Abstract: An ionization-type particle detector having an insulating base which mounts an inner electrode provided with a source of ionizing radiation which directs a generally conical beam perpendicularly away from the base; an intermediate cylindrical electrode symmetrically disposed around the inner electrode which has an open upper end through which the radiation beam exists from a compensation zone defined by the inner and intermediate electrodes; and an outer cup-shaped electrode symmetrically disposed about the intermediate electrode and which defines therewith a sensing zone into which radiation enters from the top opening in the intermediate electrode. The sensing zone is divided by the envelope of the radiation beam into an upper bipolar region and a lower unipolar region.