Geiger Counter

Geiger Counter, device that detects and records information about subatomic particles emitted by radioactive substances. These particles are invisible to the eye, and Geiger counters are used to guard against human exposure to radiation, which can cause illness or death. See also Particle Detectors; Elementary Particles.

A Geiger counter is a sealed metal tube filled with an inert gas, such as argon. One fine wire, called an electrode, runs down the center of the tube. When the Geiger counter is on, the difference in electric voltage between the positively charged electrode and the negatively charged walls of the tube is about 1000 volts. Although the tube carries such high voltage, no electric current passes between the electrode and the walls because the inert gas does not normally conduct electricity.

When a charged particle, such as an electron, enters the tube, it interacts with the gas to create ions, atoms or molecules with an electric charge. The positive ions move toward the negatively charged walls of the tube and the negative ions move toward the positively charged electrode, creating a momentary electric current between the electrode and the tube wall. The Geiger counter relays the current to the user with an audible click or a visual signal to indicate the presence of a radioactive particle. Each signal represents that one particle has entered the detector.

In 1908 German physicist Hans Wilhelm Geiger created the first version of this counter. Later, Geiger collaborated with German physicist Walther Müller to develop the Geiger-Müller counter, which counts subatomic particles with no charge, such as neutrons.