Physics in storage rings ... with stripped atoms ... with negative ions.

Strip all but one of the 92 electrons from a uranium atom and the result is a highly charged positive ion. Add an electron to a calcium atom and the result is a negative ion that barely holds on to its extra electron. Neither type of charged particle lasts long enough to be Studied in typical laboratory settings.

Researchers can now investigate the characteristics of these particles by injecting streams of them into new, specially designed storage rings. Confined and focused by magnetic fields, such beams circulate through a sequence of vacuum chambers. Continually speeding around this atomic racetrack, individual ions remain far enough from their neighbors and sufficiently isolated to survive for long periods.

... with stripped atoms

At the Institute for Heavy Ion Research (GSI) in Darmstadt, Germany, researchers are starting to use a new storage ring to look at the behavior of highly charged ions. With only one or two electrons, the positively charged nuclei of these ions exert such a strong force on the few electrons present that subtle quantum and relativistic effects - barely detectable in a hydrogen or helium atom - become greatly amplified. By detecting X-rays emitted by these tightly bound electrons as they jump from one orbit to another, researchers can generate data to help test theories of how electrons interact.

"One-electron atoms are the simplest systems we can calculate, and from the experiments, we get a very stringent test of the theory," says GSI's Paul H. Mokler.

Researchers at GSI have also observed for the first time an extremely unusual type of radioactive decay in which the electron (beta particle) produced by the decay of a "parent" nucleus stays bound to the newly created "daughter" nucleus instead of speeding off. The physicists started with highly charged dysprosium ions. Although neutral dysprosium-163 is stable, the naked nucleus -- the atom stripped of all 66 of its electrons --is unstable and decays by emitting a beta particle to create a holmium-163 nucleus, which captures the beta particle and hangs on to it as an orbiting electron. Extremely rare if not impossible in neutral atoms, "bound-state beta decay" may play an important role inside stellar plasmas during the synthesis of elements via nuclear fusion reactions.

... with negative ions

Torkild Andersen and his collaborators are using the new storage ring ASTRID at Aarhus University in Denmark to investigate the weak interactions of electrons loosely bound to atoms and simple molecules. These fragile negative ions hold together long enough in the storage ring to allow researchers, for the first time, to measure accurately how long the particles retain their charges. The lifetimes range from 10 microseconds to 100 milliseconds. The examples studied so far include singly charged, negative ions of helium, beryllium, and calcium and a molecular ion consisting of two helium atoms bound together with an extra electron ([He.sub.2.-]).

"We have been able to show that the lifetimes are considerably shorter than expected from theory, and the theory is now going to be revised," Andersen says.

Some negative ions are so delicate that the heat (blackbody radiation) of the apparatus itself at room temperature is sufficient to knock out the extra electron. "This was a surprise because... you don't expect this energetically weak blackbody radiation to remove electrons," Andersen says. But "if you go to a very weakly bound system, it will be the controlling factor." For example, the binding energy of the extra electron in a negatively charged calcium ion is sufficiently low that the ions measured lifetime of 490 microseconds is determined almost entirely by environmental blackbody radiation.