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NIST-F1 Cesium Fountain Atomic Clock The Primary Time and Frequency Standard for the United States. f = 9,192,631,770 Hz. T ~ 10 -6 K .
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NIST-F1 Cesium Fountain Atomic ClockThe Primary Time and Frequency Standard for the United States f = 9,192,631,770 Hz T ~ 10-6 K The base unit of time is officially defined as "the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom". http://tf.nist.gov/cesium/fountain.htm
Cesium 133 energy diagram http://inms-ienm.nrc-cnrc.gc.ca/research/cesium_clock_e.html