Bothe, Walther Wilhelm Georg
Bothe, Walther Wilhelm Georg
(b. Oranienburg, Germany, 8 June 1891; d. Heidelberg, Germany, 8 February 1957)
Bothe’s father was Fritz Bothe, a merchat. During the period 1908 to 1912 Walther studied at the University of Berlin, where his training included not only physics and mathematics but also chemistry. In 1914 he obtained his doctorate under Max Planck for a study of the molecular theory of refraction, reflection, dispersion, and extinction, a subject he continued to study from 1915 to 1920 while a prisoner of war in from 1915 to 1920 while a prisoner of war in Russia. On his return to Germany, where he married Barbara Below, Bothe accepted Hans Geiger’s invitation to work at the radioactivity laboratory of the Physikalische-Technische Reichsanstalt. Much of his early experimental work was done with Geiger, and Bothe later said that it was Geiger who had initiated him into his researches in physics. Bothe taught physics at the University of Berlin from 1920 until he accepted the professorship of physics at Giessen in 1931. In 1934 he became director of the Max Planck Institute at Heidelberg, where he remained until his death. Bothe received the Max Planck Prize, and he shared the Nobel Prize with Max Born in 1954.
From 1921 to 1924 Bothe was active in both theoretical and experimental work on the scattering of alpha and beta rays. He devised a statistical theory for processes involving multiple scattering through small angles, a phenomenon far more complex than large-angle scattering. Bothe studied multiple scattering of electrons experimentally by tracking the trajectories on photographic plates. He also deduced mathematical expressions for the relationships between scattering angle and foil thickness, using a nuclear model of the atom.
Among the topics that Bothe studied in 1924 was the ejection of electrons by X rays, and it was in connection with this phenomenon that he and Geiger performed an important experiment. In an effort to reconcile the particulate and wavelike properties of radiation, Bohr, Kramers, and Slater in 1924 formulated a new quantum theory of radiation. According to their hypothesis, momentum and energy-are conserved only statistically in interactions between radiation and matter. Bothe and Geiger suggested that this could be tested experimentally by examining individual Compton collisions. Bothe introduced a modification into the Geiger counter that made it appropriate for use in coincidence experiments (a very novel procedure in 1924). Using two counters, they studied the coincidences between the scattered X ray and the recoiling electron. Correlating photons with electrons, Bothe and Geiger found a coincidence rate of one in eleven; since the chance coincidence rate for the situation was 10−5, the experimental results contradicted the theoretical predictions and indicated small-scale conservation of energy and momentum.
With Werner Kolhörster, Bothe used the coincidence method again in 1929 to demonstrate that cosmic rays might be particles. Ever since their discovery in 1912, physicists had assumed that cosmic rays were high-energy photons, and Millikan’s hypothesis that they were released during the elementary synthesis of elements by fusion in the atmosphere was especially popular. In the experiments of Bothe and Kolhörster, two Geiger counters were separated by about 4 cm. of gold; and in order for a photon to produce a pulse in a counter, it would have to undergo a Compton collision and produce an ionizing electron. The known probability of Compton collisions and the average energy of the photons indicated that coincidences between the two counters were highly improbable. The high coincidence rate in the experiment, approximately 75 percent of the original single-counter rate, therefore indicated that the cosmic radiation might well be particulate.
Nuclear transmutation was a third new topic of interest for physicists at this time. In 1919 Rutherford had produced oxygen nuclei and protons by bombarding nitrogen with alpha particles, and during the following decade various laboratories worked on this type of transmutation. Bothe took up the subject in 1926, and in the following years he studied the transmutation, via alpha particles, of boron to carbon. He was among the early users of the electronic counter to detect the protons in this type of reaction. With H. Becker he searched systematically for a gamma radiation accompanying the transmutation; its existence seemed reasonable because there were some light elements, such as beryllium, that did not disintegrate. In these experiments there were problems in finding a suitable source of alpha particles (one that did not produce other radiations as well), and Bothe himself finally prepared one by a process of chemical extraction. In 1930 Bothe and Becker detected a highly penetrative radiation from beryllium bombarded by alpha particles, and they assumed that it was gamma radiation. Bothe estimated the photon energy from the degree of absorption of the secondary electrons. When physicists studied this “beryllium radiation,” estimating its energy constituted a problem, for it varied greatly according to the substance used as absorber. Chadwick later suggested that the radiation was particulate and consisted of a new particle, the neutron.
After detecting the new radiation in 1930, Bothe continued studying nuclear transmutation, and made coincidence measurements on the products of the reaction between alpha particles and beryllium. When he became director of the Max Planck Institute, he was involved in the construction of a Van de Graaf accelerator there and in the planning of a cyclotron which was finally completed in 1943. After the outbreak of World War II he did much work on uranium and on neutron transport theory, and he was one of the foremost scientists of Germany’s “uranium project” for nuclear energy.
In the early 1950’s Bothe again dealt with questions of electron scattering and cosmic rays, and of beta and gamma spectra. Thus, over a period of more than thirty years Bothe studied a broad range of highly relevant problems in a variety of ways.
I. Original Works. A few of Bothe’s more important papers are “Ein Weg zur experimentellen Nachprüfung der Theorie von Bohr, Kramers, und Slater,” in Zeitschrift für Physik, 26 (1924), 44 (written with H. Geiger); “Ueber das Wesen des Comptoneffekts; ein experimentellen Beitrag zur Theorie der Strahlung,” in Zeitschrift für Physik, 32 (1925), 639–663 (written with H. Geiger); “Das Wesen der Hoehenstrahlung,” in Zeitschrift für Physik, 56 (1929), 75–77 (written with W. Kolhörster); “Kunstliche Erregung von Kern-γ-Strahlen,” in Zertschrift für Physik, 66 (1930), 289–306 (written with H. Becker). Other papers are listed in Science Abstracts.
II. Secondary Literature. Discussions of Bothe’s work on scattering are in E. Rutherford, J. Chadwik, and C. D. Ellis, Radiations from Radioactive Substances (Cambridge, 1930), pp. 209–212, 219–220, 237–238. In connection with the work on cosmic rays, see Bruno Rossi, Cosmic Rays (New York, 1964), pp. 30–42. The context of the early research on nuclear transformations may be found in M. Korsunsky, The Atomic Nucleus (New York, 1965), pp. 137–145. M. Jammer, The Conceptual Development of Quantum Mechanics (New York, 1966), pp. 181–188, and B. L. Van der Waerden, ed., Sources of Quantum Mechanics (Amsterdam, 1967), pp. 12–14, discuss the significance of the 1925 experiment. There is an account of Bothe’s role in the German uranium project in S. A. Goudsmit, Alsos (New York, 1947). Obituaries are Lise Meitner, in Nature, 179 (1957), 654–655; and R. Fleisch-mann, in Die Naturewissenschaften, 44 (1957), 457–460.
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.
Understanding the Compton Effect
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.
Institute for Physics
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. □