Johannes Ullmann received the Giersch Excellence Grant on October 13, 2015 for his works on the determination of the hyperfine splitting energy of the ground state of hydrogen- and lithiumlike Bismuth by laser spectroscopy.
The Experiment was conducted at the experimental storage ring of the Helmholtz center for heavy ion research in Darmstadt and allows a precise test of quantum electrodynamics in extreme electric and magnetic fields, which are experienced by the ground state electron in a highly charged, heavy ion. Uncertainties in the theoretical prediction of nuclear effects on the hyperfine splitting were eliminated by measuring two charge states of the same isotope.
Although the hyperfine splitting energy in hydrogen-like Bismuth is known since 1994, the resonance in the lithium-like charge state was not found until 2011. Due to too large uncertainties in the determination of the ions' velocity, no accurate value could be determined then.
An in-situ high voltage measurement was therefore installed in collaboration with Physikalisch-Technische Bundesanstalt, the national metrology institute of Germany. In combination with extensive improvements of detection systems and data acquisition, the uncertainty could be reduced by one order of magnitude.
An improved value of the hyperfine splitting energy in hydrogen-like bismuth has been published recently in Journal of Physics B. The upcoming finalization of analysis of the lithium-like Bismuth data in connection with the published value promises the first accurate test of quantum electrodynamics in strong fields.