A new electrode geometry for a crossed-beam mass-spectrometer ion source has been developed. The geometry is particularly suited to the measurement of electron ionization cross sections for low vapor pressure materials and for materials which would normally contaminate source surfaces. Ions are generated at the intersection of an electron beam and an atomic beam in the center of a six-sided ionization chamber; each side of which is formed out of fine wire grids. By applying different electrical potentials to the grids and by passing trickle currents directly through the grid wires (resulting in small voltage drops), a variety of calculable field configurations may be set up. Passing higher currents through the grids allows the structure to be flashed clean. No insulators are exposed to any beam. The ions are extracted perpendicularly to both the electron and atomic beams; extraction in one direction shoots them into a large quadrupole mass spectrometer, extraction in the opposition direction shoots them into a total ion collector. The chamber may be run with a uniform electric extraction field, with conventional nonuniform fields, as a field-free region, in a saddle potential mode, or in a variety of pulsed modes. The source has been used in the uniform field mode to measure the absolute electron-ionization cross sections for several metals.