Hans Geiger

German physicist

• Born: September 30, 1882
• Birthplace: Neustadt an der Haardt (now Neustadt an der Weinstrasse), Germany
• Died: September 24, 1945
• Place of death: Potsdam, Germany

Geiger, working with Walther Müller, invented the Geiger counter, which detects electrically charged particles. Geiger aided Ernest Rutherford in a series of experiments that discovered that atoms consist of small, positively charged nuclei surrounded by a larger cloud of negative charge.

Early Life

Hans Geiger (hahnz GI-gur) was born in Neustadt an der Haardt, Germany. He was the oldest of five children. His father, Wilhelm Ludwig Geiger, was a professor of philosophy at the University of Erlangen from 1891 to 1920. Geiger graduated from the Erlangen Gymnasium, or high school, in 1901, and entered the University of Munich. He then studied physics and mathematics at the University of Erlangen, where he completed his thesis on electrical discharges through gases and was awarded a Ph.D. in 1906.

After he received his doctorate, Geiger accepted a research position at the University of Manchester in England. Initially, he served as an assistant to Arthur Schuster, who was a professor of physics at the University of Manchester. In 1907, Ernest Rutherford succeeded Schuster, and Geiger became Rutherford’s assistant. Shortly after his arrival at Manchester, Rutherford, who was widely known for his work on radioactivity, was awarded the Nobel Prize in Chemistry in 1908.

Life’s Work

Geiger’s work with Rutherford was significant. Rutherford had devised a series of experiments to investigate the internal structure of the atom. He used “alpha particles,” which are emitted in some radioactive decay processes, which were known to penetrate thin solid foils. Rutherford and Geiger assumed that alpha particles could pass through atoms and could serve as probes of the interior structure of atoms. In 1908, Rutherford and Geiger demonstrated that these alpha particles were doubly charged helium ions.

Geiger and Rutherford designed a device that shot alpha particles through a thin piece of gold foil and then onto a screen. Each alpha particle produced a tiny flash of light when it hit the screen, allowing both the position and the number of alpha particles striking the screen to be determined. Rutherford used this device to determine how each alpha particle was deflected by its encounter with the gold nucleus, and he developed a model of the atom that consisted of a very small positively charged nucleus surrounded by a much larger cloud of negative charge.

Counting the flashes was tedious, so Geiger decided to investigate easier and more accurate ways to count the alpha particles. His solution was an early version of what came to be called the Geiger counter , an electronic device used to detect and count the alpha particles. The device Geiger invented consisted of a tube filled with gas, with a high voltage applied across the tube. When an alpha particle passed through the tube it would briefly ionize some of the gas, allowing an electric current to pass through the tube. By monitoring this current the passage of an alpha particle through the instrument could be determined.

In 1911, Geiger, working with John Mitchell Nuttall, discovered the relationship between the decay constant the ratio between the number of radioactive atoms and the rate at which that number decreases because of radioactive decay of a radioactive isotope and the energy of the alpha particle emitted in the decay. This relationship is now called the Geiger-Nuttall law. This law shows that the shorter the half-life, which is the time required for half of the starting amount to decay, of an isotope, the more energetic the alpha particle emitted in its decay.

Geiger and Rutherford had observed that the decay of uranium produced alpha particles of two distinctly different energies. In 1912, Geiger and Nuttal demonstrated that the two different alpha particle energies observed in the decay of uranium came from two uranium isotopes, atoms of the same chemical element having different atomic weights, each having a different half-life.

Geiger returned to Germany in 1912, where he became the director of the newly established Laboratory for Radioactivity at the Physikalische-Technische Reichsanstalt (physical-technical institute) in Berlin. Geiger’s research was interrupted by World War I, in which he served as an artillery officer in the German army. Following the war, Geiger married Elisabeth Heffter.

Geiger was appointed a professor at the University of Kiel in 1925. There, he and his first Ph.D. student, Walther Müller, set about to improve on the original design for the Geiger counter, which could detect only alpha particles. In 1928, the new Geiger counter, which could detect all kinds of ionizing radiation, was unveiled by Geiger and Müller. While at Kiel, Geiger used his counter to confirm the existence of light quanta, or packets of energy, an important confirmation of one of the principles of the new field of physics called quantum mechanics.

Geiger moved to the University of Tübingen in 1929, where he served as director of research at the Physics Institute. He continued to improve the measurement capability of the Geiger counter, eventually reaching a point where it was capable of determining the position of a charged particle to about one centimeter and its arrival time to a hundred-millionth of a second. He used the improved Geiger counter to detect bursts of radiation from cosmic-ray showers, which result when a very energetic cosmic ray hits the atmosphere of the earth, producing a cascade or shower of secondary charged particles. He continued to investigate these cosmic-ray showers over the rest of his career.

Geiger returned to Berlin in 1936, accepting a position in physics at the Technische Hochschule (Institute of Technology). During this period, Geiger became active in politics after the National Socialist Party, led by Adolf Hitler, took power in Germany. He and many other German scientists opposed governmental interference in their work. In 1936, Geiger helped draft a document signed by seventy-five of Germany’s most respected physicists. The document, which was presented to the German education ministry, urged the government not to interfere with scientific research and complained that there were too few new physicists and physics students because of newspaper attacks on physics by National Socialists.

During World War II, Geiger was a member of Uranverein, the Uranium Club, a team of German scientists that attempted to build an atomic bomb . Since much of the preliminary research on uranium decay was performed by German scientists, it remains a matter of debate why this group of physicists failed in their efforts to develop an atomic bomb. Some physicists and political scientists speculate that foot-dragging on the part of German scientists, concerned over the ethics of developing such a powerful weapon, contributed to the failure of the project.

Geiger’s contribution to the atomic bomb project was limited because he was often in bed with rheumatism, which he developed during his service in World War I. He died in Potsdam a few months after the end of World War II.

Significance

Geiger is best known for his invention of the Geiger counter, an instrument that remains the main detector of ionization radiation. However, his most important achievement was his contribution to the understanding of the structure of the atom. Working with Rutherford, he observed that alpha particles occasionally are deflected through very large angles when they hit a thin gold or silver foil. This scattering experiment was critical to the development of Rutherford’s nuclear theory of the atom, demonstrating that the positively charged nucleus occupies a very small volume at the center of the atom. These early experiments led to Rutherford’s discovery of the atomic nucleus.

Geiger’s lifelong friendship with Rutherford, as well as similar friendships with other German physicists and Western scientists, may have led to the slowdown, or even the demise, of work toward building an atomic bomb for Germany. Concern about the ethics of the atomic bomb project, and the project’s lasting implications, also is part of Geiger’s legacy.

Bibliography

Powers, Thomas. Heisenberg’s War: The Secret History of the German Bomb. New York: Da Capo Press, 2000. A well-researched, six-hundred-page account of the German effort to build an atomic bomb, focusing on the question, Did German scientists deliberately stall the project? The book highlights Geiger’s contributions.

Segrè, Emilio. From X-Rays to Quarks: Modern Physicists and Their Discoveries. New York: W. H. Freeman, 1980. Segrè, a nuclear physicist, traces the history of nuclear physics, not only showing the main discoveries but also explaining how these conclusions were reached. He describes the personalities of the leading physicists and discusses the errors they made before they achieved an understanding of nuclear structures and reactions.

Von Oertzen, W., ed. Detectors in Heavy-Ion Reactions: Proceedings of the Symposium Commemorating the One-Hundredth Anniversary of Hans Geiger’s Birth. New York: Springer, 1983. A 258-page collection of articles focusing on heavy-ion detectors. Includes an excellent chapter on Geiger’s life and his contributions to the development of charged-particle detectors.