The point at which a phase transition occurs is called the critical point and is dear to every physicist’s heart. The fascination with systems at the critical point stems from the fact that a small change in one parameter, such as temperature, can trigger spectacular changes in the other properties. Indeed, it is this discontinuity in the properties at the critical point that makes phase transitions so challenging to describe.

Phase transitions arguably play an even more important role in the world of quantum mechanics. In the case of nuclei, the numbers of neutrons, N, and protons, Z, act as a control parameter in the same way that temperature affects water. Physicists can study variations in the shape of the nucleus as a function of N and Z, paying particular interest to those regions where the shape changes most rapidly. These regions are the equivalent of the critical points in a standard phase diagram.

In many examples in macroscopic and condensed-matter physics, it is easy to draw a phase diagram because huge numbers of particles participate. But nuclei contain at most a few hundred nucleons – and this makes the analysis of nuclear phase transitions more difficult.

Now Franceso Iachello at Yale University in the US has proposed a method based on nuclear symmetries that might lead to a framework to deal with nuclei at, or around, critical points. Moreover, Rick Casten and Victor Zamfir, also at Yale, have shown convincing evidence that the properties predicted by Iachello’s model are approximately observed in a samarium isotope (Phys. Rev. Lett. 2001 87 052502; 052503).

In the August issue of Physics World, Piet Van Isacker of the GANIL laboratory, France, explores the new method for modelling the often complicated transitions in nuclear phase diagrams.