  By accelerating a Bose-Einstein condensate in a controlled way across the
edge of the Brillouin zone of a 1D optical lattice, we investigate the
stability of the condensate in the vicinity of the zone edge. Through an
analysis of the visibility of the interference pattern after a time-of-flight
and the widths of the interference peaks, we characterize the onset of
instability as the acceleration of the lattice is decreased. We briefly discuss
the significance of our results with respect to recent theoretical work.
