Nobel Prize winner in physics

Bothe and Geiger developed the "coincidence method", which became a widely used technique in nuclear physics. It provides insight into the details of a reaction or decay. For their work, Geiger and Bothe also had to develop completely new electronic circuits and devices to electronically record the reaction of the counting tubes and automatically count the number of coincidence events in which both counting tubes react simultaneously, or almost simultaneously.

Their experimental set-up consists of two needle counters embedded in a hydrogen atmosphere, between whose common, non-contacting end wall an X-ray beam is directed. Hydrogen absorbs the X-rays only weakly, but scatters them strongly.

One of the counting tubes is open and therefore filled with hydrogen. The other counting tube, covered with a platinum foil, is filled with air. This counting tube does not react to electrons, as the platinum foil absorbs the recoil electrons. However, the photons penetrate the foil and release photoelectrons from the air and the foil, which the counting tube registers. The open counter tube, on the other hand, registers almost no photons, as these are hardly absorbed by the hydrogen. The recoil electrons, on the other hand, are measured.

Using the Koinizidendz method, Bothe and Geiger were able to prove beyond doubt the quantum nature of the photon and thus disprove the - at that time still widely accepted - view held by Niels Bohr and several other renowned physicists that two important physical principles, namely the conservation of energy and momentum, were only fulfilled statistically, i.e. on average, in Compton scattering, but were violated in the individual process.

Bothe summarized his work in several handbook articles, thus laying the methodological foundations for the analysis of scattering processes. When Geiger accepted an appointment at the University of Kiel in 1925, Bothe became his successor as head of the Lauboratorium for Radioactivity. At Geiger's generous suggestion, Bothe was assigned the coincidence method when the areas previously worked on together were divided up. The years of collaboration with Geiger, of whom he always spoke with great admiration and affection, marked a decisive turning point in Bothe's scientific career.

In 1925, Bothe habilitated under Max Planck at the University of Berlin with his thesis "On the elementary process of photoelectric electron triggering". In the following years, Bothe developed coincidence measurement into an important method for researching cosmic rays. He placed several of the latest Geiger-Müller counters on top of each other and arranged absorbing layers of different thicknesses between and above the counters in order to be able to determine the absorption of cosmic rays in different materials by reducing the number of coincidences accordingly. He hoped to gain profound insights from the investigation of cosmic radiation discovered by Victor Hess during balloon flights in 1912.

Together with the astronomer Werner Kolhörster, he carried out coincidence measurements in which rays were guided through Geiger counters and the response of the counting tubes was only indicated if the measurements were taken within a predetermined short time interval of each other. As this only happens when one and the same particle passes through all the counting tubes, only radiation from a specific direction is registered. With this new method of random counting, the path of a charged particle can be tracked through the counting tubes. This shows that the particles prefer to fall perpendicular to the earth's surface. If the apparatus is tilted towards the horizon, the intensity of incidence decreases. As the vertically incident particles have the shortest path through the Earth's atmosphere, they are absorbed the least, which clearly indicates the origin of the radiation under investigation outside the Earth.

With their experiments, Bothe and Kolhörster prove that cosmic radiation does not primarily consist of gamma rays, as previously assumed due to their high penetrating power, but of material particles with an energy of at least 1,000 million electron volts.

In 1930, Bothe was appointed Professor of Physics and Director of the Institute of Physics at the University of Giessen. He was the first to address quantum mechanics in his lectures and turned his institute into an important research center. When he shoots beryllium with alpha rays, he obtains an unusually penetrating radiation. However, he did not realize that it was a new particle. Two years later, Sir James Chadwick discovered the neutron and was awarded the Nobel Prize in 1935.

After the discovery of neutrons, experimental nuclear physics began to develop rapidly, thanks in part to the completion of the first accelerators and the enormous improvements in measuring and counting techniques.

In 1932, Bothe was appointed to the University of Heidelberg, but resigned from his professorship and the management of the institute after the Nazi regime came to power. In 1934, he became head of the physics department at the Kaiser Wilhelm Institute for Medical Research, which was founded in 1930 and consisted of four independent sub-institutes. This later became the Max Planck Institute for Nuclear Physics. Bothe was able to return to his basic research and work on the controlled nuclear fission chain reaction. He and his colleagues are among the first scientists to investigate nuclear reactions and the properties and structures of the atom, carry out nuclear spectroscopic studies and produce artificial isotopes.

During the Second World War, the Army Ordnance Office undertakes efforts to build nuclear weapons and has the best German nuclear physicists working on the so-called uranium project. Bothes Institute is also involved. When measuring the absorption of slow neutrons in graphite, he came to the conclusion that graphite was unsuitable as a nuclear moderator. This assessment prevented the National Socialists from producing nuclear weapons until the end of the war. It was not until 1945 that it was realized that Bothe had determined an incorrect value because the graphite he had used was contaminated. But could a scientist like Bothe, who worked so conscientiously and meticulously with such high concentration, really have made such a serious mistake? The first graphite-moderated reactor was built by Enrico Fermi in Chicago in 1945. The fission of uranium produces fast neutrons. Graphite slows them down and reflects them back into the reactor core, where they are then available for further fissions and thus maintain the nuclear chain reaction.

For his research, Bothe needs a cyclotron, a particle accelerator in which elementary particles, atomic nuclei, ionized atoms or molecules are accelerated to high speeds by electric fields. He succeeds in procuring the necessary funds and, together with his assistant Wolfgang Gentner in Heidelberg, constructs the first German cyclotron, which is used for the first time in the fall of 1943. After the Second World War, the Americans used the institute as an Aero-Medical Center and also the cyclotron. The physics department, however, was closed.

Although he was initially not allowed to work in his original field of nuclear physics, Bothe returned to the Institute of Physics at the University of Heidelberg as Director in 1945. He ensured that Hans Jensen was appointed to Heidelberg in 1949, put the cyclotron back into operation and carried out nuclear physics experiments with his students. (10) .

The Max Planck Society is founded as the successor to the Kaiser Wilhelm Society and Bothe is able to continue his work at the Institute of Physics in 1952, devoting himself to research in the fields of nuclear physics and cosmic radiation and working on the further development of nuclear spectroscopy.

However, progressive vasoconstriction, which even necessitated the amputation of a leg, eventually forced him to retire from research. From 1953, he was only able to take care of the management of the Institute of Physics.

Ten years after the death of Hans Geiger, Walther Bothe was awarded the Nobel Prize in Physics in 1954 for the development of the coincidence method and the discoveries he had made with it, together with Max Born, who was honored for his fundamental research in quantum mechanics. Unfortunately, Bothe is unable to travel to Stockholm for health reasons. At his request, his daughter Dr. Elena Riedel accepted the award.(11) The coincidence method is still one of the most important means of investigating cosmic rays and all types of nuclear and elementary particle processes.

Alongside other Nobel Prize winners, on July 15, 1955, Bothe signs an appeal to the world's politicians to renounce violence as a means of politics. On February 10, 1957, Prof. Dr. Walter Bothe (12) dies in Heidelberg after a long illness at the age of 66 and is laid to rest in the cemetery in Handschuhsheim.

Gamma rays are not produced by energy changes in the electrons in the electron shell, but by energy changes in the atomic nuclei. Here too, as with visible light, there are absorption and emission processes. However, the relationships here are much more complicated.

The exact wavelength of the emitted or absorbed radiation depends on the state of motion of the emitter or absorber. We see something very similar in the so-called Doppler effect, which was first described by the Austrian physicist Christian Doppler in 1842. We are familiar with this phenomenon from everyday life as a sudden change in pitch from high to low as a sound source approaches and then moves away. A typical example is the changing pitch of the siren of a passing fire engine.

An example from astronomy is the red shift of light as a galaxy moves away. In this way, if the frequency of the source is known, its speed relative to the observer can be determined.

In the case of gamma radiation from atomic nuclei, however, it was observed that the emitted radiation was not readily absorbed by similar nuclei. Apparently, the energy contained in the emitted gamma quantum did not match the required excitation energy of the absorber.

Mössbauer uses the Doppler effect for his experimental setup. This means that he allows the source of the gamma rays to move towards and away from the sample at different speeds. In this way, he can determine exactly at what speed absorption occurs again and convert this speed into energy.

In 1957, during his experiments with gamma radiation in iridium-191, an isotope (the Greek word "isotope" refers to different types of atoms of the same chemical element) of the element iridium, he observed minimal deviations from the actually expected measurement result. When he investigated the reason for this unclear and initially seemingly negligible effect, he discovered the phenomenon that the γ-radiation emitted by an atomic nucleus can be absorbed by nuclei of the same isotope in another sample without any loss of energy.

Mössbauer finds the solution to the puzzle. If free atomic nuclei emit gamma rays, the physical law of conservation of momentum requires that the emitted particle - i.e. the gamma quantum - is not given all of its energy, but that part of this energy is consumed in the recoil of the emitting nucleus. The wavelength of the gamma radiation changes with the strength of the recoil, and a shift occurs between the absorption and emission lines.

In the early post-war years, it was difficult for German scientists to access international journals. Jensen was therefore delighted when he was able to travel to Copenhagen again in 1948 and read in an article by Maria Goeppert-Mayer (1906-1972), Professor of Physics at the University of Chicago, in an issue of "Physical Review" that she had also found experimental evidence for the "magic numbers" and was working on an explanation, as was he.

Jensen contacted her and an intensive scientific exchange developed between them, which ultimately led to them jointly developing the models they had previously developed independently of each other into the so-called "shell model" of the atomic nucleus.

The Danish nuclear physicist Niels Bohr had a major influence on their attempts to understand nuclei. However, while the droplet model developed by Niels Bohr in 1936 compares the atomic nucleus with a drop of water behaving according to the rules of mechanics, the shell model developed from this looks at the individual nucleons and their movement according to the laws of quantum mechanics (the "Pauli principle" named after the physicist Wolfgang Pauli).

For the development of the shell model, Jensen was awarded one half of the Nobel Prize in Physics in 1963 in equal shares with Maria Goeppert-Mayer, (24) while the other half went to Eugene Wigner for the discovery of the nuclear shell structure of the atomic nucleus, (25) who showed that most of the essential properties of nuclei follow from generally valid symmetries of the laws of motion.

Despite many honorable calls from Germany and abroad, Prof. Hans Jensen (26) remained loyal to Heidelberg University and helped Heidelberg to play a leading role in nuclear physics. He retired in 1969 and died unexpectedly on February 11, 1973 at the age of 65 in his Institute for Theoretical Physics in Heidelberg, which can still be found today at Philosophenweg 16. In 2007, it was given the name "Jensen House" and a memorial plaque commemorates the important researcher. (27)