First observation of scattering of sub-GeV electrons in ultrathin Si crystal at planar alignment and its relevance to crystal-assisted 1D rainbow scattering

Here we report on the first measurements of 255 MeV electron scattering by an ultrathin 0.58 μm Si(111) crystal at angles of incidence less than the Lindhard critical angle. Computer simulations of trajectories in the ultrathin crystal explain the appearance of specific angular distributions of scattered electrons as a sequence of multiple-value connections between the points of incidence and deflection (scattering) angle. This is similar to the classical rainbow scattering (RS) of waves and particles on a three-dimensional (3D) potential. The principal difference is that scattering by the ultrathin crystal under channeling conditions is dependent on two additional parameters - the crystal thickness (longitudinal size of one-dimensional (1D) potential formed by the periodically arranged crystal planes) and the angle between the beam and crystal planes. The results of simulations agree with the experimental data. The obtained results contribute to an understanding of the physics of relativistic sub-GeV electron scattering by ultrathin crystals and allow it to be recognized as the fourth scattering type among doughnut scattering, scattering at planar alignment, and mirroring, i.e., one-dimensional 1D rainbow scattering (1D-RS).