Aage Bohr og Ben R. Mottelson Collective Co-ordinates for Nuclear Rotations
Aage Bohr and Ben Mottelson received the Nobel Prize in 1975 jointly with James Rainwater «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». At the time of the inception of their common scientific work nuclear physics phenomena were described in terms of two seemingly contradictory models, the liquid drop model (LDM) from the end of the 1930’es and the Shell Model (SM) established about 10 years later. In the LDM the nuclear constituent particles (nucleons) behaved collectively in the nuclear volume confined by a surface exhibiting surface tension in analogy with a drop of liquid. On the other hand, the SM treated the individual nucleons as the degrees of freedom and was thus considered the more fundamental of the two. Both models turned out to be successful, within their particular limitations. Although the SM successfully described nuclear properties which could be attributed to the properties of individual nucleons, such as angular momenta and to a certain extent magnetic moments, it grossly underestimated the electric quadrupole moments of several nuclei. This suggested that the nucleus as a whole deviated from a spherical shape, so that the main contribution to the quadrupole moment was a collective one. Still, particle degrees of freedom had to be taken into account, and so Bohr and Mottelson developed a unified model marrying concepts from both the LDM and the SM. The unified model was corroborated by the wealth of experimental data following the theoretical predictions. The Norwegian text attempts, with basis in the Nobel lectures and the accompanying Autobiographical notes of Bohr and Mottelson, to show how their personal and scientific histories develop and eventually get intertwined.
One point of critique of the unified model was that a clean separation of collective and individual particle motion is not possible, since the collective behavior of the nucleus is produced by the behavior of the interacting constituent particles. The separation between collective and particle degrees of freedom is the topic of the above mentioned paper. Although a clean separation cannot be made, the authors succeed in making an optimal separation. Other ways of separation are possible, but might lead to complicated couplings between the two types of motion.