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 91°µÍø 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 ().