NIST F 1 AND F 2 T . P . Heavner , T . E . Parker , J . H . Shirley , P . Kunz

The National Institute of Standards and Technology operates a cesium fountain primary frequency standard, NIST-F1, which has been contributing to International Atomic Time (TAI) since 1999. During the intervening 11 years, we have improved NIST-F1 so that the uncertainty is currently 16 0 3 10 , f f     dominated by uncertainty in the blackbody-radiationinduced frequency shift. In order to circumvent the uncertainty associated with the blackbody shift, we have built a new fountain, NIST-F2, in which the microwave interrogation region is cryogenic (80 K), reducing the blackbody shift to negligible levels. We briefly describe here the series of improvements to NIST-F1 that have allowed its uncertainty to reach the low 10 -16 level and present early results from NIST-F2.