Figure 1. Overview of the experimental setup at GSI.

The bismuth ions revolve in the ESR and the velocity spread of the ions is reduced by electron cooling. The final velocity is determined by the accelerating potential of the electron beam, which is measured with a high-precision voltage divider. Two ion bunches are formed by applying the second harmonic of the revolution frequency to a radio frequency (RF) cavity. Fluorescence and background photons are reflected by specialized mirror systems (inset) and detected with photo multiplier tubes (PMT) for ultraviolet and infrared light. A newly designed parabolic copper mirror system³⁰ allowed the detection of the resonance in the Li-like charge state¹³. The data acquisition system (DAQ) comprises fast time-to-digital converters to resolve the temporal phase of each detected photon.