We are all familiar with the change water goes through under the influence of falling temperatures, evolving from a disordered liquid to an ordered solid. What we may not know, however, is that such — known as phase transitions — can also be found within the nucleus of a single atom. In this case the ordering is found when nucleons — neutrons (or protons) — pair up to make the nucleus more stable. Physicists from Oak Ridge National Laboratory and the University of Tennessee have offered the first realistic description of phase transition in an atomic nucleus, using ORNL's Jaguar supercomputer to analyze the odd behavior of germanium-72, a medium-mass nucleus with 32 protons and 40 neutrons, as it is heated and rotated. Nuclei typically lose their pairing — and therefore their order — when they are exposed to high heat and rotation. In germanium-72, however, that pairing re-emerges and peaks at a critical temperature — nearly 2 billion degrees Fahrenheit. Any realistic nuclear theory must take this behavior into account. The team's results are documented in the Nov. 19 edition of Physical Review Letters (http://link.aps.org/doi/10.1103/PhysRevLett.105.212504).