Radioactivity on the spot.

As a by-product of a painstaking search for an extremely rare type of radioactivity, researchers have detected the step-by-step tranformation of a single atomic nucleus through four stages. Starting as a radioactive isotape of radon, this particular nucleus eventually ended up as isotape of polonium.

"This may be the first time anyone has seen four successive decays of the same nucleus," says Michael K. Moe of the University of California, Irvine.

Moe and his colleagues have spent the last few years looking for a form of radioactivity known as neutrinoless double-beta decay, characterized by the simultaneous emission of two beta particles, or electrons. Using an apparatus called a "projection time chamber," the researchers trace the paths of the two electrons released during a decay event and compute their energies.

However, several other processes produce remarkably similar tracks. For example, the trace contaminant bismuth-214 decays into polonium-214 by emitting a single beta particle, but the process sometimes dumps enough energy into the bismuth atom to force the ejection of one of the atom's orbital electrons. Such tracks are difficult to distinguish from those left by legitimate double-beta decays. Luckily, the resulting polonium-214 nucleus decays almost immediately by emitting a telltale alpha particle (helium nucleus). But sometimes the emitted alpha particle gets trapped and doesn't show up.

About eight years ago, Luis W. Alvarez of the University of California, Berkeley, suggested the possibility of eliminating bismuth-214 events by looking back in time for the recorded trails left by alpha particles or electrons created during the decay of radioactive isotopes in the chain leading to bismuth-214. Citing the large number of extraneous events normally detected in his apparatus, Moe dismissed the idea as impractical. He would have to sort through too many recorded events to be sure he had found the right tracks.

Since then, however, Moe has moved his apparatus into one of the deep tunnels serving Hoover Dam near Boulder City, Nev., where the surrounding rock helps shield the equipment from the effects of cosmic rays (SN: 9/17/88, p.188). "As the chamber got quieter, we were able to do what Alvarez proposed," Moe says.

The researchers started with a suspect two-electron event. Going back 57 minutes in their records, they noticed the twin trails of a beta particle and an emitted orbital electron coming from the same spot. The track of an alpha particle appeared in the same location 70 minutes earlier, and still another showed up 72 minutes before that. These observations represented a sequence of decays in a single nucleus, going from radon-222 to polonium-218 to lead-214 to bismuth-214 to polonium-214.

Moe and his colleagues may not be the only ones who have actually traced such a chain of events. Researchers now attempting to detect solar neutrinos at the Gran Sasso underground laboratory in Italy have had to scrutinize and characterize similar sequences of decays to eliminate false signals.