[en] The Rayleigh-Taylor instability has been investigated in accelerated planar foils. The foil in which the instability is to be observed is accelerated from one side using two laser beams, and backlit by thermonuclear alpha-particles from an adjacent implosion of a deuterium-tritium filled microballoon. The alpha-particles, after probing the unstable foil, are recorded in CR-39 plastic. Growth of the R-T instability leads to a thickness distribution across the foil, which can be directly determined from the energy of each particle recorded in CR-39, computed from high resolution measurements of the etched tracks. The technique is sensitive to small scale length instabilities ∼ 1 μm. Foils which are pre-perturbed, as well as planar foils driven by modulated laser beams, are used to seed the instability. The R-T instability has been observed in a number of foils. The range loss varies from almost zero to over 10 μm, and these variations in path length indicate the instability is present. (author)