavy ions such as zinc or calcium onto a target, consisting for example of lead. Some of the projectile nuclei hit a target nucleus in such a way that they fuse to form a superheavy element. This can be separated from other particles with the SHIP velocity filter or the TASCA gas-filled separator. To study the short-lived nuclei, they are either implanted directly into a suitable detector that measures their radioactive decay, or they must be decelerated for further study. The “brake” is for instance a cell filled with helium or argon. The scientists can then investigate the physical and chemical properties of the nuclei: the particles can be stored in traps, for example, illuminated with lasers or examined regarding their chemical binding behavior to specific substances.
Another focus of the SHE section is the investigation of the chemical properties of superheavy elements. In heavy elements, the large proton number significantly influences the chemistry of the element. The reason is that the electrons on the orbitals of the atomic shell that are near the nucleus “feel” the very strong positive charge of the proton-rich nucleus and are accelerated to up to 80 percent of the speed of light, inducing a noticeable relativistic mass increase. The electrons literally gain weight and reduce their radii of motion around the atomic nucleus. This in turn affects the shape and size of the electron orbitals, which determine the chemical properties. Among other things, such relativistic effects cause gold to have its typical color and mercury to be liquid instead of solid at room temperature.
Details on the HIM-Website.