In this paper we consider the problem of determining the structure of the electric field near the surface of a flat insulated body under conditions of a deep vacuum. It is assumed that the emitted particles are electrons leaving the body surface under the influence of ionizing radiation whose velocities distribution near the surface is isotropic. It is estimated the thickness of the screening layer under conditions of stationary emission from a flat surface. The solutio of the problem of determining a stationary self-consistent electric field near the surface is found in a simple analytical form. The thickness of the screening layer is calculated from this formula.