Laser Monochromator of a Relativistic Proton Beam with Resolution and Accuracy up to 2×105

We propose a method by which it is possible to narrow the energy distribution of relativistic protons. The method is based on the conversion of relativistic hydrogen ions into neutral hydrogen atoms, consequent velocity-selective excitation of relativistic hydrogen atoms by the $1S-2P$ transition, and photoionization of the selectively excited atoms. The method can be realized using existing lasers on present accelerators.