The FOCAL crystal spectrometer project:
Lamb shift studies at the ESR gasjet target

In the most recent 1s Lamb shift experiment for U91+ a ground state Lamb shift of 459.8 eV ± 4.6 eV was obtained which is the most precise test of quantum electrodynamics for a single electron system in the strong field regime and is at the threshold of a meaningful test of higher-order QED contributions. By using decelerated beams, further progress towards an absolute accuracy of 1 eV may be anticipated. In order to obtain a significantly improved precision, future experiments will also use a highly redundant setup at the jet-target and will focus in addition on decelerated ions combined with the crystal spectrometers or bolometers which presently are under construction. As an example, a new kind of x-ray spectrometer, setup in the FOcussing Compensated Asymmetric Laue (FOCAL) geometry, has been developed for this purpose. The spectrometer serves in measuring small wavelength differences between the fast moving x-ray source, represented by the circulating ions in the ESR, and a stationary calibration source. It is designed for energies between 50 and 100 keV or wavelengths between 25 and 12 pm leading to Bragg angles of less than 4o for a Si(220) crystal. In the experiment, this focusing transmission crystal spectrometer will be combined with a segmented germanium x-ray detectors.

Very recently first measurement with the FOCAL spectrometer has been performed at ESR using gold beam and nitrogen gas-jet target.


H. F. Beyer, Nuclear Instruments and Methods in Physics Research A 400, (1997) 137. (NIMB link)
H. F. Beyer at al., GSI Scientific Report 2000 (PDF version).
D. Protic, T. Stöhlker, H. F. Beyer et al., IEEE Trans. Nucl. Science (2000).
(IEEE link)
Th. Stöhlker et al., Nuclear Instruments and Methods in Physics Research B 205, (2003) 210–214 (NIMB link)

Talks and posters:

Poster - Applications of Position Sensitive Germanium Detectors for X-Ray Spectroscopy of Highly-Charged Heavy Ions. (PDF version)
Talk (H. F. Beyer) - X-ray precision spectroscopy at ESR (PDF version)

Talk (H. F. Beyer) (PDF version)


FOCAL - FOCAL crystal spectrometer page (H. E. Beyer personal page)
New x-ray optics for measurement of quantumelectrodynamic effects (PDF Version)
Detector development - (Research part of this page)
Lamb-Shift experiments on high-Z one-electron ions - GSI Atomic Physics QED Page