Final Report Summary - HELIOS (Heavy Element Laser Ionization Spectroscopy)
The heaviest elements of Mendeleev's table form an intriguing research laboratory of interest for nuclear and atomic physics. The heaviest element experimentally produced on earth is Oganesson (118 protons). Such heavy atoms are radioactive and short lived but theory predicts an island of stability when going to even heavier elements. Their atomic structure is influenced by strong relativistic effects making it challenging to predict their atomic and chemical properties and thus their position in Medeleev's table.The essential know-how that is missing, is the way these heavy atoms are constructed from their protons, neutrons and electrons. Therefore, it is imperative to study experimentally the heaviest atoms accessible in the laboratory and for the more exotic inaccessible ones to develop reliable nuclear and atomic models.In the framework of the present project, a novel laser-spectroscopy method to study nuclear and atomic properties of isotopes from heavy and super heavy elements has been developed. Radioactive isotopes were produced in a nuclear reaction using beams from a heavy-ion linear accelerator of the Cyclotron Center (U.C.L. - Belgium). The speedy reaction products are stopped in a gas cell filled with argon and transformed in a gas jet through a special shaped nozzle. While residing in the gas jet, the atoms of interest are irradiated with laser light, ionized and further transported to a detection system. By varying the wavelength (color) of the laser light the atomic levels are investigated. These in turn provides information on properties of the atomic nucleus which are used to validate state-of-the-art nuclear models. The goal to acquire new nuclear and atomic structure information of the heaviest isotopes to advance our understanding of the atom and its nucleus, prototype of a many-body system, was reached for actinium and nobelium.A first, pioneering experiment on short-lived, neutron-deficient actinium isotopes was performed at the Leuven Isotope Separator On Line facility (Belgium) to establish the proof-of-principle of this concept. Prior to these on-line experiments, atomic data were obtained at Mainz University (Germany) using long-lived actinium-227 which allowed to identify optimal ionization schemes. A new laser and mass separator laboratory has been constructed at KU Leuven to characterize the crucial elements and extended gas dynamics simulations to optimize the gas cell and to evaluate the gas-jet formation process led to tailored cell and nozzle designs. First atomic information on nobelium was obtained form a gas-cell based experiment at GSI (Germany).An international network with groups from France, Finland, Germany, Russia, Japan and Belgium to further exploit the possibilities of the In Gas Laser Ionization Spectroscopy (IGLIS-net) technique resulted from the current project. More specifically, at the SPIRAL2 project at GANIL (France) a new set-up based on the outcome of this project, is being commissioned.