Thallium Beam Frequency Standards

Abstract

The principle, design, and performance of thallium beam frequency standards are described. Recent measurements of the Tl²⁰⁵ 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 10¹¹. 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 10¹², or long term stability of better than the order of a part in 10¹³, is wanted without need for measuring, or elaborately controlling, the C-field in the atomic beam tube.