Abstract
A zone-refined aluminium single crystal was mounted on a Lang X-ray camera, stressed in a controlled way and photographed in the stressed state at different stress levels. Based upon the results a model of a dislocation multiplication source operating close to a surface at low stresses is proposed. Primary dislocations close to the surface cross-slip to reduce their line length, making anchoring points at which single-ended Frank-Read sources may start operating and produce rows or bands of new dislocations.