Path to new elements now looks steeper.

Fusing existing elements to form superheavy ones that don't exist naturally has never been easy. What's more, the signs of possible success are so fleeting and ambiguous that researchers sometimes fool themselves, as happened recently in efforts to forge element 118 (SN: 8/4/01, p. 68).

Now, Annette C. Berriman of the Australian National University in Canberra and her colleagues have shown that superheavy atoms are probably even harder to make than scientists thought.

When an ion--a charged atom--plows into a metal foil in a particle accelerator, the projectile's nucleus may strike a target nucleus and stick to it. Two basic outcomes are possible: Either the two nuclei fuse into a lasting larger one, or the emerging heavy nucleus quickly splits, or fissions, into two pieces.

In the Sept. 13 NATURE, the Canberra team reports unexpected results from collisions of relatively light nuclei slamming into heavy targets. The researchers found signs of so-called quasi-fission, a process in which the projectile nucleus steals protons and neutrons from the target nucleus. The two then fly apart without ever having actually combined.

Surprisingly, these encounters took place even when projectiles as light as fluorine-19 careened into gold nuclei. Prevailing theory holds that the projectile would have to be at least four times heavier than fluorine for quasi-fission to occur after impact with a gold target. The relative ease of quasi-fission may mean that would-be fusions of two nuclei into superheavy elements don't occur.

Berriman and her colleagues say their data harbor pointers for making superheavy elements. They suggest that the best way to encourage fusion is to combine the lightest projectiles possible witht heaviest possible targets. That's the recipe they've found for minimizing quasi-fission.