Geiger, Hans Wilhelm

Geiger, Hans Wilhelm (1882-1945), German physicist and inventor of the Geiger counter, a type of particle detector. Geiger researched the detection of alpha particles (nuclear particles consisting of two neutrons and two protons), ionization (the formation of electrically charged atoms or molecules), and radioactivity (the emission of nuclear particles, such as alpha particles, or energy by atoms). His work contributed significantly to the development of nuclear physics.

Born in Neustadt, Geiger studied physics at the Universities of Munich and Erlangen. After receiving his doctorate from Erlangen in 1906, he accepted a fellowship at the University of Manchester in England, where he became an assistant to British physicist Ernest Rutherford in 1907. The following year, Geiger built his first device to detect radioactive particles (particles spontaneously emitted by atoms). His device became known as the Geiger counter.

The detection component of the Geiger counter is a metal tube filled with gas under low pressure. The tube and a copper cathode (see Electrode) are under a high voltage. When an alpha particle emitted from the nucleus of a radioactive atom passes through the tube, it ionizes a gas molecule, leaving the molecule with an electric charge. The electrically charged molecule, or ion, is then attracted to the cathode. As it moves toward the cathode, the ion collides with other gas molecules and produces more ions. This brief cascade of ions creates sufficient energy to produce a momentary electrical current. The Geiger counter records each cascade electronically and indicates a cascade with a click. Rapidly repeating clicks indicate the presence of a large number of alpha particles and, therefore, an increased level of radioactivity. Rutherford subsequently identified the alpha particle as the nucleus of a helium atom.

In 1909 Geiger and British physicist Ernest Marsden studied the influence of metal reflectors on alpha particles emitted by the element radium. Although most alpha particles passed straight through the metal, some were deflected at wide angles. After further research, Geiger and Marsden demonstrated that deflection of alpha particles depended on the metal used, and that the degree of deflection was reduced as the atomic weights of the metals decreased. They determined that the positively charged alpha particles were being deflected by the positively charged nuclei of the metal in the reflectors similar to the way that electrical charges repel each other. Their research helped Rutherford develop his theory that an atom consists of a nucleus with a positive charge and an electron shell with a negative charge.

In 1912 Geiger was appointed head of the radioactivity laboratories at the Physikalische Technische Reichsanstalt in Berlin, where he organized a team of researchers and continued his study of atomic structure. His work was interrupted when he served in the German artillery during World War I (1914-1918). When Geiger returned to research following the war, he worked with German physicist Walther Bothe to improve the Geiger counter. They used it in 1924 to research the increase in the wavelength of high-energy electromagnetic radiation when it collides with electrons, called the Compton effect after its discoverer, American physicist Arthur Compton.

Geiger received an appointment as professor of physics at the University of Kiel in 1925. While there he worked with German physicist Walther Müller to improve the sensitivity and durability of the Geiger counter, which became known as the Geiger-Müller counter. In addition to alpha particles, the Geiger-Müller counter detected beta particles (high-speed electrons, or positrons) and ionizing electromagnetic photons. In 1929 Geiger was appointed professor of physics at the University of Tübingen, where he began his study of cosmic radiation. He became head of the physics department at the Technical University of Charlottenberg-Berlin in 1936 and was named editor of Zeitschrift für Physik, a technical publication. Illness slowed the pace of Geiger’s work from 1940 until his death.

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.

Geiger, Hans

b. Sept. 30, 1882, Neustadt an der Haardt, Ger.

d. Sept. 24, 1945, Potsdam

byname of JOHANNES WILHELM GEIGER, German physicist who introduced the first successful detector (the Geiger counter) of individual alpha particles and other ionizing radiations.

Geiger was awarded the Ph.D. by the University of Erlangen in 1906 and shortly thereafter joined the staff of the University of Manchester, where he became one of the most valuable collaborators of Ernest Rutherford. At Manchester, Geiger built the first version of his particle counter and used it and other radiation detectors in experiments that led to the identification of the alpha particle as the nucleus of the helium atom and to Rutherford's correct proposal (1912) that, in any atom, the nucleus occupies a very small volume at the centre.

Moving in 1912 to the Physikalisch-Technische Reichsanstalt ("German National Institute for Science and Technology") in Berlin, Geiger continued his studies of atomic structure. During World War I he served as an artillery officer in the German army. With Walther Bothe, Geiger devised the technique of coincidence counting and used it in 1924 to clarify the details of the Compton effect. In 1925 Geiger accepted his first teaching position, at the University of Kiel. There, he and Walther Müller improved the sensitivity, performance, and durability of the particle counter; the Geiger-Müller counter detects not only alpha particles but beta particles (electrons) and ionizing electromagnetic photons. In 1929 Geiger took up a post at the University of Tübingen, where he made his first observation of a cosmic-ray shower. He continued to investigate cosmic rays, artificial radioactivity, and nuclear fission after accepting a position in 1936 at the Technische Hochschule in Berlin, which he held until he died.

Atom counting

The concept of the atom is an ancient one; the Greek philosopher Democritus (c. 460-c. 370 BC) proposed a form of "atomism" that contained the essential features of the chemical atom later introduced by the British chemist John Dalton in 1810. The British physicist Ernest Rutherford spoke of counting the atoms and in 1908, with the German physicist Hans Geiger, disclosed the first electrical detector for ionizing radiations. The development of wavelength-tunable lasers has made it possible to carry out Rutherford's concept of counting atoms. As stated above, RIS can be used to remove one electron from each of the atoms of a selected type, and the modern version of the electrical detector, known as the proportional counter, can even be made to count a single electron. Thus, all that is required for the most elementary form of atom counting is to pulse the proper laser beam through a proportional counter.

Experimental demonstrations of atom counting can be performed by introducing low concentrations of cesium vapour into proportional counters, commonly used for nuclear radiation detection, that contain a "counting" gas composed of a mixture of argon and methane. Pulsed laser beams used to implement the RIS scheme of Figure 14A

Figure 14A: Figure 14: Resonance-ionization schemes. Photons from lasers are tuned so that their...

can be directed through a proportional counter to detect individual atoms of cesium without interference from the much larger number of argon atoms and methane molecules.