1. X-ray Imaging Center (XIC), Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang 790-784, Korea; Institute of Physics, Academia Sinica, 128 Academia Rd, sec. 2, Nankang, Taipei 11529, Taiwan; and Faculté des Sciences de Base, Ecole Polytechnique Fédérale, CH-1015 Lausanne, Switzerland