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NIST F1 and F2



Thomas P. Heavner, Tom Parker, Jon H. Shirley, Steven R. Jefferts


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. At the time of the last Symposium on Frequency Standards and Metrology in 2001, the uncertainty of the standard was df/f=1x10-15 which was at that time the state-of-the art. During the intervening 7 years we have improved NIST-F1 so that the uncertainty is currently df/f=3x10-16 dominated by uncertainty in the Blackbody radiation induced 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 (80K) reducing the blackbody shift to negligible levels. We briefly describe here the series of improvements to NIST-F1 which have allowed its uncertainty to reach the low 10^-16 level and present the first results from NIST-F2.
Proceedings Title
Proc. 2008 Symposium on Frequency Standards and Metrology
Conference Dates
October 5-10, 2008
Conference Location
Pacific Grove, CA, US


atomic clock, primary frequency standards, cesium fountain


Heavner, T. , Parker, T. , Shirley, J. and Jefferts, S. (2008), NIST F1 and F2, Proc. 2008 Symposium on Frequency Standards and Metrology, Pacific Grove, CA, US, [online], (Accessed July 20, 2024)


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Created October 4, 2008, Updated October 12, 2021