Walther Bothe was a prominent German physicist during the "Golden Age of Physics," circa 1900-1930. He was one of the few graduate students to study with physicist Max Planck in Berlin and began his professional career with Hans Geiger, inventor of the Geiger counter. During the 1920s he developed the coincidence counting technique for tracking the collisions between electrons and electromagnetic quanta, for which he was awarded the Nobel Prize in 1954. Bothe also contributed many ideas to nuclear reaction theory.
Walther Wilhelm Georg Bothe was born in Oranienburg, Germany, on January 8, 1891, the son of Fritz and Charlotte Hartung Bothe. His father was a merchant. At age 23, Bothe received his Ph.D. degree from the University of Berlin. His dissertation concerned the interactions between light and molecules as they related to the reflection, refraction, dispersion, and absorption of light. After graduation he began work at the Physical-Technical Institute under Hans Geiger, the inventor of the Geiger counter.
World War I was just beginning, and Bothe immediately entered the army as a machine gunner. He was soon captured and spent the rest of the war as a Russian prisoner in Siberia, where he managed to study Russian and work on physics.
After World War I, Bothe returned to research with Geiger and taught at the University of Berlin. He became interested in quantum theory, which was generally accepted by 1920 but had not yet been confirmed experimentally. The theory suggested that electromagnetic energy, such as light, displays characteristics of particles as well as of waves. The work of Arthur Holly Compton in 1923 had shown that electrons could scatter X rays in patterns as if they were particles, and that in the process the X rays would transfer some energy and momentum to the electrons, which would recoil from their atoms. This phenomenon became known as the Compton effect.
After Compton's discoveries, Niels Bohr and others had hypothesized that the electron's recoil would bear a statistical relationship, not a direct relationship, to a wave quantum, and that the energy and momentum of the participating objects would be conserved in the sum of many collisions rather than at the level of each interaction. In 1924 Bothe and Geiger tested this hypothesis using two of Geiger's particle counters, one to detect the recoil of electrons from hydrogen atoms, and the other to track the scattering of X-ray quanta. They found that the recoil and scattering events occurred together much more often than could be explained by chance, and concluded that each scatter event corresponded with a specific electron recoil. This showed that energy and momentum were conserved in each interaction, and that the laws of classical physics applied at the subatomic level. It was this work that earned Bothe his share of the 1954 Nobel Prize with Max Born. The technique became known as coincidence counting and turned out to be useful in the study of nuclear reactions and cosmic rays.
During the 1930s life became difficult for German physicists as the National Socialists gained power under Adolf Hitler. Nazi ideology associated theoretical physics, especially Albert Einstein's, with Jewish intellectualism. At least 11 Nobel laureates in physics left the country before the end of World War II. As one of those who stayed, Bothe was faced with the decline of physics education and a slowing of research momentum. He opposed National Socialism and was disliked by the Nazis because he continued to support the theory of relativity. In 1932 Bothe was chosen to replace the Nazi favorite Philipp E. A. Lenard at the University of Heidelberg. But Bothe stayed only two years in the Reich-dominated department before transferring to the Kaiser Wilhelm Institute (later the Max Planck Institute). Bothe continued teaching and research, convinced that staying involved in the educational system would be a form of resistance to National Socialism. In the late 1920s Bothe's study of radioactivity laid the groundwork for James Chadwick's later discovery of the neutron, and in 1929 he used the coincidence counting method to detect cosmic rays. At the Max Planck Institute for Medical Research, Bothe supervised construction of Germany's first cyclotron, a circular particle accelerator, completed in 1943.
During the war, Bothe worked with Werner Karl Heisenberg on Germany's efforts to develop an atom bomb. Like the Americans, the Germans had to develop a reactor in order to acquire a large enough supply of plutonium, and the reactor required a substance that could moderate the speed of the neutrons. At the time, the choice for this medium lay between heavy water (deuterium oxide) and graphite. The former was difficult to obtain, whereas the latter was relatively plentiful.
Between 1940 and 1941, Bothe measured graphite's ability to absorb neutrons and concluded that graphite would be inferior to heavy water. His calculations were wrong, but no one detected the error at the time. As a result, the Germans chose to rely on heavy water, while the Americans, who had made more accurate measurements, chose graphite. Thus the German bomb project, already slowed by the Nazis' inability to make long-range research commitments, fell far behind the American effort. Some people have speculated that Bothe's error was intentional, since he was known as an especially careful experimenter. He himself never accounted for it.
After World War II, Bothe continued work at the Max Planck Institute and the University of Heidelberg. Bothe died in Heidelberg on February 8, 1957.
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