The Performance and Capability of Cesium Beam Frequency Standards at the National Bureau of Standards

NBS II, the older of the two cesium atomic beam frequency standards which are used alternatively as the United States Frequency Standard, has been operating for more than five years. The contribution to inaccuracy produced by uncertainties in the C field has been reduced by a factor of 30 to ±2 × 10-13. The average precision of measurement (standard deviation of the mean) has been demonstrated to be 1 × 10-12 for averaging times of 1 hour and 2 × 10-13 for 12 hours. The overall accuracy is considered to be ±8 × 10-123¿. A new cesium standard, NBS III with an interaction length of 3.66 meters is in operation and has demonstrated an improved precision of 1 × 10-13 over 2 hours and an accuracy of ±5 × 10-123¿. The C field contributions to inaccuracy in this machine have been reduced to ±1 × 10-13. Considerable effort has been devoted to the detection and elimination of small frequency shifts produced by various electronic components of the excitation systems. In spite of the various improvements effected, a small unexplained difference in frequency of about 1 × 10-12 continues to exist between the standards. The extremely high stability of the difference frequency, however, suggests that resolution of the difficulties should result in an accuracy capability of perhaps ±1 × 10-123¿.