In the early 20th century, many scientists were looking for ways to measure radiation, which had been discovered near the end of the previous century. Radioactivity is the spontaneous emission of radiation (waves or subatomic particles) exhibited by certain elements due to either the unstable nuclei of its atoms or a nuclear reaction. To investigate radioactivity adequately, researchers needed to detect and quantify radiation. There was, however, one problem: Radiation cannot be detected by the unaided human senses.
Enter Hans Geiger, a young German scientist on the staff at the University of Manchester. Having received his Ph.D. from the University of Erlangen in 1906, he moved to England and began studying radioactive emissions with up-and-coming nuclear physicist Ernest Rutherford, chair of the physics department. Rutherford had an ardent interest in special particles, called “alpha particles,” that are emitted from radioactive materials (alpha particles are nuclei of helium-4 atoms). Knowing that alpha particles can travel through very thin solids, Rutherford and Geiger developed a method of detecting the particles using gold foil and a screen: After passing through the foil, the particles hit the screen, producing on impact barely detectable flashes of light. The scientists (Geiger especially) spent innumerable hours counting these flashes in a dark room. As Rutherford recounted in a 1908 letter to a friend, “Geiger is a good man and work[s] like a slave.... [He] is a demon at the work and could count at intervals for a whole night without disturbing his equanimity.”
In 1908, hoping to reduce the time, strain and imprecision involved in the work, Geiger built a device that automatically counted individual particles. This early form of what became known as the Geiger counter detected only alpha particles. A later version developed in the late 1920s by Geiger and his student Walther Müller was sensitive to all types of ionizing radiation.
The Geiger counter typically consists of a cylinder (called a Geiger tube) capped with a thin material such as mica through which radiation can penetrate. The cylinder is filled with inert gas and connected to a voltage source. Down the middle of the cylinder runs a wire or needle that functions as a positive electrode; the cylinder wall is a negative electrode. When a particle of radiation penetrates the cap of the tube, it knocks a charged particle off a gas atom inside, thus ionizing it. This displaced particle then heads, depending on its charge, either toward the positively charged wire or the negatively charged cylinder wall. This sets in motion an ionizing avalanche: More and more particles get knocked off gas atoms as the first one rushes toward the electrode attracting it. This flurry of particles creates an electrical pulse when it hits the electrode, which is then amplified so it can be observed, usually as a clicking sound. The number of clicks generated per second by a Geiger counter indicates the intensity of the radiation it is measuring.
Using the Geiger counter, Geiger and Rutherford performed many groundbreaking experiments involving alpha rays and alpha particles. Upon the outbreak of World War I, however, Geiger returned to his motherland to serve in the armed forces. After the war, he taught at a number of German universities and continued perfecting his radiation detector. Its final form is generally known as the Geiger-Müller counter in recognition of Müller’s contribution. Other scientists also made modifications to the design. One of the most notable adaptations makes the device capable of detecting neutrons and other non-ionizing radiation.
With the much faster and more sensitive detection device he built with Müller, Geiger made several important scientific contributions. In 1925, he used the counter to experimentally verify the Compton effect, which is the scattering of X-rays and their decrease in energy upon interaction with matter. A few years later, he used several Geiger counters at once to observe showers of cosmic rays, a phenomenon which became the focus of his research.