Creating, cooling, trapping francium atoms.

At number 87 in the periodic table, francium is the least stable of the first 103 elements. Typically found in trace amounts in uranium deposits, francium atoms decay rapidly into lighter elements.

Now, researchers have not only created but also, for the first time, slowed down and captured francium atoms in a glass bulb, holding them in place with a web of magnetic fields and beams of precisely tuned laser light. Such accumulations of francium set the stage for detailed studies of the atomic characteristics of this rare radioactive element.

Luis A. Orozco, Gene D. Sprouse, and their coworkers at the State University of New York at Stony Brook reported the team,s findings at an American Physical Society meeting held last week in Indianapolis.

"We're very excited about what we have achieved," Orozco says.

Francium atoms can serve as miniature laboratories for probing interactions between electrons and quarks, which make up the protons and neutrons of the nucleus. These subtle effects are enhanced in the heavy nuclei of atoms such as francium.

Applying techniques similar to those used earlier to capture radioactive rubidium atoms (SN: 5/7/94, p. 303), the researchers produced francium by hurling oxygen-18 atoms at a gold target heated almost to its melting point. Collisions between gold and oxygen nuclei created atoms of francium-210, which has 87 protons and 123 neutrons. Diffusing to the surface, these newly generated nuclei escaped from the gold target as ions and were then focused electrically into a beam (see diagram). Later neutralized, the atoms were sent into a glass bulb, where they bounced back and forth between the container's specially coated walls, losing energy with each bounce. An array of six laser beams at a wavelength of 718 nanometers, together with a magnetic field, captured slowly moving atoms to form a cluster at the trap's center.

Orozco and his team generated about 1 million francium-210 ions per second and held 1,000 or more atoms at a time in their trap. Although the atoms remained in the trap for only about 20 seconds before decaying or escaping, a steady stream of fresh atoms replaced those lost, keeping the number of trapped atoms roughly constant for minutes or longer.

Enough francium was trapped that a video camera could capture the light given off by the atoms as they fluoresced (see image). The atoms appeared as a glowing sphere about 1 millimeter in diameter. "This was the very first time that anyone had ever seen the fluorescence from francium" Orozco says.

The researchers can now make extremely sensitive measurements of the light emitted and absorbed by the trapped atoms, providing the first experimental results on various transitions between atomic energy levels in francium. Initial measurements show very good agreement between experimental values and calculations based on quantum theory, Sprouse says.

Such high-precision atomic data are necessary for later detecting the tiny influence of the weak nuclear force on the behavior of electrons bound to an atom.