Laser Spectroscopy of the Ground State Hyperfine Splitting in Lithiumlike Bismuth

Laser spectroscopic measurements of the ground state hyperfine splitting (HFS) in hydrogenlike heavy ions have triggered great interest because they can be used to test QED effects in extremely strong electric and magnetic fields. However, the interpretation of the experimental value is difficult because the uncertainty of the contribution of the nuclear magnetization distribution (Bohr-Weisskopf effect) hinders to test the size of the QED contributions. It has been suggested to overcome this limitation by measuring the HFS in both hydrogenand lithium-like heavy ions of the same species [1]. Thus, tests of the QED effects on the level of a few percent become feasible [2]. Bismuth is the only stable isotope where the ground state hyperfine transitions of both hydrogenand lithiumlike ions are in a range accessible by laser spectroscopy. The HFS-transition wavelength for hydrogenlike bismuth ( Bi) has already been measured with a relative accuracy of 1.6× 10−4 by Klaft et al. in 1993 [3]. We are currently commissioning an experiment at the ESR to measure the HFS in lithiumlike bismuth at an energy of 400 MeV/u (which is equivalent to 70 % of the speed of light). Due to the relativistic Doppler shift at β = 0.7 , the HFS-transition wavelength is shifted from λ0 ≈ 1555 nm in the rest frame of the ion to the visible range λ ≈ 640 nm. Laser light at this wavelength can be easily provided by a dye laser. We have acquired a new laser system, consisting of a frequency-doubled pulsed Nd:YAG laser-pumped dye laser, which delivers now up to 120 mJ at 30 Hz at the desired wavelength. This laser light is transported to the ESR and overlapped collinearly with a lithiumlike bismuth ion beam at the electron cooler section where the ions are electron cooled and bunched at the second harmonic of the revolution frequency. One bunch is illuminated with the laser light while the other one is used as a reference to correct for ion-beam-induced background. Since the transition is rather long-lived (≈ 80 ms), the fluorescence is emitted along the entire length of the ESR. We have developed a new light collection system consisting of a copper mirror that can be moved close to the trajectory of the ion beam allowing the fluorescence photons to be efficiently directed into a photomultiplier [4, 5]. We have tested this device successfully using both a Li beam (at 59 MeV/u) [6] and a U beam (at 200 MeV/u) in 2010. Figure 1 shows an example of the signals recorded