Walther Bothe

German physicist

• Born: January 8, 1891
• Birthplace: Oranienburg, Germany
• Died: February 8, 1957
• Place of death: Heidelberg, West Germany (now in Germany)

Bothe was awarded the Nobel Prize in Physics for his invention of the coincidence counting technique and for discoveries made using it, including the nature of cosmic rays and the fashion in which X rays interact with electrons. He was one of Germany’s leading atomic scientists and constructed its first cyclotron.

Early Life

Walther Bothe (VAHL-tehr BOH-tah) was born to Fritz Bothe, a merchant, and Charlotte Bothe. Walther studied physics, chemistry, and mathematics at the University of Berlin. As a graduate student, he became one of the few ever to study under the famous Max Planck. After obtaining his doctorate in 1914, he began to work for Hans Geiger in the radioactivity laboratory of the Physikalische-Technische Reichsanstalt (physical-technical institute).

World War I soon intervened, and Bothe became a machine gunner in the German army. Captured by the Russians in 1915, he spent the next five years as a prisoner of war in Siberia. While there, he was able to continue his studies in mathematics and physics as well as learn Russian. He married a Russian woman, Barbara Below, in 1920 and returned to the Reichsanstalt in Berlin. He and his wife had two daughters.

While performing research at the Reichsanstalt, Bothe began a simultaneous teaching career at the University of Berlin. It was an exciting, if somewhat confusing, time for anyone in the field of physics. The nature of radioactivity and the structure of the atom were two topics that defied understanding within the confines of the familiar (classical) rules of physics.

Among the most accurate measurements of the day were those of the spectroscopists, those who measured the various colors of light emitted by atoms. Niels Bohr had proposed a very curious model of the atom to explain these measurements. In Bohr’s model, electrons orbited a nucleus of protons (neutrons were yet to be discovered). Strangely, the electrons could have only certain specific values (quanta) of energy or angular momentum. For unknown reasons, other values were not allowed. Bohr’s conjectures were accepted only because they worked, not because they made sense in terms of known physics.

To extend the theory, Bohr and others calculated how an X ray and an electron would interact. It was supposed that since X rays are electromagnetic waves, the energy and the momentum of the wave would be spread out all along the wave. It was at this point that Bothe and his coworker, Geiger, entered the picture.

Life’s Work

Bothe and Geiger decided to test Bohr’s new theory. They wanted to compare it with a theory of Arthur Holly Compton. In the Compton effect, an X ray (treated as a particle) strikes an electron with the result that the electron recoils in one direction while the X ray scatters in a different direction and also has less energy.

According to Compton, the total energy of the electron and incident X ray (before the collision) should be the same as that of the recoil electron and scattered X ray (after the collision). This is called conservation of energy and momentum and is believed to be fundamental in nature. Yet according to Bohr, the energy and momentum of the X ray are spread all along the wave and are not localized at the site of the electron. Bohr predicted energy and momentum would be conserved only on the average and for a large number of collisions. A recoil electron might have absorbed some energy from many X-ray waves and would therefore not be related to any specific incident X ray.

In the experiment of Bothe and Geiger, X rays struck gas atoms inside a Geiger counter tube. The tube was connected to a pointer that moved when a recoil electron was detected. A second counter tube was placed beside the first, with the hope that it would detect the scattered X ray. If it did, its pointer also moved. Both pointers were continuously photographed on film moving through a camera at about ten meters per minute. Many hundreds of meters of film had to be developed, hung from the ceiling to dry, and then painstakingly inspected.

Bothe and Geiger were able to show that the tubes did indeed record coincident events, two events occurring at essentially the same time. The conclusion was that the recoil electron and the scattered X ray were produced simultaneously, and, hence, Bohr’s latest theory was wrong. In fact, energy and momentum were conserved in individual collisions.

In 1926, Bothe joined many other physicists in studying radioactivity. Heavy elements such as radium and uranium were known to emit alpha particles. These particles were allowed to strike targets of carbon, boron, or some other light element. Bothe was keenly interested in any radiation that the target might now emit, hoping that it would give him some insight into the structure of the atomic nucleus.

Some of these reactions emitted protons, and Bothe was among the first to use electronic counters to detect them. In 1930, Bothe and Hans Becker studied the radiation from a beryllium target. They found it to be surprisingly penetrating and assumed that it consisted of high-energy gamma rays. They missed another major discovery, for not long after James Chadwick showed that this radiation must consist of neutral particles, which he named neutrons.

At about the same time as his work with beryllium, Bothe worked with Werner Kolhörster in analyzing cosmic rays. Using Bothe’s coincidence method, they placed one counter tube above a second counter tube and placed absorbing material between the tubes. If both tubes signaled a count at the same time, it was assumed that a single cosmic ray had gone through both tubes and had not been trapped in the absorber. Somewhat to their surprise, their results were not those expected from high-energy gamma rays. Instead, their experiments implied that cosmic rays are charged particles. Bothe pointed out that to penetrate the earth’s atmosphere, these particles would need to have an energy equivalent to being accelerated through a thousand million volts. Since this was a thousand times more energy than any process known then could produce, it was a very radical suggestion. History, however, has shown it to be correct.

During the 1930’s, Bothe continued his studies of multiple scattering of electrons and began similar studies with neutrons. An article by him on both the experimental and theoretical aspects of scattering appeared in the 1933 edition of the Handbuch der Physick (handbook of physics).

When the Nazis came to power, they urged that the scientific theories of Jews, such as Albert Einstein, no longer be taught. Bothe could not agree with the restriction, and in 1932 he moved his work to the politically more tolerant Heidelberg, eventually becoming a professor at the university and simultaneously the director of the Physics Institute of the Max Planck Institute for Medical Research.

Bothe’s reputation for excellence was such that in 1939 he was called to be one of the chief scientists in the German atomic bomb project. One road to the bomb was to construct a nuclear reactor in which to make the element plutonium. Bothe was called on to decide the key issue of whether carbon could be used to slow neutrons in the nuclear reactor. According to Bothe’s measurements, carbon would absorb too many neutrons and was unsuitable. This result was in error, and it is very curious that someone of Bothe’s ability would have made this mistake. Bothe had no liking for Adolf Hitler, and it may be that he purposely reported the wrong results, for this became one of the key reasons for Hitler’s failure to develop the bomb.

To better study nuclear reactions, Bothe needed a cyclotron, a machine that can accelerate particles, such as protons, and cause them to collide with target nuclei. Bothe supervised the construction of the first cyclotron in Germany, completing it in 1944. After the war, Bothe used the cyclotron he had constructed at Heidelberg to produce radioactive isotopes for medical studies. He continued his work with cosmic rays and was senior author of Nuclear Physics and Cosmic Rays (1948) as well as of many scientific articles.

Bothe devoted the same intensity to his hobbies that he gave to his scientific studies. He was an excellent pianist and took special pleasure in playing the works of Johann Sebastian Bach and Ludwig van Beethoven. On holidays he liked to go to the mountains and paint. His oil and watercolor paintings were quite professional looking. Bothe is described as being a strict taskmaster in the laboratory but hospitable and relaxed at home. He was awarded the Nobel Prize in Physics in 1954 and died on February 8, 1957.

Significance

Bothe was a physicist’s physicist. Other physicists consulted his works for definitive statements of a problem, its current status, and experimental techniques for studying it. As such, his influence among physicists was great, although he was little known to the public. Perhaps it was the growing awareness and acknowledgment of this influence that led to his somewhat belated award of the Nobel Prize. When it came, Bothe was in failing health and unable to attend the ceremony, but at least he had the satisfaction of living to see his work widely recognized.

The coincidence method pioneered by Bothe is now a common and widely used technique in nuclear physics. Electronic counters are used instead of film to record the results, and it was Bothe who pioneered the use of these electronic counting circuits. Among his other achievements, he was the first to show that high-energy cosmic rays are particles and the first to introduce a widely used notation for nuclear reactions. In addition to the Nobel Prize, Bothe was decorated a Knight of the Order of Merit for Science and Arts (1952), awarded the Max Planck Medal (1953), and awarded the Grand Cross of the Order for Federal Services of Germany (1954).

Bibliography

Beyerchen, Alan D. Scientists Under Hitler: Politics and the Physics Community in the Third Reich. New Haven, Conn.: Yale University Press, 1977. This excellent work describes the political involvement (or noninvolvement) of many of Germany’s top scientists. Nobel laureates become public figures and are often pressured into some kind of political action.

Dardo, Mauro. Nobel Laureates and Twentieth-Century Physics. New York: Cambridge University Press, 2004. Chronicles major developments in physics since 1901, the year the first Nobel Prize in Physics was awarded. Includes information about the work of Bothe and other prize winners.

Goudsmit, Samuel A. Alsos. New York: Henry Schuman, 1947. Reprint. Los Angeles: Tomash, 1983. Alsos was the code name for the intelligence mission to discover the status of the German atomic bomb project. Includes Goudsmit’s interview and appraisal of Bothe at the war’s end. Readers should be cautioned that Werner Heisenberg believed that Goudsmit undervalued the German achievements. Keeping this in mind, the book is quite useful.

Irving, David. The German Atomic Bomb: A History of Nuclear Research in Nazi Germany. Reprint. New York: Da Capo Press, 1983. A fascinating and easy-to-read account. Details Bothe’s work on the project from start to finish. Gives an exciting account of how the Allies sabotaged German efforts to procure heavy water and explores other reasons for the lack of success of the German project. Highly recommended.

Nobel Foundation. 1942-1962. Vol. 3 in Physics. New York: Elsevier, 1964. This work is among the very few that describe Bothe’s Nobel work in an accessible manner for general readers.

Rhodes, Richard. The Making of the Atomic Bomb. New York: Simon & Schuster, 1986. One of the most comprehensive books on the subject that is readily accessible to general readers. Includes references to Bothe from his production of neutrons in 1930 to his completion of Germany’s first cyclotron in 1944.

Weart, Spencer R. Scientists in Power. Cambridge, Mass.: Harvard University Press, 1979. Weart describes the French World War II atomic bomb project and follows its postwar development to the first plutonium production in 1949. He discusses Bothe’s use of the French cyclotron as well as Bothe’s error in measuring the absorption of neutrons by carbon.