Construction : A cylindric metallic tube is filled with 90% Argon and 10% ethanol and a tungsten electrode is connected as shown in the figure. And further electrode is connected to a external circuit which contains high voltage battery and circuit is completed by connecting battery to metallic tube. This creates an electricfield directed towards an electric field from tungsten electrode.

Working : Whenever a nucleus enters the metallic tube, it leads to creation of a positive and an electron.
In the electric field $\overrightarrow{\mathrm{E}}$ electron accelerates towards tungsten electrode while positive ion moves towards metallic tube. When electrons accelerates it knocks out some other electrons from other atom's orbit resulting electrons do the same creating an avalanche effect. All electrons move towards anode and complete the circuit giving potential difference across R_L And recombine with +ve ions collected at point C, which completes the circuit. The signal generated across R_L is amplified and sent  to pulse counter. It Scouts this as 1 nucleus detected. During this period if any other nucleus enters metallic tube it is not counted, this is called dead period.