1. Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-6102, Chemical Sciences and Technology Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8313, Physics Department, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213-3890, Department of Biomedical Engineering, Johns Hopkins University, Baltimore, Maryland 21218, and Department of Physiology and Biophysics, School of Medicine, University of California at...