The geiger counter, also known as Geiger-Mueller counter, or G-M counter, is a device for electrically recording and measuring the presence of radioactivity. The tube of the G-M counter is a metal cylinder with a wire, insulated electrically from the cylinder, fixed in the cylinder's axis. An electric voltage, sometimes in excess of 1,000 volts, is impressed on the tube, the positive charge being on the central wire. The cylinder is filled with a gas, such as a halogen gas or a mixture of an inert gas and methane, at reduced pressure. Radiation, entering the tube through a window, collides with one or more gas partióles, knocking off electrons, which may, as they are rapidly accelerated toward the positively charged wire, knock more electrons off other gas atoms or molecules. The many electrons liberated by the passage of the radiation are sufficient to cause a measurable electric current to flow. Although the Geiger counter is not so sensitive to radiation as is the scintillations counter, its low cost makes it readily available for uranium hunters or civil-defense radiation monitors.
