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THE atomic nucleus behaves so much like a drop of liquid, it can actually
boil, say physicists who have measured the properties of nuclear 鈥渧apour鈥� for
the first time. Their discovery is helping to explain how heavy elements such as
gold, lead and uranium are made inside supernovae.

Strong nuclear forces hold protons and neutrons together in nuclei in much
the same way that electromagnetic forces bind the molecules in a droplet of
water. In nuclear reactions the minuscule 鈥渄roplet鈥� can spin, bulge or split,
but until now no one had found a way to discover whether it can boil. 鈥淵ou can鈥檛
stick a thermometer in the nucleus,鈥� James Elliott at Lawrence Berkeley National
Laboratory in California points out.

Now Vic Viola of Indiana University in Bloomington and his colleagues have
cracked the problem. At Brookhaven National Laboratory in New York, they
accelerated particles called pions to 99.9 per cent of the speed of light and
smashed them into gold nuclei. By looking at the size of the nuclear blobs that
were thrown out, they were able to measure the nuclei鈥檚 transition from liquid
to vapour.

As the energy of the pions increased, so did the size of the blobs. But
eventually they stopped getting bigger鈥攕howing that the additional energy
was being used to change the state of the nuclei from liquid to gas. And when
the researchers cranked up the energy even further, the chunks suddenly got
smaller. This indicates that all the nuclei have been vaporised, says Viola. 鈥淚f
you vaporise a drop of water and look at the gas coming off, you see small
clusters of just two or three molecules,鈥� he says.

When researchers at Lawrence Berkeley and Michigan State University used
Viola鈥檚 data to calculate the boiling points of different nuclei, they found
they are typically billions of times greater than those of atoms, around 100
billion degrees kelvin. And when they measured the density of the nuclear
vapour, its pressure was proportional to its temperature, just as in an ordinary
gas.

This new understanding of nuclear matter is already helping other researchers
to model supernovae鈥攅xploding stars that generate heavy elements such as
gold from nuclei no heavier than iron. No one fully understands how the nuclei
capture the extra neutrons to form heavy elements, but Chikako Ishizuka and
colleagues at Hokkaido University in Sapporo, Japan, say that when nuclei boil,
it鈥檚 easier for them to incorporate extra neutrons. Then they can condense into
heavy elements as they cool.

Viola says knowing how nuclei change from one state to another is crucial to
understanding a range of processes. 鈥淣ow we can describe nuclear physics under
any conditions,鈥� he says.