Résumé : The deuteron-emission channel in the β decay of the halo nucleus Li11 was measured at the Isotope Separator and Accelerator facility at TRIUMF by implanting postaccelerated Li11 ions into a segmented silicon detector. The events of interest were identified by correlating the decays of Li11 with those of the daughter nuclei. This method allowed the energy spectrum of the emitted deuterons to be extracted, free from contributions from other channels, and a precise value for the branching ratio Bd=1.30(13)×10-4 to be deduced for Ec.m.>200keV. The results provide the first unambiguous experimental evidence that the decay takes place essentially in the halo of Li11 and that it proceeds mainly to the Li9+d continuum, opening up a new means to study the halo wave function of Li11. © 2008 The American Physical Society.