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Wolfgang Paul in memoriam
Wolfgang Paul in memoriam
A few months after his 80th birthday, Wolfgang Paul passed away on December 7, 1993 . A great scientist's lifelong work of excellence has come to an end. In his Nobel lecture on December 8, 1989, Wolfgang Paul quoted an aphorism by the Gbttingen physicist Georg Lichtenberg, "one has to do something new in order to see something new", and furthermore Lichtenberg's remark that "it is a sad situation in all our chemistry that we are unable to suspend the constituents of matter free". In perspective, Wolfgang Paul put serious attention to both these statements during his scientific life . In the turbulent time following the last world war Paul and his colleagues in Germany were faced with immense problems to reconstruct what was left of science in Germany. After finishing his doctoral thesis on spectroscopic hyperfine structure studies of nuclear moments for Professor Hans Kopfermann, he was appointed a "Privatdozent" at Göttingen University in 1944. With this background he soon found his way to mass spectroscopy and isotope separation . At about that time the betatron concept was developed by Donald Kerst in the USA which opened up the field of oscillatory orbital movements of charged particles in inhomogeneous magnetic configurations. Realizing the potentialities of this approach, Paul and other groups started to construct betatron accelerators for nuclear physics work, in particular with the aim of studying the charge structure of nuclei by means of electron scattering. Paul had now acquired the necessary knowledge, and developed a taste for this kind
ii
Wolfgang Paul in memoriam
of physics, which enabled him to go further in looking for different types of field configurations with possible applications in other fields of physics and chemistry . So, when he was called to a new professorship at the University of Bonn as the Director of the Physics Institute in 1952, he organized a group of young and able students to assist him in exploring this promising field. He then faced the first of Lichtenberg's mentioned two quoted points. As it turned out somewhat later, he was actually already on his way to realizing Lichtenberg's second remark : to "suspend the constituents of matter free". By systematically examining the different possible field forms for keeping molecular ion beams together he thus invented the quadrupole and sextupole long lenses which have become of such a basic interest in both mass spectroscopy and in high energy physics . More generally, multipole field configurations exhibit several features which can be applied to many different fields of research . It is interesting to note that the first spectacular applications were found in molecular physics, i.e. in low energy physics and chemistry and, as it turned out, were later on equally indispensable in high energy elementary particle physics, complementing the huge particle accelerators based on strong focussing . In order to obtain stable orbits suitable for the quadrupole lens mass spectrometer dominating nowadays one needs a combination of a radio frequency field and a static quadrupole field. This yields two-dimensional focussing. Paul found already at an early stage of his research that three-dimensional trapping of ions would be possible for a certain type of potential configuration . For this purpose an hyperbolically-shaped ring and two rotationally symmetric hyperbolically-shaped caps are required . Again a periodic field is needed for the stabilization of the orbits . This ion trap, known as the Paul trap, was conceived and realized in 1954. At his Nobel lecture in 1989 Paul amused his audience by making an experiment demonstrating the trapping principle by means of a mechanical model . Together with his many students, some of whom later on formed their own research groups, the new tools were applied to detailed studies in molecular physics with rich outcome . During more recent years Paul turned his interest to the trapping of neutral particles, in particular neutrons. He showed that several different multipole configurations are actually feasible for achieving this goal. The preferred configuration was in this case a magnetic toroidally closed trap in which sextupoles, octupoles, and even decapoles were used. The first successful results of these remarkable experiments, which were performed at the Grenoble high flux reactor, were reported by Paul during a lecture in Uppsala when he was awarded an honorary doctorate in 1978 . A neutron storage time of 20 min was achieved . In more recent experiments with an improved neutron flux by a factor of 40 the neutrons were stored 90 min. This is about six times the decay time of the neutrons due to radioactive decay. In fact the lifetime of the neutrons could be measured to be 877 ± 10 s. In a later, very elegant experiment which was typical of the skillful experimentalist Wolfgang Paul, he could even weigh the neutrons in his trap. Because ofthe small magnetic moment ofthe neutrons, which are bound elastically to the symmetry plane of the magnetic field in the trap, this restoring field is not much more than the Earth's gravitational field. The neutrons are literally "hanging" by the "string" of the trap's field which is "stretched" by gravity . The equilibrium position of the neutrons is therefore shifted downwards somewhat . By measuring the profile of the circulating neutron beam with a neutron counter it was actually possible to give a gravitational mass of the neutron, namely mg = (1 .63 ± 0.06) x 10-24 g. This is consistent within 4% with its inertial mass. Wolfgang Paul was not only a superb experimental physicist but also an efficient research administrator. In this capacity he was in great demand for a number of important tasks in German and European scientific undertakings . Thus, he was for a long period of time scientific advisor at CERN, was equally concerned with the Hamburg high energy physics performed at DESY, etc . A particularly important leadership was entrusted to him when he accepted the post of President of the Humboldt Foundation in 1979. In this capacity he acted for ten years . With special pride we finally note that Wolfgang Paul was a member of the Editorial Board of our journal NIM all the time since its beginning in 1957. The scientific community has lost one of its most authoritative and distinguished representatives whose influence on science was far-reaching .
Kai Siegbahn
A few months after his 80th birthday, Wolfgang Paul passed away on December 7, 1993 . A great scientist's lifelong work of excellence has come to an end. In his Nobel lecture on December 8, 1989, Wolfgang Paul quoted an aphorism by the Gbttingen physicist Georg Lichtenberg, "one has to do something new in order to see something new", and furthermore Lichtenberg's remark that "it is a sad situation in all our chemistry that we are unable to suspend the constituents of matter free". In perspective, Wolfgang Paul put serious attention to both these statements during his scientific life . In the turbulent time following the last world war Paul and his colleagues in Germany were faced with immense problems to reconstruct what was left of science in Germany. After finishing his doctoral thesis on spectroscopic hyperfine structure studies of nuclear moments for Professor Hans Kopfermann, he was appointed a "Privatdozent" at Göttingen University in 1944. With this background he soon found his way to mass spectroscopy and isotope separation . At about that time the betatron concept was developed by Donald Kerst in the USA which opened up the field of oscillatory orbital movements of charged particles in inhomogeneous magnetic configurations. Realizing the potentialities of this approach, Paul and other groups started to construct betatron accelerators for nuclear physics work, in particular with the aim of studying the charge structure of nuclei by means of electron scattering. Paul had now acquired the necessary knowledge, and developed a taste for this kind
ii
Wolfgang Paul in memoriam
of physics, which enabled him to go further in looking for different types of field configurations with possible applications in other fields of physics and chemistry . So, when he was called to a new professorship at the University of Bonn as the Director of the Physics Institute in 1952, he organized a group of young and able students to assist him in exploring this promising field. He then faced the first of Lichtenberg's mentioned two quoted points. As it turned out somewhat later, he was actually already on his way to realizing Lichtenberg's second remark : to "suspend the constituents of matter free". By systematically examining the different possible field forms for keeping molecular ion beams together he thus invented the quadrupole and sextupole long lenses which have become of such a basic interest in both mass spectroscopy and in high energy physics . More generally, multipole field configurations exhibit several features which can be applied to many different fields of research . It is interesting to note that the first spectacular applications were found in molecular physics, i.e. in low energy physics and chemistry and, as it turned out, were later on equally indispensable in high energy elementary particle physics, complementing the huge particle accelerators based on strong focussing . In order to obtain stable orbits suitable for the quadrupole lens mass spectrometer dominating nowadays one needs a combination of a radio frequency field and a static quadrupole field. This yields two-dimensional focussing. Paul found already at an early stage of his research that three-dimensional trapping of ions would be possible for a certain type of potential configuration . For this purpose an hyperbolically-shaped ring and two rotationally symmetric hyperbolically-shaped caps are required . Again a periodic field is needed for the stabilization of the orbits . This ion trap, known as the Paul trap, was conceived and realized in 1954. At his Nobel lecture in 1989 Paul amused his audience by making an experiment demonstrating the trapping principle by means of a mechanical model . Together with his many students, some of whom later on formed their own research groups, the new tools were applied to detailed studies in molecular physics with rich outcome . During more recent years Paul turned his interest to the trapping of neutral particles, in particular neutrons. He showed that several different multipole configurations are actually feasible for achieving this goal. The preferred configuration was in this case a magnetic toroidally closed trap in which sextupoles, octupoles, and even decapoles were used. The first successful results of these remarkable experiments, which were performed at the Grenoble high flux reactor, were reported by Paul during a lecture in Uppsala when he was awarded an honorary doctorate in 1978 . A neutron storage time of 20 min was achieved . In more recent experiments with an improved neutron flux by a factor of 40 the neutrons were stored 90 min. This is about six times the decay time of the neutrons due to radioactive decay. In fact the lifetime of the neutrons could be measured to be 877 ± 10 s. In a later, very elegant experiment which was typical of the skillful experimentalist Wolfgang Paul, he could even weigh the neutrons in his trap. Because ofthe small magnetic moment ofthe neutrons, which are bound elastically to the symmetry plane of the magnetic field in the trap, this restoring field is not much more than the Earth's gravitational field. The neutrons are literally "hanging" by the "string" of the trap's field which is "stretched" by gravity . The equilibrium position of the neutrons is therefore shifted downwards somewhat . By measuring the profile of the circulating neutron beam with a neutron counter it was actually possible to give a gravitational mass of the neutron, namely mg = (1 .63 ± 0.06) x 10-24 g. This is consistent within 4% with its inertial mass. Wolfgang Paul was not only a superb experimental physicist but also an efficient research administrator. In this capacity he was in great demand for a number of important tasks in German and European scientific undertakings . Thus, he was for a long period of time scientific advisor at CERN, was equally concerned with the Hamburg high energy physics performed at DESY, etc . A particularly important leadership was entrusted to him when he accepted the post of President of the Humboldt Foundation in 1979. In this capacity he acted for ten years . With special pride we finally note that Wolfgang Paul was a member of the Editorial Board of our journal NIM all the time since its beginning in 1957. The scientific community has lost one of its most authoritative and distinguished representatives whose influence on science was far-reaching .
Kai Siegbahn