There’s a lot we don’t know about the atomic nucleus, even though it was discovered a century ago this year.
We have, of course, learned much. We can get energy by splitting the nucleus in a process known as fission or smashing nuclei together in a process known as fusion. While we can’t say exactly when an unstable nucleus will decay on its own—spontaneously transforming from one isotope to another—we can say how fast a large group of nuclei will do so. In fact, we can confidently determine the half-life of a nucleus—the time in which 50 percent will decay—even in cases in which that half-life is greater than the age of the universe. The nucleus displays oddities the understanding of which will help explain our world. One of these is the tendency of protons and neutrons that make up the atomic nucleus—known collectively as nucleons—to bond together in pairs.
Physicists from Oak Ridge National Laborator, the University of Tennessee, and Germany’s GSI in Darmstadt recently used ORNL’s Jaguar supercomputer to explore the pair bonding of neutrons in one uncommon isotope—germanium-72. In doing so they discovered that changes in temperature and rotation take the nucleus through at least two physical phases. Their work, which offers the first realistic description of this kind of phase transition in an atomic nucleus, was featured in the Nov. 19, 2010, edition of Physical Review Letters.
In our mundane lives we witness phase transition anytime we see water chill into ice or boil into steam. Those three states of water—solid, liquid and gas—are the three phases, and the transitions depend on both pressure and temperature. In the concealed quantum world of the atomic nucleus, however, phase transitions are more subtle.
Germanium-72 has 32 protons (like all germanium isotopes) and 40 neutrons. Those 40 neutrons pair off strongly when the nucleus is cold and calm, but pairing weakens as you increase the temperature or rotation. What the team discovered, however, was that the relationship is not straightforward. When rotation is high, the pairing weakens as temperature rises, spikes back up at one small range of temperatures, and then weakens as temperature continues to rise. That spike indicates the transition between phases.