The document discusses the proportional counter, which is a gaseous state particle detector used to detect nuclear particles and radiation. It consists of a metallic tube filled with argon and methane gas and a thin metallic wire acting as an anode. When radiation enters the chamber, it ionizes the gas, creating positive ions and electrons. An applied voltage causes the electrons to accelerate and collide with other gas atoms, producing an avalanche effect and amplifying the signal. The proportional counter can count and determine the energy of low-energy particles and be used as a spectrometer. However, its amplification factor depends on maintaining a constant applied voltage.