[cdn-nucl-l] Atom Smasher Probes Realm of Nuclear 'Gas'

["Adam McLean" <adam.mclean@utoronto.ca>]

Posted in Volume 295, Number 5555, Issue of Science Magazine of 25 Jan
2002, pp. 603-604. 
http://www.sciencemag.org/cgi/content/full/295/5555/603a
Fission of gold!  Well, kinda...

Adam

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Atom Smasher Probes Realm of Nuclear 'Gas'
Charles Seife

"Oh, that this too too liquid nucleus would evaporate." If Hamlet were a
nuclear physicist, he might be feeling a bit more cheerful. Strange as
it may seem, atomic nuclei do sometimes act like liquids, and when
blasted apart at high enough energies they can sizzle into gas. Now
scientists working at Brookhaven National Laboratory in Upton, New York,
have charted the conditions under which gold nuclei make that leap,
information that might help unravel the secrets behind the birth of a
neutron star.

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 just knock off a chip or two; it splits the
nucleus neatly in two. Physicists realized that the uranium nucleus is
behaving like an oversized drop of water. When it is struck by a
neutron, the nucleus oscillates, stretches out, and then blurps into two
roughly equal parts (throwing off a few smaller fragments, such as
neutrons, in the process). "Everyday garden-variety nuclei behave like a
liquid," says Victor Viola, a physicist at Indiana University,
Bloomington. "It's a very successful description."

Viola and colleagues decided to take the liquid analogy one step further
by determining the nucleus's equation of state--the relations between
pressure and temperature that govern when the nucleus behaves like a gas
and when it behaves like a liquid. At Brookhaven, they shot 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 the particles that flew
off.

The physicists analyzed the readings in two different ways. The first
starts with the distribution of the sizes of chunks that fly out of the
nucleus. "In boiling water, you don't get individual water molecules
coming off," says Viola. "You get dimers, trimers, tetramers. The
temperature of the vapor is related to the relative numbers of those
clusters." By comparing the energy added to the nucleus (hence 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 the liquid phase can no
longer exist, which they calculate at about 7 million electron volts
(MeV). The second analysis directly models the breaking and making of
nuclear bonds and comes up with a slightly higher critical temperature,
slightly above 8 MeV.

"I do think it's a really nice piece of work they've done," 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 expands and breaks up after the
collision. "I have some ideas."

Even though wrinkles need to be ironed out, the results have given
physicists a new tool for understanding the "evaporation" of nuclei.
They might also shed light on the reverse process, the condensation of
nuclei from smaller parts. "It's relevant to what happens in the
formation of neutron stars," says Viola. If so, the work is likely to be
a hit--a palpable hit.

Volume 295, Number 5555, Issue of 25 Jan 2002, pp. 603-604.