Researchers at SUNY-Stony Brook have successfully trapped the world's rarest naturally occurring element, francium, setting the stage for high-precision tabletop measurements on how the weak nuclear force manifests itself at the atomic level. The Stony Brook team developed a technique to trap more than 10,000 francium atoms in a volume about the size of a head of a pin, using six laser beams and an inhomogeneous magnetic field.

Francium is the heaviest alkali and the least stable of the first 103 elements on the periodic table. Less than 30 grams of it exists on the Earth at any one time, in uranium deposits. It appears, atom by atom, as heavier atoms decay, and it disappears in less than 20 minutes as francium itself decays. While creating francium artificially has not been a problem, it has been a major challenge to trap francium atoms and study them. Researchers at Stony Brook, Berkeley, and elsewhere have previously used magneto-optic traps to collect radioactive atoms, but a challenge with francium has been to figure out how to tune the trapping lasers, since there are no known stable isotopes of francium to use as a reference. Recent developments were described by Gene Sprouse in a DNP Mini-symposia on Friday at the Joint APS/AAPT Meeting.

The SUNY team, headed by Luis Orozco, can now produce a million ions per second of francium-210, which has a half-life of about three minutes, by bombarding a gold target very close to melting point with beams of oxygen from the superconducting linear accelerator at Stony Brook. "You can't have a bottle of francium or a pellet of francium," said Orozco. "You have to be making it all the time in order to work with it."

Because the atoms were created with too much energy to be immediately trapped with lasers, the Stony Brook team devised methods to remove energy from them quickly and efficiently. After converting the ions into neutral atoms and slowing them down considerably, they send the francium into a magneto-optical trap, a device employing six laser beams - which had to be tuned to the correct frequency to slow and confine the atoms - and a nonuniform magnetic field. Inside the traps, the atoms bounce back and forth between specially coated glass walls, slowing down some atoms enough to be caught at the center of the trap.

Now that this rare element can be concentrated and confined, the research team plans to study the atomic properties of francium atoms, which opens new horizons for the understanding of the atomic structure of a very heavy element. For example, a new energy level has been observed for the first time, and lifetime measurements are in progress.

Studies of trapped francium can also ultimately lead to high-precision measurements of a phenomenon known as parity nonconservation, which would then provide information on the inter-relationship between the electromagnetic and weak force. The francium energy transition is forbidden by the electromagnetic interaction because it violates parity, but is permitted by the parity-violating weak interaction. The effects of parity violation are at least 18 times more pronounced in francium than in cesium, another atom in which parity violation has been studied.