Promising path to heavy nuclei.

Making heavy, artificial atomic nuclei has long been part of the program for answering fundamental questions about matter and the universe, but it hasn't been easy. Physicists are looking toward techniques that forge hefty nuclei by fusing lighter, radioactive ones. Now, results of an accelerator experiment at Oak Ridge (Tenn.) National Laboratory suggest that the approach may prove even more fruitful than many nuclear scientists had anticipated.

Since the 1940s, researchers have created various types of heavy nuclei that don't exist naturally on Earth. They've produced those particles mainly in accelerators by slamming stable, nonradioactive nuclei together. Yet thousands of potential heavy isotopes remain beyond the capacity of that process.

So, nuclear physicists are turning to unstable, radioactive nuclei. Besides providing information about nuclear structure, that strategy might lead to novel radioisotopes for medicine, improved understanding of nuclear-weapons blasts, and insights into astrophysical phenomena such as supernovas. Last year, nuclear physicists in the United States named as their top construction priority a proposed $900-million device devoted to creating and studying unstable nuclei. Researchers in Europe and Japan are developing similar facilities.

Less electrostatic repulsion occurs during collisions including nuclei that are unstable because they have excess neutrons than during collisions of stable nuclei. So, scientists expect collisions including unstable nuclei to produce fusions more readily than collisions of stable isotopes do.

Light nuclei such as tin and nickel can also fuse at collision energies even smaller than those needed to overcome the particles' mutual electrostatic repulsion. Scientists attribute those fusions to a quantum mechanical phenomenon called tunneling, which permits neutrons or protons to pass between the colliding nuclei.

In the new experiment, J. Felix Liang of Oak Ridge and his colleagues smashed unstable nuclei of tin-132 into stable nickel-64 to make platinum-196 nuclei. The team found that low-energy reactions took place at a rate roughly 10 times as high as that when the researchers use stable tin-124 projectiles. The enhancement, to be reported in an upcoming Physical Review Letters, is "very exciting, very encouraging," Liang says.

"I was hoping for something like this," comments Robert V.F. Janssens, who heads a research team using an accelerator at Argonne (Ill.) National Laboratory. "Every time we've looked at neutron-rich [unstable] nuclei, we have found surprises," he says.

The details of the low-energy fusion enhancement in the Oak Ridge experiment remain unclear. Moreover, the researchers don't yet know whether the fusion enhancement will persist when they use other projectiles and targets, Liang adds.

The new results raise the prospect that future accelerators will create exotic nuclei more readily than have those facilities handling only stable nuclei. Team member Walter Loveland of Oregon State University in Corvallis says that if some of those exotic nuclei prove to be highly stable, as many theorists have predicted (SN: 6/12/99, p. 372), "the whole of chemistry and physics of heavy atomic nuclei would open up."