The most outstanding contributions of the German physicist Walther Bothe (1891-1957) were the invention of the coincidence method for the study of individual atomic and nuclear processes and the discovery of a nuclear radiation later identified as neutron emission.
Walther Bothe was born on Jan. 8, 1891, in Oranienburg near Berlin. He went to the University of Berlin, where, in addition to physics and mathematics, he did considerable work in chemistry. He was a pupil of Max Planck and wrote under Planck's mentorship his doctoral dissertation on the molecular theory of refraction, reflection, dispersion, and extinction.
After serving as an officer in World War I, Bothe returned to Berlin, where he started research with the rank of Regierungsrat (government counselor) at the Physikalisch-Technische Reichsanstalt, the German equivalent of the National Bureau of Standards in Washington, D.C. His immediate superior was Hans Geiger, director of the laboratory of radioactivity and inventor of the Geiger counter. Bothe's first research in Geiger's laboratory concerned the single and multiple scattering of electrons, for which he developed comprehensive mathematical formulas.
The most crucial contribution of Bothe to the understanding of the Compton effect was made in collaboration with Geiger. Their work was based on the coincidence method developed by Bothe for the use of two or more Geiger counters. When a coincidence circuit of Geiger counters was coupled with a cloud chamber, it became possible to ascertain the time parameters of the ionization paths visible in the cloud chamber. This represented an important advance, and Bothe and Geiger used it to good advantage in the debate that ensued in the wake of the discovery of the Compton effect.
In 1925 Geiger accepted an invitation to the University of Kiel, and Bothe succeeded him as director of the laboratory of radioactivity at the Reichsanstalt. Four years later Bothe provided further evidence of the enormous potentialities of his coincidence method. This time it did not consist in establishing the simultaneous occurrence of two phenomena but in the follow-up of the motion of one single particle amid a great number of simultaneous ionization effects. On such a basis Bothe demonstrated that the hard component of cosmic rays was not gamma radiation but a stream of particles, such as protons and light nuclei.
Simultaneously, Bothe began studying the bombardment of light elements by alpha particles. He found that when boron was hit by alpha particles a carbon isotope was formed with the simultaneous emission of a proton. Later he observed similar results with lithium, iron, sodium, magnesium, aluminum, and beryllium. In these processes two types of radiation also occurred, only one of which was isotropic. The isotropic one consisted of low-energy gamma rays. Far more elusive was the other type of radiation, which Bothe and Becker investigated more closely in their experiments with beryllium exposed to alpha particles from polonium. Two years later the Joliot-Curies showed that the anisotropic radiation could produce secondary protons, but it was James Chadwick, a few weeks later, who showed that the radiation emitted from beryllium, as originally observed by Bothe, consisted of neutrons. Thus Bothe played a pivotal role in ushering in the age of nuclear energy to which the knowledge and control of neutrons are crucial.
In 1930 Bothe became professor of physics and director of the Institute of Physics at the University of Giessen. Two years later he succeeded in the same capacity the Nobel laureate P. Lenard at the University of Heidelberg. In 1934 he became director of the Institute of Physics at the Kaiser Wilhelm Institute for Medical Research in Heidelberg. He energetically set about improving the research facilities of the institute. First he installed a Van de Graaff generator, with which he produced, in collaboration with W. Gentner, the first clear evidence of nuclear isomerism in the course of work with bromium-80. His second greatest achievement at the institute was the installation of a cyclotron in 1943. It was the only one of its kind to remain operational in Germany throughout the war.
Bothe's part in the German uranium project consisted in the study of neutron diffusion, and he became the first, with a paper published in 1941, to outline the socalled transport theory of neutrons. This and Bothe's derivation of the "disadvantage factor" in connection with the measurement of neutron density represented the two chief German wartime contributions to nuclear reactor theory. Bothe also developed noteworthy ideas on the multiplication of thermal neutrons in uranium and on the effect of the splitting of uranium atoms on the efficiency of the reactor.
When the Kaiser Wilhelm Institute was taken over by the occupation powers, he assumed the directorship of the Institute of Physics at the University of Heidelberg. Later he acted as director of both institutes, but finally he confined his work to the Kaiser Wilhelm Institute, which was renamed the Institute for Physics of the Max Planck Institute for Medical Research. In 1954 he received the Nobel Prize in physics in recognition of his coincidence method, which proved an invaluable tool in atomic, cosmic-ray, and nuclear physics. He also was awarded the Max Planck Medal from the German Physical Society and the Grand Cross for Merit from the Federal Republic of Germany, and he became a knight of the Order of Merit for Science and the Arts. He died on Feb. 8, 1957.
The history of modern physics as the background for Bothe's work is discussed in George Gamow's often-anecdotal Biography of Physics (1961). J. Yarwood, Atomic Physics (1958), contains a well-documented and technical account of Bothe's chief scientific discoveries. Volume 3 of Nobel Lectures: Physics, 1942-1962 (1964), published by the Nobel Foundation, includes a biography of Bothe as well as his Nobel lecture.