Evaluation of a Thallium Atomic Beam Frequency Standard at the National Bureau of Standards

The original NBS cesium standard (NBS I) has been converted to a thallium standard and was operated for one and one-half years with a typical precision of 2 × 10-12 and an accuracy of 1 × 10-11. Experiments are described which were performed to establish these precision and accuracy estimates. These results, which are comparable to those obtained with longer cesium standards, are considered sufficiently encouraging to justify the conversion of a longer cesium standard to thallium for a more thorough evaluation.