Electrostatic x-ray imaging apparatus of the type having a gas-filled chamber bounded by a photocathode and a charge-receiving sheet anode has a control mesh positioned in the gas-filled gap and maintained at a positive voltage with respect to the cathode to facilitate high avalanche amplification and has a voltage between the mesh member and the anode chosen in the plateau region of the x-ray photocurrent versus voltage curve, to achieve a nearly constant efficiency of charge collection at the anode, independent of accumulation of charge thereat.