Uranium fission spawns exotic nuclei.

Exploiting a new method for studying the fragments resulting from the fission of uranium, researchers have identified more than 100 types of unstable nuclei rich in neutrons. Though known to be generated routinely in nuclear fission reactors, these short-lived nuclei had never previously been observed.

This experiment represents "the first direct observation of every single type of isotope produced in fission," says Monique Bernas of the Institute of Nuclear Physics in Orsay, France.

A chemical element is characterized by the number of protons in its nucleus.

Isotopes of an element differ in the number of neutrons present in the nucleus. About 270 stable isotopes are known to exist on Earth. During the 5 decades since the discovery of nuclear fission, physicists have identified more than 400 unstable isotopes among fragments produced by the splitting of uranium nuclei.

To complete this list, Bernas and her coworkers used a particle accelerator at the Society for Heavy Ion Research in Darmstadt, Germany, to accelerate uranium ions to 80 percent of the speed of light. In flight, the uranium-238 nuclei break up into two main fragments, which can then be identified before they decay into other isotopes on their way to reaching a stable combination of neutrons and protons.

Because the fragments move at nearly the uranium beam's velocity and because they are totally stripped of electrons, "the fission products are much easier to detect than in previous experiments," Bernas explains.

The new, neutron-rich isotopes, all for the elements between vanadium and ruthenium in the periodic table, don't survive for long. Their half-lives range from 20 to 700 milliseconds.

"The new method opens up a wide field for nuclear structure investigations," Bernas says. For example, researchers can now study nickel-78, which contains 28 protons and 50 neutrons. These numbers of protons and neutrons correspond to an unusually stable nuclear arrangement.

The production and investigation of exotic nuclei allow theorists to test models of how nuclei are put together. Such information also serves as a check on astrophysical theories concerned with the creation of elements in supernova explosions and the abundance of these elements in the solar system.