D4c: A very high precision diffractometer for disordered materials

The disordered-materials diffractometer D4 at the Institut Laue-Langevin (ILL) has been thoroughly upgraded through improvements in detectors, collimation and shielding to become the D4c instrument. A larger solid angle of detection has increased the total counting rate by a factor of 5, thereby reducing random error in the diffraction measurement, and a corresponding factor of 5 improvement in detector stability has reduced the principal cause of systematic error. The overall precision of the instrument has therefore been increased by a factor of 5 as compared to its previous version, D4b. We present an overview of the D4c instrument's design as well as some results of the successful very high precision experiments performed at D4c since its commissioning in May/June 2000.