Low-temperature X-ray detectors for precise Lamb shift measurements on hydrogen-like heavy ions

The precise determination of the Lamb shift in heavy hydrogen-like ions provides a sensitive test of quantum electrodynamics in very strong Coulomb"elds, not accessible otherwise. For the investigation of the Lyman-a transitions in 208Pb81‘ or 238U91‘ with su$cient accuracy a high resolving calorimetric detector for hard X-rays (E)100 keV) is presently developed. The detector modules consist of arrays of silicon thermistors and of X-ray absorbers made of high Z material to optimize the absorption e$ciency. The detectors are housed in a specially designed 3He/4He dilution refrigerator with a side arm which "ts to the geometry of the internal target of the storage ring ESR at GSI Darmstadt. T…