APPLICATION OF LOW-TEMPERATURE CALORIMETERS FOR PRECISE LAMB SHIFT MEASUREMENTS ON HYDROGEN-LIKE VERY HEAVY-IONS

The precise determination of the 2s(1/2), 2p(1/2), 2p(3/2) --> 1s(1/2) X-ray transitions in hydrogen-like very heavy ions such as Pb-208(81) or U-238(91+) provides a sensitive test of QED, especially of higher order contributions to the self-energy which are not accessible by al ternative methods. Such measurements became possible recently by X-ray spectroscopy using highly charged ions stored and cooled in heavy ion storage rings. A better energy resolution of the X-ray detector is mo st essential in order to improve the experimental accuracy, which is p resently about one order of magnitude worse than the theoretical uncer tainty. An energy resolution of Delta E = 30-50 eV for E(y) = 50-100 k eV, and a photopeak efficiency above 30% may be provided by a low temp erature calorimeter, It will be constructed on the basis of already ex isting arrays of silicon microcalorimeters, but with larger volume and high Z absorbers. Such a detector will allow a more precise determina tion of the 1s-lamb shift and for the first time the direct investigat ion of the 2s-Lamb shift in hydrogen-like heavy ions. In the present c ontribution the experimental scenario for QED experiments at storage r ings, the present status of experimental and theoretical investigation s and the design of an appropriate low temperature detector are discus sed.