Experimental evidence of the superfocusing effect for axially channeled MeV protons

Sub-\AA{}ngstr\"om focusing of megaelectronvolt (MeV) ions within axial channels was predicted over 10 years ago, but evidence proved elusive. We present experimental angular distributions of axially channeled MeV protons in a 55-nm-thick (001) silicon membrane through which multiple scattering is negligible. Fine angular structure is in excellent agreement with Monte Carlo simulations based on three interaction potentials, providing indirect evidence of the existence of the superfocusing effect with flux enhancement of around 800 within a focused beam width of $\ensuremath{\sim}20\phantom{\rule{0.16em}{0ex}}\mathrm{pm}$.