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Hal Kibbey
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Last modified: Tuesday, January 15, 2002

Atomic nucleus can "boil" like water

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Note: A news release on this project from the Jan. 8 issue of "Physics News Update," a publication of the American Institute of Physics, can be seen at http://www.aip.org/physnews/update/. It includes two illustrations. Nuclear scientist Victor Viola of Indiana University, co-spokesman for the project, can be reached at 812-855-2878 or 812-855-6537 or viola@indiana.edu.

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BLOOMINGTON, Ind. -- For over two decades, nuclear scientists have sought experimental confirmation of a transition between the liquid and gas phases in hot atomic nuclei, as predicted by theory. Now a series of nuclear physics experiments and separate theoretical analyses of these data have provided strong evidence for this phenomenon. The results will appear in four papers to be published during the next month in The Physical Review and Physical Review Letters.

The nuclear liquid-gas phase transition is analogous to the boiling of water and condensation of steam and may have relevance to our understanding of the formation of neutron stars and black holes during supernova events. Supernovas are the most spectacular events that astronomers observe in space, involving the explosion of massive stars during the final stages of stellar evolution. The extreme pressure of gravity in the core of a supernova causes the gaseous nuclear matter to condense into a giant drop of nuclear liquid -- a neutron star, one of the densest forms of matter in the universe. For example, if our planet Earth were compressed to neutron star density, it would fit inside a football stadium. Hence, the present studies are germane not only to the microscopic world of nuclear physics, but also to fundmental cosmological processes.

It has long been known that the nuclei of heavy elements such as gold or uranium behave very much like a drop of liquid water. However, experimental attempts to bring this nuclear liquid to a boil and vaporize it encounter major complications. First, in order to make nuclei boil, temperatures of nearly 100 billion degrees are required. Such extreme conditions can only be achieved on Earth by bombarding nuclei with beams traveling at near the speed of light, produced by high-energy particle accelerators. Second, nuclei are microscopic subatomic objects, 10,000 times smaller than an atom. Finally, the nuclear phase transition occurs on a time scale of less than a billion-trillionth of a second. Thus, complex particle detectors are required to sift through the debris created in these nuclear collisions and sort out those events that provide experimental signals for a phase transition.

The experimental measurements were performed with the Indiana Silicon Sphere (ISiS) detector array at the Brookhaven (NY) National Laboratory AGS nuclear particle accelerator. Beams of 8-10 GeV protons and negative pi mesons were used to bombard a target of gold nuclei located inside the ISiS detector array. Each event was registered by a spherical array of detectors and stored via a complex electronic and computer system. In the most violent collisions the gold nuclei were fragmented into over 40 pieces, consisting of light chemical elements such as hydrogen, helium, carbon and oxygen. In order to understand the nature of these events, it was necessary to reconstruct each collision, identifying each piece, its direction and how fast it was ejected -- much as a forensic scientist would re-create the explosion of a bomb.

Examination of the results by the ISiS collaboration found that the experimental observables for the hottest nuclei showed all the characteristics that one would expect from a liquid-to-gas phase transition. These conclusions are presented in the current issue of The Physical Review (Phys. Rev. C 64, 064603 and Phys. Rev. C 64, 064604). Another important experimental signal was the application of a nuclear-clock technique that showed the disintegration of the hottest nuclei proceeded almost instantaneously, recently published in Physical Review Letters.

The theoretical analyses were performed by separate groups at Michigan State University and Lawrence Berkeley National Laboratory and are reported in papers to be published in Physical Review Letters (http://www.aip.org/physnews/select). By studying the number and sizes of the fragments formed in each reaction event as a function of collision violence, both groups demonstrated that the data were in excellent agreement with the theoretical expectations for the conversion of the nuclear liquid to a gas of clusters, protons and neutrons.

The spokesmen for the ISiS collaboration are Kris Kwiatkowski (now at Los Alamos National Laboratory) and Victor Viola (Indiana University; viola@indiana.edu). Other key members of the collaboration were Luc Beaulieu (now at Laval University), Thomas Lefort (now at LPC Caen), Wen-chien Hsi (Rush Presbyterian Hospital, Chicago), Ananya Ruangma and Sherry Yennello (Texas A&M), Ralph Korteling (Simon Fraser University), Herbert Breuer (University of Maryland), Ludwik Pienkowski (University of Warsaw) and Romualdo deSouza (Indiana University). The Lawrence Berkeley team included James Elliott (jbelliott@lbl.gov), Luciano Moretto, Larry Phair and Gordon Wozniak. The Michigan State team was composed of Wolfgang Bauer (bauer@pa.msu.edu), Marko Berkenbusch and Scott Pratt.