Dec. 19, 2000 Watching ice melt and water vaporize by increasing the temperature would not surprise anybody. Watching the nucleus of an atom transform from solid to liquid to gas is less common but is now possible and could lead to a better understanding of the properties of atomic nuclei.
To study how nuclei undergo changes from solid to liquid to gas, Texas A&M University scientists smash atomic nuclei together in an on-site accelerator, the Cyclotron. The two nuclei mix together, creating a very hot intermediate state where some nuclei are liquefied and others are vaporized.
"We are talking about two things colliding, mixing for a very short period of time, and breaking up, and then we see the pieces of that," says Sherry Yennello, associate professor of chemistry at Texas A&M.
"The phase transition - the passage from a solid state to a liquid state or a liquid state to a gas state - exists only for less than a billionth of a billionth of a second," Yennello says. "So we have to backtrack and say: 'How did this thing happen?'"
To study the fragments of the collision and look for hints of a phase transition, Texas A&M physicists have built a detector called NIMROD.
"With the NIMROD detector, we can study all of the charged fragments that are emitted in the reaction," says Joseph Natowitz, professor of chemistry at Texas A&M and director of the Cyclotron Institute. "There have been three versions of this detector since 1989, and the last version started working about a year and a half ago."
NIMROD is cylindrical in shape, and consists mainly of concentric layers of numerous small detectors used to detect charged particles, and a large detector surrounding the small detectors, used to detect neutral particles.
Scientists working with the NIMROD detector try to determine the nature of the phase transition of nuclei from liquid to gas.
"A nucleus undergoes a phase transition that has strong similarities with a change from liquid water to water vapor," says Natowitz." This is called a first order phase transition, which is a very sharp phase transition."
"But nuclei could also undergo a second phase transition, which is more gradual," he says. "Nuclei would go from a liquid phase to a gas phase more smoothly."
In fact, during the collision, nuclei expand and release the particles they contain: neutrons and protons, which undergo a liquid-gas phase transition. Neutrons and protons can tell if the transition occurred sharply or smoothly.
"If there is a different number of neutrons and protons, all the protons may undergo a phase transition before all the neutrons, or vice-versa," says Yennello. "So as you start making the transition to the gas phase, neutrons may make that transition first. Then, the intermediate state explodes, and you end up with a liquid-gas mixture and a neutron-rich gas."
Though physicists have not analyzed all collected data yet, there is some evidence that a neutron-rich gas is present during the collisions, according to Yennello, suggesting a smooth liquid-gas transition of the nucleus.
Understanding the liquid-gas transition of nuclei can be used to determine how nuclei behave under various temperature and pressure conditions.
"Like a gas, for which the relationship between the pressure, volume and temperature is well known, nuclei form a particular type of matter, called nuclear matter, for which the thermodynamic properties can be investigated," Natowitz says.
Studying liquid-gas transitions of nuclei also has applications in astrophysics and chemistry.
"These studies have significant relevance to astrophysical questions, like the understanding of supernovae explosions and neutron star formation," says Natowitz.
"There are also applications in the study of chemical clusters of, say, 50 atoms or even 1000 atoms," he adds. "This a very interesting area of exploration because chemical systems have lots of potential applications in terms of catalysis for instance."
"NIMROD is just beginning," Yennello says. "This detector is a very powerful state-of-the-art device that will give us opportunities to look at lots of great science for many years to come."
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