Research Profile

by Ben Johnson

Leo James Rainwater

Nobel Prize in Physics 1975 together with Aage N. Bohr and Ben R. Mottelson
"for the discovery of the connection between collective motion and particle motion in atomic nuclei and the development of the theory of the structure of the atomic nucleus based on this connection".

The physicist James Rainwater was born in Council, Idaho, USA on December 9, 1917. The death of his father in the 1918 influenza epidemic caused the family to move to California where Rainwater would eventually excel in physics, mathematics and chemistry while still in school. Later he was admitted to the California Institute of Technology as a chemistry major before switching to physics and graduating with a B.S. In 1939.

Furthering his studies at Columbia University, Rainwater came into contact with many celebrated physicists such as Isidor Rabi, Enrico Fermi and Edward Teller. However, before finishing his thesis, the outbreak World War II led him to join the Manhattan Project as a member of the research and development team. His work during this time, after being declassified in 1946, earned him his Ph.D.

Continuing on at Columbia, Rainwater became heavily involved in experimental physics and the construction of new large-scale facilities for the investigation of subatomic particles.
The year 1949-1950 proved to be especially monumental when the danish physicist Aage Bohr came to Columbia and shared an office with Rainwater. In 1975 the two men would share the Nobel Prize amongst themselves and Ben Roy Mottelson for expanding the model of the atomic nucleus beyond the liquid drop and shell models and introducing non-spherical effects which could better account for experimental observations in a single model.

Rainwater’s Nobel Prize-winning work was actually published in 1950 [J. Rainwater, Physical Review 79, 432-434 (1950)] as a short article describing a particle (nucleon) in a spheroidal box and was able to make generally accurate predictions about nuclear excited states.
Put into historical perspective, Rainwater’s achievement was the culmination of the ground-
breaking work of several generations of physicists. Beginning in 1909, Ernest Rutherford,
Hans Geiger and Ernest Marsden were able to show the existence of the atomic nucleus through the Geiger-Marsden (gold foil) experiment and took the first step toward our modern concept of the nuclear atom. After the discovery of wave mechanics in 1926 in the context of atomic physics, its concepts were applied to the nucleus as well. In a discussion of the structure of atomic nuclei before the Royal Society in London in 1929, Ernest Rutherford offered a review of the then current conceptions of the nucleus based mainly on empirically obtained atomic weights, mass defects and α-particle emission. The main theoretical insight was provided by George Gamow and his liquid drop model in which he made predictions about the energy of the nucleus, the nature of the forces holding the nucleus together and the quantum conditions to which the nucleus is subject. At the time, the nucleus was considered to be built up mainly by α-particles, with nuclear electrons balancing the protons to the extent necessary to arrive at the desired atomic charge and weight. The notion that electrons existed in the nucleus at all was provided by the observation of β-decay. Closing the Royal Society meeting, Ralph Fowler, building on ideas from Gamow, discussed the much-studied phenomenon of α-emission from the nucleus in terms of quantum mechanical tunneling to explain how an α-particle could escape the apparent enormous potential barrier which must exist in the nucleus.

Interestingly, the absence of the neutron in these first models of the atomic nucleus is a stark reminder of the scientific progress still to come. This discovery came in 1932 with James Chadwick’s announcement of the discovery of the the neutron and paved the way for the ability of the liquid drop model to explain nuclear fission, which was first observed in 1938 by Otto Hahn and Fritz Strassmann.

Throughout the 1930s several groups, most notably Lise Meitner and Otto Frisch as well as Niels Bohr and John Wheeler, worked on increasingly powerful theories of the nucleua, which would then be used in describing the processes involved in nuclear fission. The culmination of the liquid drop model arrived at the end of this decade with the publication of several papers from these groups, which gave detailed energy and stability diagrams of heavy nuclei and remained current through World War II.

However, the observation and description of unstable nuclei brought with it the question of why stable nuclei also exist, especially since it had been noticed that certain magic numbers of protons and/or neutrons in a nucleus are very stable compared to their isotopic or isotonic neighbors. This led to the notion of a shell model of the nucleus, not unlike that for the atomic electrons. The energetic levels resulting out of the nucleons’ movement in the potential of an isotropic oscillator led to nucleonic shells which, after accounting for the effects of spin-orbit coupling, contained the observed magic numbers of particles. Any additional particle would have a relatively small binding energy, rendering the closed shell configuration especially stable. Two papers were published in close proximity in 1949, first by Maria Goeppert-Mayer and then by Otto Haxel, J. Hans D. Jensen and Hans E. Suess correctly describing such a scenario (for which Mayer and Jensen would later win a Nobel Prize of their own).
Coming with an outsiders perspective, Goeppert-Mayer was convinced that the liquid drop model, describing collective nucleon motion, and the shell model, based on individual nucleon motion and spin-orbit coupling, both had their place in nuclear physics. But could a unification be found between these two models, more basic physics which accounted for all of the observed evidence?

This seemed plausible. Although able to account for the ground state of the nucleus, neither the liquid drop model nor the shell model could give any information about excited states of the nucleus and also failed to account for other empirical observations. Most disturbing, as pointed out by several scientists including Charles Townes, Henry Foley and William Low in 1949, was the existence of large electric quadrupole moments of nuclei lying mid-way between the closed shells and zero or very small quadrupole moments at the magic numbers. In fact, the large size of many of the quadrupole moments could not be accounted for by single odd nucleons, as proposed in the shell model and was the first evidence that the spherical nature of the nucleus as described by the liquid drop model was not correct. Foley and Low were at Columbia at this time with Rainwater, and their thinking no doubt had an effect an his ideas about the nucleus.

One year later, Rainwater proposed his nuclear model which extended beyond that of the nucleus as a lone particle in an isotropic potential and included a small perturbation which rendered the nucleus spheroidal. He also solved the corresponding eigenvalue problem. Now, possibly subject to rotations and vibrations of the nucleus, all nucleons would feel the effects of this perturbed potential and could act collectively to achieve large electric quadrupole moments in certain cases. Of course, in the case of closed shells, the nucleus would still be very nearly spherical and have a very small quadrupole moment.

Apart from describing excited nuclear states and moving beyond the liquid drop and shell models, Rainwater’s model also showed the collective action of the nucleons and resulted in the entire body of work based on his initial insight being referred to as “collective models” of the nucleus.

Rainwater’s discovery and career also mark several changes in scientific research at the time, above and beyond producing a new concept of the nucleus. Rainwater was an experimental physicist whose foray into theory proved to be short and potent and also one of the last of its kind. After this period the split between theory and experiment became so immense that hardly a scientist could bridge the gap. This can be seen in the case of Rainwater’s Nobel co-winners Aage Bohr and Ben Roy Mottelson who based a large body of theoretical work on Rainwater’s idea and relied mainly on the experimental work of others, notably Maurice Goldhaber and R.D. Hill, for confirmation. In fact, it was Bohr and Mottelson who attacked the problem of unification of the liquid drop and shell models and were able to develop a new and functioning nuclear theory based on Rainwater’s initial idea. Their fabled teamwork resulted in a number of influential papers based on the new theory in the 1950s and beyond. The association of Aage Bohr in the story also clearly indicates a generational shift from the early pioneers of quantum theory (Niels Bohr) to the ones who bequeathed a mature theory (Aage Bohr and Rainwater).

Although Rainwater went back to experiment after his publication on the spheroidal nucleus (in 1953 he published important studies on X-rays from μ-mesonic atoms) his nuclear model continued to be the basis for nuclear research for years. This included refined descriptions of rotational and vibrational states including band models, transition probabilities and also Coulomb excitation, the excitation of nuclei via the electromagnetic field of an incident particle without the particle entering the nucleus.

Rainwater married Emma Louise Smith in 1942 and became a full professor at Columbia in 1952 where he remained active in science nearly until his death in 1986.

References

[1] Nobelprize.org

[2] Nobel Prize Winners, Tyler Wassan, Ed., The H.W. Wilson Company, New York (1987)

[3] The Defining Years in Nuclear Physics, M. Mladjenovic, IOP Publishing, Ltd, London (1998)

[4] Ernest Rutherford, The Scattering of Alpha and Beta Particles by Matter and the Structure of the Atom, Philosophical Magazine 21, 669-688 (1911)

[5] Ernest Rutherford, et al., Discussion on the Structure of Atomic Nuclei, Proceedings of the Royal Society of London A 123, 373-390 (1929)

[6] James Chadwick, Possible Existence of a Neutron, Nature 129, 312 (1932)

[7] Niels Bohr and John Archibald Wheeler, The Mechanism of Nuclear Fission, Physical Review 56, 426-450 (1939)

[8] Maria Goeppert-Mayer, On Closed Shells in Nuclei II, Physical Review 75, 1969-1970 (1949)

[9] James Rainwater, Nuclear Energy Level Argument for a Spheroidal Nuclear Model, Physical Review 79,
432-434 (1950)


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