Thallium Beam Frequency Standards

The principle, design, and performance of thallium beam frequency standards are described. Recent measurements of the Tl 205 hyperfine structure separation are given. The values, obtained with different frequency standards, are near 21, 310, 833, 946 Hz in the A. 1 time scale. The range covered is 7 parts in 10 11 . The marked advantage of thallium frequency standards over cesium standards is that their sensitivity to magnetic effects is about one-fiftieth as great. Thallium standards would be expected to be superior to cesium standards in applications where precision better than the order of a part in 10 12 , or long term stability of better than the order of a part in 10 13 , is wanted without need for measuring, or elaborately controlling, the C-field in the atomic beam tube.