Lifetimes of 21 excited states in atomic Yb were measured using time-resolved fluorescence detection following pulsed laser excitation. The lifetime of the 4 f 14 5d6s 3 D 1 state, which is of particular importance for a proposed study of parity nonconservation in atoms, was measured to be 380͑30͒ ns.

We describe an experiment which demonstrates the feasibility of trapping signi6cant quantities of short-lived radioactive atoms with laser light. A thermal beam of 22.5 sec half-life 'Na atoms was produced on-line at the LBL 88" Cyclotron. After decelerating the beam using a Zeeman-tuned slowing technique we stored about 4X103 2'Na atoms in a magneto-optical trap. The number of trapped atoms is large enough to be used in experimental studies of the beta decay of 'Na. The basic method can be adapted for other rare isotopes. PACS numbers: 32.80.Pj, 23.90.+w The recently developed methods [1-3] for manipulating atoms with laser light have promising applications in several areas of science and technology. So far these techniques have only been used for stable isotopes, but adapting them to radioactive species would be extremely useful in several experiments with implications for nuclear and particle physics. For example, measurements in a series of isotopes of the mixing between opposite-parity atomic levels is recognized as a method for reducing the present systematic uncertainty in the Weinberg angle at low momentum transfer [4]. Implementing this program will require precision measurements with limited supplies of rare radioactive atoms. Trapped radioactive atoms are ideal for certain experimental tests of fundamental symmetries and searches for time-reversal-invariance breaking electric dipole moments. In beta-decay studies, systematic errors associated with source scattering could be completely eliminated and the recoiling daughter nucleus would be directly observable.Precisely characterized nuclear orientations should be achievable with traps. Trapped radioactive sources would enable beta-decay correlation measurements with unprecedented precision, leading to new tests of the fundamental weak interaction and searches for physics beyond the standard model [5].Many of the methods already developed with stable atoms can obviously be applied to radioactive isotopes but there are significant new difficulties. The total number of atoms available for trapping is many orders of magnitude smaller than so far considered.For very short-lived atoms the experiment must be done on-line at the accelerator. Removing an adequate fraction of the limited number of reaction products from the target and then quickly transporting them to a high vacuum region where they can be captured and loaded into a trap is a significant challenge. To get adequate numbers of trapped atoms, losses must be minimized during each step of the process. Experiments to study the decay process itself are especially sensitive since atoms which are lost are potential sources of background. Despite the diSculties developing the appropriate tools for loading rare and exotic atoms into traps would provide new opportunities with great scientific potential. This Letter reports the successful loading of a magneto-optical trap (MOT) [1] with 'Na atoms produced on-line at the Lawrence Berkeley Laboratory 88" Cyclotron. Sodium-21 decays by positron emission to it...

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