Status of the international effort on the x-ray crystal density work and its progress towards a measurement of the Avogadro constant

The present status of an international attempt of several national metrology institutes to determine the Avogadro constant with a relative standard uncertainty better than 2ldr10
^-8
is presented. To this end, a world-wide collaboration has been set up to produce a 5 kg
²⁸
Si single crystal with an enrichment factor greater than 99.985%. Technological steps for the growth of the
²⁸
Si single crystal, manufacturing of two 1 kg
²⁸
Si spheres, and first experimental results are reported.