A charged particle beam system for imaging and processing targets is disclosed, comprising a charged particle column, a secondary particle detector, and a secondary particle detection grid assembly between the target and detector. In one embodiment, the grid assembly comprises a multiplicity of grids, each with a separate bias voltage, wherein the electric field between the target and the grids may be adjusted using the grid voltages to optimize the spatial distribution of secondary particles reaching the detector. Since detector lifetime is determined by the total dose accumulated at the area on the detector receiving the largest dose, detector lifetime can be increased by making the dose into the detector more spatially uniform. A single resistive grid assembly with a radial voltage gradient may replace the separate grids. A multiplicity of deflector electrodes may be located between the target and grid to enhance shaping of the electric field.