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Highly-charged heavy ions like U 91+ provide unique conditions for the investigation of relativistic and quantum electrodynamical effects in strong electromagnetic fields. We present two X-ray detectors developed for high-resolution spectroscopy on highly-charged heavy ions. Both detectors consist of metallic magnetic calorimeters (MMCs) forming linear eight-pixel arrays. The first detector, maXs-20, is developed for the detection of X-rays up to 20 keV with an energy resolution below 3 eV. The second device, maXs-200, is designed for X-ray energies up to 200 keV with an energy resolution of 40 eV. The results of characterization measurements of single detectors of both arrays will be shown and discussed. In both cases, the performance of the detectors agrees well with their design values. Furthermore, we present a prototype MMC for soft X-rays with improved magnetic flux coupling. In first characterization measurements the energy resolution of this device was 2.0 eV (FWHM) for X-rays up to 6 keV.

The Thorium-229 isotope features a nuclear isomer state with an extremely low energy. The currently most accepted energy value, 7.8±0.5 eV, was obtained from an indirect measurement using a NASA x-ray microcalorimeter with an instrumental resolution 26 eV. We study, how state-of-the-art magnetic metallic microcalorimeters with an energy resolution down to a few eV can be used to measure the isomer energy. In particular, resolving the 29.18 keV doublet in the γ-spectrum following the α-decay of Uranium-233, corresponding to the decay into the ground and isomer state, allows to measure the isomer transition energy without additional theoretical input parameters, and increase the energy accuracy. We study the possibility of resolving the 29.18 keV line as a doublet and the dependence of the attainable precision of the energy measurement on the signal and background count rates and the instrumental resolution.