FOCAL - precision x-ray spectroscopy for extreme fields in high-z ions

H.F. Beyer1, D. Banaś2, K.-H. Blumenhagen8, F. Bosch1, C. Brandau4, W. Chen1, Chr. Dimopoulou1, E. Forster3,8, T. Gasner1,8, A. Gumberidze4, S. Hagmann1,5, R. Hes1, P.-M. Hillenbrand1, P. Indelicato6, P. Jagodzinski2, T. Kampfer8, Chr. Kozhuharov1, M. Lestinsky1, D. Liesen1, Yu.A. Litvinov1, R. Loetzsch8, B. Manil7, R. Martin8, F. Nolden1, N. Petridis4,5, M.S. Sanjari1,4, K.S. Schulze8, M. Schwemlein1, A. Simionovici10, U. Spillmann1, M. Steck1, Th. Stohlker1,8, C.I. Szabo6, M. Trassinelli10, S. Trotsenko8, I. Uschmann3,8, G. Weber8, O. Wehrhan3,8, N. Winckler1, D. Winters1, N. Winters1, and E. Ziegler11 1GSI Helmholtzzentrum, Darmstadt, Germany; 2Institute of Physics, Swietokrzyska Academy, Kielce, Poland; 3Inst. fur Optik und Quantenelektronik, Friedrich-Schiller-Universitat, Jena, Germany; 4Extreme Matter Institute, EMMI, GSI Helmholtzzentrum, Darmstadt, Germany; 5Institut fur Kernphysik, Goethe-Universitat, Frankfurt am Main, Germany; 6Lab. Kastler Brossel, Universite P. et M. Curie, Paris, France; 7Laboratoire de Physique des Lasers (LPL) UMR 7538 CNRS Universite Paris 13, Villetaneuse, France; 8Helmholtz-Institut Jena, Jena, Germany; 9LGIT, Observatoire des Sciences de l’Univers de Grenoble, Grenoble, France; 10Institut des Nanosciences de Paris, Universite Pierre et Marie Curie-Paris 6 and CNRS-UMR 7588, Paris, France; 11ESRF, Grenoble, France Introduction and Motivation The extraordinarily strong electric field provided by the nucleus of a very heavy one-electron ion exposed to its inner electrons is the testing ground for bound-state quantum electrodynamics (QED) in a largely unexplored domain. Experimentally the QED contribution to the 1s binding energy is accessible via a direct measurement of the K-shell transitions with sufficient accuracy. The corresponding Lyman transitions in highZ ions lie in the hard x-ray region. Previously the x-ray energy has been measured with the aid of germanium xray detectors of limited resolution [1]. The present experiment marks the leap to wavelength-dispersive spectroscopy of substantially higher spectral resolving power simultaneously coping with the low x-ray intensity.