Using a microcalorimeter to measure the Lamb shift in hydrogenic gold and uranium on cooled, decelerated ion beams

A precise determination of the Lamb shift from photons emitted by highly charged, one electron ions represents one of the most sensitive tests of QED in strong electromagnetic fields. Recent progress in the production and cooling of intense beams of fully stripped Au and U in the SIS/ESR synchrotron storage ring at GSI, Darmstadt has made it possible to obtain precision spectroscopy of these ions. A fully stripped beam of either Au79+ or U92+ ions is injected, stored and cooled in the ESR and interacts with an internal gas target. The capture of an electron and the subsequent population of a 2p or 3p state will lead to a decay by either Lyman or Balmer X-ray emission. Although measurements of the 1 s Lamb shift in U with Ge ionization detectors accurate to ∼3% have provided a test of QED for the high Z domain, the experimental errors (±13 eV) are about one order of magnitude larger than the accuracy theoreticians presently claim (±1 eV). We present the results from initial broad band experiments using NTD Ge microcalorimeters to measure the 2 s Lamb Shift in Au and U at the ESR. The broad band coverage of the microcalorimeter makes it possible to reduce the systematic uncertainties in the Doppler corrections while the high-energy resolution reduces the statistical error in the absolute energy calibration.