Abstract
X-ray spectroscopy of highly charged heavy ions is an important tool for the investigation of many topics in atomic physics. Such highly charged ions, in particular hydrogen-like uranium, are investigated at heavy ion storage rings, where high charge states can be produced in large quantities, stored for long times and cooled to low momentum spread of the ion beam. One prominent example is the determination of the 1s Lamb Shift in hydrogen-like heavy ions, which has been investigated at the Experimental Storage Ring (ESR) at the GSI Helmholtz Centre for Heavy Ion Research. Due to the large electron binding energies, the energies of the corresponding photon transitions are located in the X-ray regime. To determine the transition energies with high accuracy, highly resolving X-ray spectrometers are needed. One concept of such spectrometers is the concept of microcalorimeters, which, in contrast to semiconductor detectors, uses the detection of heat rather than charge to detect energy. Such detectors have been developed and successfully applied in experiments at the ESR. For experiments at the Facility for Antiproton and Ion Research (FAIR), the Stored Particles and Atoms Collaboration (SPARC) pursues the development of new microcalorimeter concepts and larger detector arrays. Next to fundamental investigations on quantum electrodynamics such as the 1s Lamb Shift or electron–electron interactions in two- and three-electron systems, X-ray spectroscopy may be extended towards nuclear physics investigations like the determination of nuclear charge radii.
Highlights
High-precision X-ray spectroscopy of highly charged ions has provided important input in many fields of physics
For the use in X-ray spectroscopy, we developed and fabricated three different general-purpose detector arrays that are optimized for different energy ranges
After the new pulse-tube-cooled cryostat was commissioned in the laboratory, it was for the first time applied at the Experimental Storage Ring (ESR) in a test experiment on hydrogen-like xenon using the detector array which was used in the Lamb Shift experiment [60,61]
Summary
High-precision X-ray spectroscopy of highly charged ions has provided important input in many fields of physics. At the ESR, the X-ray spectrometer FOCAL (FOcusing Compensated Asymmetric Laue) [32] has been developed for X-ray energies around 70 keV, optimized for experiments on the 1s Lamb Shift of hydrogen-like ions For these high X-ray energies, FOCAL obtains an excellent energy resolution of ∆E/E ∼ 10−3. The application of microcalorimeters for high-precision X-ray spectroscopy at the ESR to investigate the 1s Lamb Shift in hydrogen-like heavy ions was proposed by Egelhof et al in 1996 [34]. Aside from systematic uncertainties connected to the high energies of the ions in the storage ring, i.e., the correction of the Doppler shift, the energy resolution of the germanium X-ray detectors is one current limitation of the accuracy of these investigations.
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