For the first time, scientists have staked out,
in the form of a diagram, how nuclear matter goes from
the liquid phase to the gas phase. Liquid-gas phase
diagrams are a staple of chemistry, where they anatomize
the energy frontier between, say, liquid water and water
vapor. Altering the pressure or the temperature can send
one back and forth across the two forms of existence.
Do the protons and neutrons sheltering together inside
a nucleus act like molecules in an ordinary gas or
liquid? Theorists have thought as much, but it's been
hard to prove owing to the extreme finiteness of a
nucleus (with perhaps 100-200 constituent protons and
neutrons) compared to a macroscopic liquid (with
1024 or more molecules).
In an experiment at Brookhaven 8 GeV pions are slammed
into gold nuclei. What happens next can be compared to
the evaporation or boiling processes in chemistry. First,
some nucleons are ejected, leaving behind an agitated
nucleus; it now casts off more fragments of various sizes
and can be said to possess a virtual "vapor
pressure."
By looking at collisions of various degrees of
violence, and by counting the number and size of
fragments thrown off, an equivalent nuclear "pressure"
and "temperature" can be calculated for these events (see
sequence
of figures).
Such an experiment has been carried out at Brookhaven
with the Indiana Silicon Sphere (ISiS) detector as the
thermometer and pressure gauge. The ISiS scientists
(Indiana/Laval/Los Alamos/Simon Fraser/Texas
A&M/Maryland; contact Vic Viola, viola@indiana.edu,
812-855-6537) have collaborated with two different teams
of scientists, one at LBNL (contact James Elliott,
jbelliott@lbl.gov, 510-486-7962,) and one at Michigan
State University (Wolfgang Bauer, bauer@pa.msu.edu,
517-353-8662) to survey, for the first time, an
experimentally based Mason-Dixon line between nuclear
liquid and vapor on a previously uncharted
pressure-vs-temperature plot. Indeed this represents the
first time an experimentally derived phase diagram has
ever been made for a system of particles that wasn't held
together by the electromagnetic force.
It is interesting to note that the vapor from an
excited nucleus, if you take into account the sticky
interactions among nucleons, behaves approximately like
an ideal gas (loosely conforming to Boyle's law: PV=nRT).
While the absolute scales of the nuclear and atomic
forces are quite different, the shape of these two types
of interactions (repulsive at very short range,
attractive at longer range) are qualitatively
similar.
Just to appreciate the difference in scales being
compared here, take the case of a group of krypton atoms
and a krypton nucleus. For the atoms, the critical
temperature (boiling point) is 209 K and the critical
density about 0.01 moles per cubic cm. For the nucleus,
the critical temperature would be about 7 MeV, or
8*1010 K, and the critical density about .05
nucleons per cubic fermi, or 8*1013
moles/cm3. Finally, the experiment is germane
to astrophysics since the opposite of nuclear
boiling-namely nuclear condensation-is what happens
during a supernova when a neutron star forms. (Two papers
in Physical Review Letters for: Elliott et
al. (LBNL) in the next few weeks; and Berkenbusch
et al. (MSU) in the 14 Jan 2002 issue; for the
ISiS experimental results see Lefort
et al., Physical Review C, 1 December
2001.)
