A Watched Nucleus Will Indeed Boil

Luciano Moretto and colleagues at the Lawrence Berkeley National Laboratory, in collaboration with scientists at Indiana University, have shed light on a long-standing mystery about the nature of nuclei. By reanalyzing existing experimental data from a new perspective, Moretto, professor of chemistry, and coworkers found the strongest evidence to date that nuclei can experience a “phase transition” and go from a liquid to a vapor state.

“We have recently produced the first phase diagram for finite nuclear matter,” said Gordon Wozniak (Ph.D. ’74), coauthor of the study that appeared in the January 28 issue of Physical Review Letters. A phase diagram is a graph that shows the phases of a substance as they relate to temperature and pressure. As water is boiled, for example, it goes from liquid to vapor as the temperature increases.

“In this study, we measured the liquid/vapor coexistence line for atomic nuclei from low temperatures up to the critical point. This is a very important thermodynamic quantity,” said Wozniak. “It is striking that the phase diagram for liquid krypton is very similar to the one for its nucleus even though the nucleus is held together by the strong interaction while the atoms are bound by the electromagnetic interaction. This represents the first experimental measurement of any phase diagram not bound together by electromagnetic forces.”

To explain the nature of atomic nuclei, scientists have long described the nucleus as a fluid and have characterized its physical properties and behaviors. If this theory is correct, scientists believed that under the appropriate conditions—such as in the high impacts of particle accelerator collisions—the protons and neutrons inside an atomic nucleus should behave like ordinary molecules, and the nuclei should change from liquid to vapor phases. This study is the first evidence that the liquid to vapor transition actually takes place for nuclei.

With their phase diagram, the researchers say that, given the size of the fragmented nuclei and the energy that was used to fragment them, they can predict the temperatures and pressures under which the nuclei will change from the liquid to the vapor phase. As with materials scientists, the next step for nuclear scientists will be to determine how the properties of atomic nuclei clusters compare to the properties of bulk nuclear matter.

Back to April 2002 Newsletter

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