An active area of research in physics in the early 1900s was the attempt to develop instruments to detect and count various types of radiation. Since radiation cannot be discerned by any of the human senses unaided, such devices were essential to any study of radioactivity, discovered in 1896. One of the most important among the early detection instruments invented during this time was the Geiger counter, first developed by Hans Geiger in 1908.
Born in Neustadt, Rheinland-Pfalz, on September 30, 1882, Geiger began his studies in physics at the University of Munich, and completed his doctorate at Erlangen in 1906. In the same year, he accepted a position as research assistant at the University of Manchester, where Ernest Rutherford became director in 1907. Over the next decade, Rutherford and Geiger collaborated on a number of fundamental research studies on the nature of alpha rays and alpha particles. At the outbreak of World War I, Geiger returned to his homeland to serve in the German army. After the war, he was appointed professor of physics at the University of Kiel. He took on a post at the University of Tübingen in 1929 and, in 1936, was appointed chairman of the physics department at the Technical University at Charlottenberg-Berlin. After losing his home and all his possessions in World War II, he died in Potsdam on September 24, 1945.
Geiger constructed the earliest form of his radiation counter in 1908. The device consisted of a wire extending down the center of a sealed metal tube with a glass or mica window at one end. The wire and the metal tube were connected to a power source through an external circuit. When radiation passed through the tube, it created a track of ion pairs. The positive ions in each pair were attracted to the negatively charged metal tube, while the negatively charged electrons were attracted to the central wire.
As the ions and electrons passed through the gas, they collided with other gas molecules, producing further ionization. Because of this avalanche effect, the number of ions and electrons reaching the outside circuit was sufficient to initiate an electric current that could be recorded as a sound or observed as a flash of light.
Geiger continued to work on the design of his counter over the next two decades. In 1928, he made further modifications based on the studies of a German colleague, Walther Müller. The final design, often referred to as the Geiger-Müller counter, soon became one of the two or three most widely used radiation counters available to scientists.
A number of variations of the design have been developed, among them the ionization counter and proportional counter. These variations can be used to identify the types of radation and the characteristics of particle that pass through a counter. The Geiger-Müller counter can also be modified to detect and count non-ionizing radiation, such as neutrons. In this case, boron fluoride gas is used to fill the chamber. The reaction between neutrons and boron nuclei results in the formation of alpha particles, which can then be detected like any other alpha particle.
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