Accurate X-Ray Spectroscopy with FOCAL ∗

Laboratoire de Sciences de la Terre, ENS, Lyon, FranceThe twin crystal-spectrometer assembly, Bi-FOCAL,operated in the FOcusing Compensated Asymmetric Lauegeometry [1] has been arranged for accurate x-ray spec-troscopy at the ESR gas jet as schematically depicted in“gure 1. The goal of this experiment, conducted in a jointeffort with the development of suitable calorimetric low-temperature detectors [2] is the measurement of the 1sLamb shift in a heavy one-electron ion as a test of QEDin the domain of strong “elds.Figure 1: The Bi-FOCAL crystal spectrometer arrange-ment at the ESR gas jet.In March 2006 the two kinds of spectrometers weresuccessfully operated together during a beam time usingbare Pb