Status of the g-factor experiment for highly-charged calcium

Measurements of the anomalous magnetic moment of the electron bound in hydrogen-like ions with spinless nuclei have proven to be highly-sensitive tests of corresponding calculations based on bound-state quantum electrodynamics (BS-QED). The calculations of the g-factors of the electron bound to hydrogen-like carbon and oxygen and their corresponding measurements performed by the collaboration of GSI and the University of Mainz [1, 2] agreed down to the 9 significant digit. Presently, a triple-trap Penning-trap experiment on highly-charged calcium is prepared [3, 4], which is expected to yield a precision on the level of 10 for the electronic g-factor of Ca. This experiment makes use of a cryogenic in-trap electron-beam ion source (EBIS) at T = 4K, where the charge breeding of the ions is performed by electron-impact ionization [5]. A new cathode was developed for the EBIS based on a single field emission point (FEP) made from tungsten. The emission of electrons is achieved by applying a voltage between the FEP and an extraction electrode. The electron beam is reflected several hundred times between a reflector and the FEP until it widens up and hits the target, which consists of carbon with a layer of calcium. The electron current on the target is measured by monitoring the voltage drop across the input resistance of an oscilloscope. Atoms from the target get ionized by electron impact and are trapped in the so-called creation trap. After charge breeding the ions are transported either to the analysis trap or the precision trap. For recording a mass spectrum as shown in figure 1, the electric trap potential is ramped continuously to bring the ions in resonance with the tank circuit. For detection of these ion signals a special device was developed to filter and amplify the signal near the resonance frequency of the tank circuit. This signal is rectified and integrated, yielding the spectral power of the detection signal, which shows peaks of ions with different charge-to-mass ratios. The spectrum in figure 1 was measured by averaging over several single spectra taken in the precision trap. Assuming that the peak at U = −10.25 V corresponds to a charge-to-mass ratio of q m = 0.5, the other peaks can be identified. In this way it was possible to detect mainly several oxygen, nitrogen and carbon ions. The resolution of the spectra is not high enough to discriminate ions with similar charge-to-mass ratios like O, N and C. For the high-precision g-factor measurements ultra-