The micro segregation of doped silicon crystals grown by the float zone technique was investigated by using a heat pulse technique. The heat pulses (3-10 Hz) generated finely spaced periodic time markers which overlay the dopant inhomogeneities caused by convective instabilities in the melt. The microscopic growth rate was determined by measuring the distance of the markers. For a flow regime governed by time-dependent thermocapillary convection, the average growth rate of 2 mm/min fluctuated from 1.27 to 2.63 mm/min with fluctuation frequencies between 0.01 and 0.15 Hz. The microscopic growth rate becomes constant when applying a static axial magnetic field of 500 mT. The impact of concentration fluctuations within the solute boundary layer ahead of the crystal-melt interface was observed qualitatively by comparing the interface profile given by the pulse markings with the contours of the convectively induced striations. (C) 2002 Elsevier Science B.V. All rights reserved.