Wolfgang Paul The Nobel Prize in Physics

autobiography

was born in Lorenzkirch (Saxony) on 13 August 1913, but grew up in Munich. After studying at the Technical University of Munich and gaining a physics diploma in Berlin, he went to Kiel and Göttingen as a pupil.

He became a lecturer at the University of Göttingen in 1944. In 1952 he was made professor at the University of Bonn, and also appointed director of the Institute of Physics, a position he held until he was granted his emeritus status in 1980. In the 1957/58 academic year

was dean of the mathematics and science faculty of the University of Bonn, the following year a visiting scientist at CERN, from 1960 to 1962 chairman of the board of the nuclear research center in Jülich, 1963/64 chairman of the scientific council there, from 1965 to 1967 director of the Nuclear Physics Division at CERN, 1968/69 chairman of the committee for nuclear counter experiments at CERN, in 1969 chairman of the research commission advising on the development of universities in North Rhine Westphalia, from 1971 to 1973 managing director of DESY, and subsequently chairman of the DESY scientific council, then from 1979 to 1989 president of the Alexander von Humboldt- Foundation. He spent time as a visiting scientist in Chicago, Harvard, Rio de Janeiro, and Tokyo. Among his many honours were the Nobel Prize for Physics in 1989 (together with

from the USA), the Pour le Mérite order, membership of the German Academy of Sciences (Leopoldina) in Halle, and honorary degrees at the University of Uppsala and the RWTH Aachen

died on 7 December 1993 at the age of 80 The interview shown here in shortened form took place with

with his cyclotron, where the particles don't just go once through the field but many times. Millions of times, in fact. That led to a rapid development in accelerators. It worked beautifully with protons. The problem with electrons is that their mass increases when they get more energetic. They don't get any faster, but simply more massive. So it gets harder and harder to keep them on a circular path and accelerate them. If I want them to go faster I have to keep increasing the magnetic field, and I have to do it in synchronization with the electrons. Then we have what's called a synchrotron. The mass can easily be calculated from the energy: that just follows from relativity. The problem is to find a trick so that the electrons keep moving in a circle automatically. It was 1946 before the principle was discovered, by M an outstanding engineer. When we heard about it, it was half finished, and of course we expressed great interest in it.

We wanted it for physics research, whereas Siemens were interested only in medical uses. But luckily they saw an opportunity in collaborating with physicists: Gund was an electrical engineer, and had never learned the methods for measuring high-energy radiation. we agreed that I would build the equipment to investigate the functioning of the betatron. I moved to Erlangen in 1944 to carry out the work. The war came to an end, and Germany was occupied. The Siemens betatron was confiscated by the army, and there was an order issued by the military government to destroy it. We managed to prevent that with the help of the British officer responsible for science in the British-occupied zone

I myself wasn't particularly interested in the medical side. I wanted to do physics. But the condition had been imposed. Now a friend of mine, who was interested in radiation biology, was senior physician at the gynaecological clinic in Göttingen. I got together with him, and we announced that we wanted to see how suitable high-energy particles are for radiation therapy. Of course we didn't try it out on humans. We started with the famous fruit fly, Drosophila. We also did some physics on the side. The interactions between electrons and matter were important for medicine, naturally. Later we did try radiation therapy against skin cancers. The electrons penetrate by about two success, and later it became a standard procedure. In 1952 I was appointed professor in Bonn, so I gave up my medical activities because my job was to be in charge of a physics institute.

Oh, yes, the minor modification. It was like this. That betatron was the first electron accelerator in which it was possible to eject the electrons in a focused beam. It was Gund who thought it out, but we were the first to make use of it in Göttingen. The electron paths are very sensitive to magnetic fields, so to get a nice directional beam you can't afford to have just any old magnetic fields outside the accelerator. You have to make sure the field points in the right direction, which you can do with pieces of iron. I used my bunch of keys to fine-tune the beam – in those days doorkeys were all long iron rods – and at one position I tried Yes, and then I made an aluminium bracket and glued it to the betatron. When I laid my bunch of keys on the bracket everything went fine. That became a standard feature. I'd come into the lab, put my keys on the bracket, and we'd start. That was how we worked in those days. Nobody would dream of doing that sort of thing now!

I'd intended building a synchrotron in Göttingen. I had some old magnets from a mass spectrometer, which would have been suitable. In the war I was supposed to produce a series of ten mass spectrometers, and had already been given the materials for six of them. I could use those magnets as the basis of an accelerator, using the »classic« synchrotron model, though I couldn't see any chance of getting the money for it. But in 1952 there was a development in the USA which made it possible to get by with much smaller magnets than before. You could go for higher energies at less cost. I'm afraid it's not easy to explain the principle to a non-physicist.

heard about it and told us about it in a physics colloquium at Göttingen. »That's something that could be very important in future«, he said. I jumped at that. If I go to Bonn, I decided, then I want to build a synchrotron with a higher energy than I'd originally hoped for. In November 1952 I submitted an application to the German Research Association and also wrote them an informal letter in which I said I intended building a synchrotron with an energy of 100 MeV and would like 100 thousand marks to do it. Then I'd have got another 50 thousand from the regional government. I said I know that's 10% of the entire budget for physics research, but perhaps the other gentlemen haven't set their sights so high. And if it wasn't possible then I had two other projects which would need only 50 thousand and 20 thousand respectively.

Then I had a phone call from H Mesons are particles which are found in cosmic rays and are also responsible for the bonds between the particles that make up an atomic nucleus. Now we know that mesons consist of quarks, but in those days we'd only known of the existence of mesons for a few years. About my application: I was asked how expensive a machine for meson physics would be. I had no experience in such matters; I estimated at least 500 thousand marks, but not more than 900 thousand. Three months later I had a letter approving the DM 100 thousand and saying that I could stick to my original plan to build a 100-MeV machine, or I could go for higher energies, but there was no certainty that any more money would be forthcoming. Of course I immediately said I'll build the 500-MeV machine! In the end I kept the cost below 1 million marks. We started building done by students and staff at the university workshops. Brown-Boveri built the magnets. The vacuum chamber proved difficult: a torus with an elliptical cross-section and a diameter of six metres. First we made it out of glass, but it didn't stand up to the high radiation. Then we found a company that built a ceramic torus in segments.

It's easiest to understand by taking an optical analogy. If you put together a whole row of lenses, alternately converging and diverging, you might think it achieves nothing. But if you do the mathematics you find there is an overall effect and you do get focusing. Now the magnetic lenses focus in one plane but defocus in the other. You can't make their focal length shorter than a certain amount, but – skipping the complicated physics – it turns out that even with relatively small magnets the effect is enough to keep the size of the electron beam small.

At first Geiger counters and scintillation counters: when a high-energy particle passes through fluorescent plastic or a liquid it produces light, which you can measure with a photo-electric cell. Then we had a cloud chamber: when a particle passes through saturated water vapour you can see the trace it leaves. Cloud chambers were later replaced by bubble chambers: if you fire electrons through a superheated liquid it ionizes the molecules; under reduced pressure it forms tiny bubbles which show up the tracks of the particles. We were looking for various things. First, absorption; secondly deflection, or rather the distribution of angles; and thirdly the energy loss. To know how much that is, you have to determine the mass, which is related to the speed; and you can measure that with a magnetic field.

I was born on August 10, 1913 in Lorenzkirch a small village in Saxony, as the forth child of Theodor and Elisabeth Paul nee Ruppel. All in all we were six children. Both parents were descendants from Lutheran ministers in several generations. I grew up in München where my father has been a professor for pharmaceutic chemistry at the university. He had studied chemistry and medicin having been a research student in Leipzig with Wilhelm Ostwald, the Nobel Laureate 1909. So I became familiar with the life of a scientist in a chemical laboratory quite early. Unfortunately, my father died when I was still a school boy at the age of fifteen years. But my interest in sciences was awaken, even my parents were very much in favour of a humanistic education. After finishing the gymnasium in München with 9 years of latin and 6 years of ancient greek, history and philosophy, I decided to become a physicist. The great theoretical physicist, Arnold Sommerfeld, an University colleague of my late father, advised me to begin with an apprenticeship in precision mechanics. Afterwards, in the fall 1932, I commenced my studies at the Technische Hochschule München. Listening to the very inspiring physics lectures by Jonathan Zenneck with lots of demonstrations - 6 full hours a week - I felt being on the right track. After my first examination in 1934 I turned to the Technische Hochschule in Berlin. I was lucky in finding in Hans Kopfermann a teacher with a feeling for the essentials in physics but also a very liberal man, who had taken a fatherly interest in me. He, a former Ph.D. student of James Franck, had just returned from a three years stay at the Niels Bohr Institute in Copenhagen, working in the field of hyperfine spectroscopy and nuclear moments. All in all I worked 16 years with him.

As a theorist Richard Becker taught at the TH Berlin whom I met later at the University of Göttingen again. Both men had the strongest influence on my scientific thinking. But it was not only the scientific aspect. In the Germany of these days just as important was the human and the political attitude. And I am still a little bit proud having been accepted by these sensitive men in this respect. Here are the roots for my later engagement in the anti nuclear weapon discussion and for having signed the declaration of the so-called "Göttinger Eighteen" in 1957 with its important consequences in german politics. In 1937 after my diploma exam with Hans Geiger as examinator I followed Kopfermann to the University of Kiel where he had just been appointed Professor Ordinarius. For my doctor thesis I had chosen the determination of the nuclear moments of Beryllium from the hyperfine spectrum. I developed an atomic beam light source to minimize the Doppler effect. But just before the decisive measurements I was drawn to the air force a few days before the war started. Fortunately, a few month later I got a leave of absence to finish my thesis and to take my doctor exam at the TH Berlin. In 1940 I was exempted from military service. I joined again the group around Kopfermann which 2 years later moved to Gottingen. There in 1944 I became Privatdozent at the University.

In these years I worked in mass spectrometry and isotope separation together with W. Walcher. When we heard of the development of the betatron by D. Kerst in the United States and also of a similar development by Gund at the Siemens company, Kopfermann saw immediately that scattering experiments with high energy electrons would enable the study of the charge structure of nuclei. He convinced me to turn to this new very promising field of physics and I soon participated in the first test measurements at the 6 MeV betatron at the Siemens laboratory. Later after the war we succeeded in getting this accelerator to Gottingen. But due to the restriction in physics research imposed by the military government I turned for a few years my interest to radiobiology and cancer therapy by electrons in collaboration with my colleague G. Schubert from the medical faculty. Besides we performed some scattering experiments and studied first the electric disintegration of the deuteron, and not to forget for the first time we measured the Lamb shift in the He-spectrum with optical methods.

In 1952 I was appointed Professor at the University of Bonn and Director of the Physics Institute, with very good students waiting for a thesis advisor. I was very lucky that my best young collaborators followed me 0. Osberghaus, H. Ehrenberg. H.G. Bennewitz, G. Knop and H. Steinwedel as a "house theoretician". Here we started new activities: molecular beam physics, mass spectrometry and high energy electron physics. It was a scanty period after the war. But in order to become in a few years competitive with the well advanced physics abroad we tried to develop new methods and instruments in all our research. In this period these focusing methods in molecular beam physics with quadrupole and sextupole lenses having already started in Gottingen with H. Friedburg, were further developed and enabled new types of experiments. The quadrupole mass spectrometer and the ion trap were conceived and studied in many respects by research students. And with the generous support of the Deutsche Forschungsgemeinschaft we have built a 500 MeV electron synchrotron, the first in Europe working according to the new principle of strong focusing. It was followed in 1965 by a synchroton for 2500 MeV. My colleagues H. Ehrenberg, R.H. Althoff and G. Knop were sharing this success with me.

In recent years my interest turned to neutron physics with a new device, a magnetic storage ring for neutrons. U. Trinks and K.J. Kügler and later my two sons Lorenz and Stephan, joined me in our experiments with stored neutrons at the ILL in Grenoble. My experience in accelerator physics brought me in close contact to CERN. I served there from the very early days on as an advisor. Having spent the year 1959 in Genève I became director of the nuclear physics division for the years 1964 - 67. I was for several years member and later chairman of the Scientific Policy Committee and for many years scientific delegate of Germany in the CERN-Council. For a short period I was chairman of ECFA, the European Committee for Future Accelerators. Together with my friends W. Jentschke and W. Walcher in 1957 we started the German National Laboratory DESY in Hamburg which I joined as chairman of the directorate 1970 - 73. For several years I was chairman of its scientific council. In the same positions I served in the first years of the Kernforschungsanlage Jülich.

In 1970 I spent some weeks as Morris Loeb lecturer at Harvard University. 1978 I was lecturing as distinguished scientist at the FERMI Institute of the University of Chicago and in a similar position at the University of Tokyo. Since 1981 I am Professur Emeritus at the Bonn University. In the past decades of recovery of German Universities and Physics research I was engaged in many advisory bodies. I have served as a referee and later as member of senate to the Deutsche Forschungsgemeinschaft. I was member and chairman of several committees: for reforming the university structure and for research planning of the federal government. Ten years ago I was elected President of the Alexander von Humboldt Foundation which since 130 years fosters the international collaboration among scientists all over the world in the universal spirit of its patron Humboldt.