The astonishing regularity of atomic nuclei - 27 Mars 2009

Nuclear physics: towards improved modeling of complex systems

The atomic nucleus is a complex system consisting of several particles, nucleons, which interact with one another. The fundamental properties of the nucleus, in particular its mass, shape, and lifetime (if it is radioactive), as well as changes in its structure in response to an external stress, have been the subjects of many studies, both theoretical and experimental. This is the field of nuclear physics research.

Two different approaches are generally followed in order to model an atomic nucleus:

• if it has a small number of nucleons (less than about fifteen), then it can be described sufficiently accurately on the basis of what is known about the interactions of two nucleons only.
• beyond this size (more than fifteen nucleons), that is, for most atoms found on Earth, or which can be created in a laboratory, it is generally necessary, because of the complexity of the mathematical equations involved, to make use of approximations. These are founded, in particular, on a classification of the collective modes of nucleon interactions. The description of the nucleus is then equivalent to that of several nucleon pairs, called "bosons", whose interactions can explain the principal collective properties of the nucleus.

The exact description of such a system relies on the conservation laws (or "symmetry properties") obeyed by nuclear forces; they may in theory be ranked hierarchically by associating, with the various forces at play, mathematical operators that satisfy specific algebraic rules.

When the interaction modes of nucleons are accounted for in this manner, they are amenable to an exact mathematical description. This, however, may sometimes strongly deviate from experimental observations, either because the interaction modes are in reality more complex, or because identified symmetry properties are not followed exactly.

To remove this difficulty, three theoreticians have proposed a new method to identify, among the different possible modes of interaction between nucleons, those which are the most relevant to the low-energy properties of the nucleus (i.e. which can be tested experimentally).


This newly proposed theoretical tool has been successfully applied to the modeling of the rotation and vibrational modes of the stable platinum-196 nucleus, which comprises no less than 196 nucleons. It thus enables scientists to gain a better understanding of the collective properties of complex systems such as atomic nuclei or molecules.
This theoretical breakthrough was published in Physical Review Letters, on March 20th 2009. Publication reference: J.E. García-Ramos, A. Leviatan, and P. Van Isacker, Physical Review Letters, March 2009
http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normalid=PRLTAO000102000011112502000001&idtype=cvips&gifs=Yes