an ionization smoke detector is disclosed in which an electric field is established in an ionization chamber which includes an outer cup - shaped electrode defining the chamber and having side wall apertures for permitting airflow therethrough and a circular inner electrode within the chamber . a radiation source is arranged to produce bipolar and unipolar regions in the chamber . a cylindrical control screen coaxial with the outer electrode and spaced therefrom is electrically connected to and supported by the inner electrode in the chamber .