Number 269 (Story #2), May 6, 1996 by Phillip F. Schewe and Ben Stein THE WORLD'S RAREST ELEMENT has successfully been trapped, setting the stage for high-precision tabletop measurements on how the weak nuclear force manifests itself at the atomic level. Francium is the least stable of the first 103 elements on the periodic table; less than an ounce of it exists on the Earth at any one time. Creating francium artificially has not been a problem; however, it has been a major challenge to trap francium atoms and study them. In work described at last week's APS Meeting, researchers at SUNY-Stony Brook (Luis Orozco, lorozco@ccmail.sunysb.edu) produce a million ions per second of francium-210 (half-life=3 minutes) at their accelerator. 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 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. With their setup, the researchers can confine approximately 10,000 francium atoms at a time. 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. Future studies of the 7S-8S francium energy transition, forbidden by the electromagnetic interaction because it violates parity but permitted by the parity-violating weak interaction, could then be used to gain precise information on the weak force. 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. (See also J.E. Simsarian et al, Physical Review Letters, 6 May 1996.)