Presentation Speech by professor Sven Johansson of the Royal Academy of Sciences
Translation from the Swedish text
Your Majesties, Your Royal Highnesses, Ladies and Gentlemen,
At the end of the 1940’s, nuclear physics had advanced to a stage where a more detailed picture of the structure of the atomic nucleus was beginning to emerge and it was becoming possible to calculate its properties in a quantitative way. One knew that the nucleus consists of protons and neutrons, the so-called nucleons. They are kept together by nuclear forces, which give rise to a potential well, in which the nucleons move. The details of the nuclear structure were, however, unknown and one had to a great extent to rely upon models. These models were rather incomplete and partly contradictory. The oldest is the drop model in which the nucleus is regarded as a liquid drop, the nucleons corresponding to the molecules of the liquid. This model could be used with a certain success for a description of the mechanism of nuclear reactions, in particular for fission. On the other hand, one could not find any excited states of the nucleus corresponding to rotations or vibrations of the drop. Neither could certain other properties of the nucleus, particularly those associated with the “magic nubers”, be explained by means of the drop model. These show that individual nucleons in a decisive way affect the behaviour of the nucleus. This discovery, which is systematized in the shell model, was awarded the 1963 Nobel Prize for Physics.
It was soon found that the nucleus has properties, which cannot be explained by these models. Perhaps the most striking one was the very marked deviation of the charge distribution from spherical symmetry, which was observed in several cases. It was also pointed out that this might indicate that certain nuclei are not spherical but are deformed as an elipsoid, but no one could give a reasonable explanation of this phenomenon.
The solution of the problem was first presented by James Rainwater of Columbia University, New York, in a short paper submitted for publication in April 1950. In this, he considers the interaction between the main part of the nucleons, which form an inner core, and the outer, the valence nucleons. He points out that the valence nucleons can influence the shape of the core. Since the valence nucleons move in a field which is determined by the distribution of the inner nucleons, this influence is mutual. If several valence nucleons move in similar orbits, this polarizing effect on the core can be so great that the nucleus as a whole becomes permanently deformed. Expressed very simply, it can be said that as a result of their motion, certain nucleons expose the “walls” of the nucleus to such high centrifugal pressure that it becomes deformed. Rainwater also attempted to calculate this effect and got results that agreed with experimental data on the charge distributions.
Aage Bohr, working in Copenhagen, but at this time on a visit to Columbia University, had, independently of Rainwater, been thinking along the same lines. In a paper, submitted for publication about a month after Rainwater’s, he formulates the problem of the interaction of a valence nucleon with the core in a general way.
These relatively vague ideas were further developed by Bohr in a famous work from 1951, in which he gives a comprehensive study of the coupling of oscillations of the nuclear surface to the motion of the individual nucleons. By analysing the theoretical formula for the kinetic energy of the nucleus, he could predict the different types of collective excitations: vibration, consisting of a periodic change of the shape of the nucleus around a certain mean value, and rotation of the whole nucleus around an axis perpendicular to the symmetry axis. In the latter case, the nucleus does not rotate as a rigid body, but the motion consists of a surface wave propagating around the nucleus.
Up to this point, the progress made had been purely theoretical and the new ideas to a great extent lacked experimental support. The very important comparison with experimental data was done in three papers, written jointly by Aage Bohr and Ben Mottelson and published in the years 1952-53. The most spectacular finding was the discovery that the position of energy levels in certain nuclei could be explained by the assumption that they form a rotational spectrum. The agreement between theory and experiment was so complete that there could be no doubt of the correctness of the theory. This gave stimulus to new theoretical studies, but, above all, to many experiments to verify the theoretical predictions.
This dynamic progress very soon led to a deepened understanding of the structure of the atomic nucleus. Even this further development towards a more refined theory was inspired and influenced in a decisive way by Bohr and Mottelson. For example, they showed together with Pines that the nucleons have a tendency to form pairs. A consequence of this is that nuclear matter has properties reminiscent of superconductors.
Drs Bohr, Mottelson and Rainwater,
In your pioneering works you have laid the foundation of a theory of the collective properties of atomic nuclei. This has been an inspiration to an intensive research activity in nuclear structure physics. The further development in this field has in a striking way confirmed the validity and great importance of your fundamental investigations.
On behalf of the Royal Academy of Sciences I wish to convey to you our warmest congratulations and I now ask you to receive your prize from the hands of His Majesty the King.