Density of a Single-Crystal Natural Silicon Sphere

The density of a single-crystal natural silicon sphere identified as ldquoAvo#3rdquo has been redetermined at the National Measurement Institute, Australia (NMIA), as part of an international collaboration to determine the value of the Avogadro constant. A systematic review of the measurement system found that a significant bias was introduced in the measurement of the volume of the sphere, which is calculated from the interferometric measurements of the mean diameter. It was found that the dynamic response of the laser that is used to generate the frequency steps for the phase-shifting interferometry introduced a bias to the returned phase value. We determined the magnitude of the bias and then applied corrections to obtain the final measured value. The density reported here is based on some new and some recalculated measurements taken between April 2000 and May 2006 during the ongoing development of the measurement system.