Ionization-type particle detector

An ionization-type particle detector comprising:

• an inner electrode,
• a single radioactive source of ionizing radiation mounted proximate said inner electrode for directing a generally conical beam of ionizing radiation along a predetermined axis of symmetry, said beam having an envelope,
• a generally cylindrical outer electrode symmetrically disposed about said inner electrode, said outer electrode having a longitudinal axis of symmetry substantially conincident with said predetermined axis of symmetry of said beam, said outer electrode having an opening therein communicating with the environment through which particles enter from the environment,
• a source of potential connected between said inner and outer electrode,
• a generally cylindrical intermediate electrode disposed symmetrically about said inner electrode to define therebetween a compensation zone having a substantially saturated ion current which is substantially unaffected by changes in particle concentration therein, said intermediate electrode being generally concentric to said outer electrode to define therebetween a particle sensing zone having an unsaturated ion current which decreases with increasing particle density therein, said intermediate electrode having an opening in the top thereof co-extensive in area to the cross-sectional area of said intermediate cylindrical electrode, said opening being defined by an upper circular edge through which radiation from said source is emitted with the envelope of said beam dividing said sensing zone into an upper bipolar region interiorly of said beam and a lower unipolar region exteriorly of said beam, said intermediate and outer electrodes having diameters and heights designed relative to each other to approximately equally divide the potential of said source between said compensation and sensing zones and produce, in response to an increase in particles in said sensing zone, an ionization current decrease in said unipolar region which exceeds the ionization current decrease in said bipolar region by a factor of approximately three, thereby providing enhanced insensitivity of the detector to variations in environmental air velocity.