The document describes the proportional counter, which is a gaseous state particle detector used to detect nuclear particles and radiation. It consists of a cylindrical metal tube filled with argon and methane gas and a thin metal wire running down the center as an anode. When radiation enters the tube, it ionizes the gas, producing electron-ion pairs. An applied voltage between the wire and tube causes gas amplification through avalanching, resulting in a pulse signal. The proportional counter can be used for particle counting and energy determination, and has advantages like low-energy detection but requires stable applied voltages.