Radium Atoms Trapped

Physicists at Argonne National Laboratory, near Chicago, have laser-cooled and trapped radium atoms for the first time.

Surprisingly, room temperature blackbody photons -- thermal radiation over a wide spectrum emitted by the apparatus itself -- were found to play a critical role in the laser-trapping of this rare and unstable element. This represents the heaviest atom ever trapped by laser light.

Using only 20 nanograms of radium-225 (halflife of 15 days) and one microgram of radium-226 (halflife of 1,600 years), the Argonne scientists held tens of radium-225 and hundreds of radium-226 atoms in the laser trap. It was particularly challenging to trap radium because quantities are scarce and the atomic structure is not well studied and understood.

Why go through the trouble of trapping radium atoms? Because it might provide a chance to detect a violation of time-reversal symmetry (abbreviated with the letter T), which would manifest itself as an electric dipole moment; that is, even though the atom as a whole is charge neutral, there might exist a slight offset between the negative and positive charge within the atom along its spin axis.

Electric dipole moment searches have been ongoing for over 50 years and continue to yield smaller and smaller limits on the size of these T-violating interactions. These limits place constraints on theories beyond the Standard Model of particle physics and explanations for the matter-antimatter asymmetry in the universe.

Next-generation electric dipole moment searches may take advantage of rare isotopes such as radium-225, which are expected to be extremely sensitive to T-violation owing to their non-spherical 'egg'-shaped nucleus. For the rare and unstable radium atoms, a laser trap offers a promising path to such a measurement.

Guest et al., Physical Review Letters, upcoming article
See the lab Web site