Rudolf Mössbauer's study of the recoilless emission of gamma rays and nuclear resonance florescence led to the discovery of methods for making exact measurements in solid-state physics, archeology, biological sciences, and other fields. His measurement method was used to verify Albert Einstein's theory of relativity and is known as the Mössbauer effect. He was honored with a 1961 Nobel Prize in physics for his work.
Rudolf Ludwig Mössbauer was born on January 31, 1929, in Münich, Germany. He was the only son of Ludwig and Erna (Ernst) Mössbauer. Ludwig Mössbauer was a phototechnician who printed color post cards and reproduced photographic materials. Mössbauer grew up during a difficult time in Germany, during the disruptions accompanying the rise of Adolf Hitler's National Socialism ( Nazi party) and the onset of World War II. Still, he was able to complete a relatively normal primary and secondary education, graduating from the Münich-Pasing Oberschule in 1948. His plans to attend a university were thwarted because, due to Germany's loss in the war, the number of new enrollments was greatly reduced.
Through the efforts of his father, Mössbauer was able to find a job working as an optical assistant first at the Rodenstock optical firm in Münich and later for the U.S. Army of Occupation. Eventually Mössbauer saved enough money from these jobs to enroll at the Münich Institute for Technical Physics. In 1952 he received his preliminary diploma (the equivalent of a B.S. degree) from the institute, and three years later he was awarded his diploma (the equivalent of an M.S. degree). For one year during this period, 1953-54, he was also an instructor of mathematics at the institute.
After receiving his diploma in 1955, Mössbauer began his doctoral studies at the Münich Institute for Technical Physics. His advisor there was Heinz Maier-Leibnitz, a physicist with a special interest in the field of nuclear resonance fluorescence. As a result of Maier-Leibnitz's influence, Mössbauer undertook his thesis research in that field. His research was not carried out primarily in Münich, however, but at the Max Planck Institute for Medical Research in Heidelberg, where Mössbauer received an appointment as a research assistant.
Nuclear resonance fluorescence is similar to widely-known phenomena such as, for example, the resonance in tuning forks. When one tuning fork is struck, it begins to vibrate with a certain frequency. When a second tuning fork is struck close to the first, it begins vibrating with the same frequency, and is said to be "resonating" with the first tuning fork. Fluorescence is a form of resonance involving visible light. When light is shined on certain materials, the atoms that make up those materials may absorb electromagnetic energy and then re-emit it. The emitted energy has the same frequency as the original light as a result of the resonance within atoms of the material. This principle explains the ability of some materials to glow in the dark after having been exposed to light.
The discovery of fluorescence by R. W. Wood in 1904 suggested some obvious extensions. If light, a form of electromagnetic radiation, can cause fluorescence, scientists asked, can other forms of electromagnetic radiation do the same? In 1929, W. Kuhn predicted that gamma rays, among the most penetrating of all forms of electromagnetic radiation, would also display resonance. Since gamma rays have very short wavelengths, however, that resonance would involve changes in the atomic nucleus. Hence came the term nuclear resonance fluorescence.
For two decades following Kuhn's prediction, relatively little progress was made in the search for nuclear resonance fluorescence. One reason for this delay was that such research requires the use of radioactive materials, which are difficult and dangerous to work with. A second and more important factor was the problem of atomic recoil that typically accompanies the emission of gamma radiation. Gamma rays are emitted by an atomic nucleus when changes take place among the protons and neutrons that make up the nucleus. When a gamma ray is ejected from the nucleus, it carries with it a large amount of energy resulting in a "kick" or recoil, not unlike the recoil experienced when firing a gun. Measurements of gamma ray energy and of nuclear properties become complicated by this recoil energy.
Researchers looked for ways of compensating for the recoil energy that complicated gamma ray emission from radioactive nuclei. Various methods that were developed in the early 1950s had been partially successful but were relatively cumbersome to use. Mössbauer found a solution to this problem. He discovered that a gamma emitter could be fixed within the crystal lattice of a material in such a way that it produced no recoil when it released a gamma ray. Instead, the recoil energy was absorbed by and distributed throughout the total crystal lattice in which the emitter was imbedded. The huge size of the crystal compared to the minute size of the emitter atom essentially "washed out" any recoil effect.
The material used by Mössbauer in these experiments was iridium-191, a radioactive isotope of a platinum-like metal. His original experiments were carried out at very low temperatures, close to those of liquid air, in order to reduce as much as possible the kinetic and thermal effects of the gamma emitter. Mössbauer's first report of these experiments appeared in issues of the German scientific journals Naturwissenschaften and Zeitschrift fur Physic in early 1958. He described the recoilless release of gamma rays whose wavelength varied by no more than one part in a billion. Later work raised the precision of this effect to one part in 100 trillion.
In the midst of his research on gamma ray emission, Mössbauer was married to Elizabeth Pritz, a fashion designer. The couple later had three children, two daughters and a son. In 1958, Mössbauer was also awarded his Ph.D. in physics by the Technical University in Münich for his study of gamma ray emission.
The initial reactions to the Mössbauer papers on gamma ray emission ranged from disinterest to doubt. According to one widely-repeated story, two physicists at the Los Alamos Scientific Laboratory made a five-cent bet on whether or not the Mössbauer Effect really existed. When one scientist was in fact able to demonstrate the effect, the scientific community gained interest.
Physicists found a number of applications for the Mössbauer Effect using a system in which a gamma ray emitter fixed in a crystal lattice is used to send out a signal, a train of gamma rays. A second crystal containing the same gamma ray emitter set up as an absorber so that the gamma rays travelling from emitter to absorber would cause resonance in the absorber. Therefore, the emission of gamma rays stays resonating and constant until a change, or force such as gravity, electricity, or magnetism enters the field. By noting the changes in the gamma ray field, unprecedented measurements of these forces became available.
One of the first major applications of the Mössbauer Effect was to test Einstein's theory of relativity. In his 1905 theory, Einstein had predicted that photons are affected by a gravitational field, and therefore an electromagnetic wave should experience a change in frequency as it passes near a massive body. Astronomical tests had been previously devised to check this prediction, but these tests tended to be difficult in procedure and imprecise in their results.
In 1959, two Harvard physicists, Robert Pound and Glen A. Rebka, Jr., designed an experiment in which the Mössbauer Effect was used to test the Einstein theory. A gamma ray emitter was placed at the top of a sixty-five-foot tower, and an absorber was placed at the bottom of the tower. When gamma rays were sent from source to absorber, Pound and Rebka were able to detect a variation in wavelength that clearly confirmed Einstein's prediction. Today, similar experimental designs are used for dozens of applications in fields ranging from theoretical physics to the production of synthetic plastics.
Mössbauer's Nobel Prize citation in 1961 mentioned in particular his "researches concerning the resonance absorption of gamma-radiation and his discovery in this connection of the effect which bears his name." The Nobel citation went on to say how Mössbauer's work has made it possible "to examine precisely numerous important phenomena formerly beyond or at the limit of attainable accuracy of measurement." The physics community acknowledged the enormous scientific and technical impact the discovery would eventually make.
After receiving his Ph.D. in 1958, Mössbauer took a position as research fellow at the Institute for Technical Physics in Münich. Two years later he was offered the post of full professor at the Institute, but, according to his entry in Nobel Prize Winners, declined the offer because he was "frustrated by what he regarded as the bureaucratic and authoritarian organization of German universities." Instead, he accepted a job as research fellow at the California Institute of Technology and was promoted to full professor a year later, shortly after the announcement of his Nobel award.
In 1964, Mössbauer once more returned to Münich, this time as full professor and with authority to reorganize the physics department there. In 1972, he took an extended leave of absence from his post at Münich to become director of the Institute Max von Laue in Grenoble, France. After five years in France, he returned to his former appointment in Münich. In addition to the Nobel Prize, Mössbauer has received the Science Award of the Research Corporation of America (1960), the Elliott Cresson Medal of the Franklin Institute (1961), the Roentgen Prize of the University of Giessen (1961), the Bavarian Order of Merit (1962), the Guthrie Medal of London's Institute of Physics (1974), the Lomonosov Gold Medal of the Soviet Academy of Sciences (1984), and the Einstein Medal (1986).
Abbott, David, The Biographical Dictionary of Scientists— Physicists, P. Bedrich, 1984, p. 118.
Frauenfelder, Hans, The Mössbauer Effect, W. A. Benjamin, 1962.
Halacy, D. S., Jr., They Gave Their Names to Science, Putnam, 1967, pp. 118-129.
McGraw-Hill Modern Scientists and Engineers, McGraw-Hill, Vol. 2, 1980, pp. 331-332.
Weber, Robert L., Pioneers of Science: Nobel Prize Winners in Physics, American Institute of Physics, 1980, pp. 184-185. □