Strange as it may seem, atomic nuclei sometimes act like liquids, and when blasted apart at high enough energies they can sizzle into gas. Now scientists have charted the conditions under which gold nuclei make that leap.
The work builds on a model that physicists cooked up in the 1930s to explain the fission of uranium. A neutron striking a nucleus more than 200 times its mass doesn't knock off a chip or two. The nucleus behaves like an oversized drop of water, eventually splitting in two. The nucleus also throws off a few smaller fragments, such as neutrons, in the process.
Victor Viola, a physicist at Indiana University, Bloomington, and his colleagues decided to determine the nucleus's equation of state--what level of pressure and temperature force the nucleus to behave like a gas or a liquid. Working at Brookhaven National Laboratory in Upton, New York, the group shot subatomic particles, including protons, pions, and antiprotons, at thin gold foil, adding energy that brought the gold nuclei to a boil. Meanwhile, a device called the Indiana Silicon Sphere (ISiS)--a beach-ball-sized sphere studded with 450 detectors--kept careful track of the size and energy of particles from the nucleus that flew off.
By comparing the energy added to the nucleus (its "temperature") with the relative abundances of fragments, the physicists figured out the properties of the nuclear "liquid," including its critical temperature, the point above which it becomes gaseous. They pinpointed the critical temperature at about 7 million electron volts (MeV), while a second analysis modeling the breaking and making of nuclear bonds calculated it at just over 8 MeV. The study appears in the 14 January issue of Physical Review Letters.
"It's a really nice piece of work," says Joseph Natowitz, a physicist at Texas A&M University in College Station, who thinks that physicists will resolve the discrepancy once they get a better grip on how the nucleus behaves after collisions. Even though wrinkles need to be ironed out, the results have given physicists a new tool for understanding the "evaporation" of nuclei. They could also shed light on the formation of neutron stars, which might condense like gigantic nuclei.