A close, cheap shave for heavy atoms.

By stripping nearly all of the electrons from a heavy atom such as uranium, physicists can create a unique environment for studying interactions between atomic nuclei and electrons. The highly charged, heavy nuclei of the resulting ions exert such a strong electrical force on the few remaining electrons that subtle quantum and relativistic effects -- normally barely detectable--are greatly amplified.

Researchers have usually generated such ions in particle accelerators or storage rings, in which beams of these highly charged ions circulate a high speeds (SN: 5/1/93,p.287). Now physicists have an alternative source. Recently completed, the high-energy electron-beam ion trap, dubbed SuperEBIT. at the Lawrence Livermore (Calif.) National Laboratory source of such ions.

In contrast heavy-ion accelerators and storage rings, which cost hundreds of millions of dollars to build, the construction of the SuperEBIT apparatus required a "mere" $1.5 million, says Livermore's Peter Beiersdorfer. More-over, because ions stored in SuperEBIT are practically stationary, researchers also avoid having to correct for relativistic effects caused by the motion of ions at close to the speed of light.

The apparatus has a chamber into which neutral atoms or ions having a small charge can be injected. When an intense electron beam, highly compressed by a magnetic field, passes through the chamber, it traps these atoms or ions, keeping them inside the beam. Beam electrons then strip electrons, one by one, from the ions or atoms present, leaving behind bare or nearly bare atomic nuclei.

In addition to plucking electrons from the ions in the chamber, the electron beam can also excite one or more of an ion's remaining electrons to higher energies. These excited electrons then emit X-rays to get rid of this excess energy. Ions can even capture beam electrons, a process that also generates X-rays of characteristic wavelengths.

"The beam does virtually all of the work," Beiersdorfer says. "It ionizes all the things that we put in, it electro-statistically confines them, and it excites them."

In first experiment involving SuperEBIT, Beiersdorfer and his co-workers monitored X-ray emissions from highly charged uranium ions having just three electrons (compared with 92 electrons in the neutral atom). By precisely measuring the wavelengths of these X-rays, the researchers could calculate a quantity known as the Lamb shift, a small change in atomic energy levels caused by interactions between electrons bound to a nucleus but still moving around.

The researchers found execellent agreement between their calculated value and theoretical predictions of the Lamb shift. They will publish their results in an upcoming PHYSICAL REVIEW LETTERS.

SuperEBIT may prove useful for a variety of atomic and plasma-physics