An asymmetric cylinder electron capture detector comprises an electrode configured to define an ionization volume, a source of ionizing radiation disposed within the ionization volume, a collector electrode, with a gas flowing past the collector electrode into the ionization volume. The electron current to the collector electrode provides an indication of the presence of electronegative constituents in the gas passing into the ionization volume. In order to provide wide dynamic range, the face of the collector electrode is disposed as close as possible to the electrode defining the ionization volume, yet is located so as to minimize field-free background current. The collector electrode and the electrode defining the ionization volume are each of cylindrical configuration, and are coaxially aligned but are spaced apart with respect to each other. The two electrodes are mechanically connected via an intermediately disposed cylindrical insulator cylinder. The collector electrode is received in one end of the insulator cylinder, and the electrode defining the ionization volume is received within the other end of the insulator cylinder. The collector electrode has an elongate portion extending into the interior of the insulator cylinder, but spaced apart from the inner surface of the insulator cylinder. This configuration provides a long insulative path to prevent electrical leakage between the electrodes. A transverse gas exit port is provided in that portion of the collector electrode within the insulator cylinder in order to impart turbulence to the flow of gas through the insulator cylinder into the ionization volume.
