NIST-7, The U.S. Primary Frequency Standard: New Evaluation Techniques

Abstract : Primary frequency standards achieve their accuracy by direct reference to the definition of the second and evaluation of all known sources of systematic terror that may perturb the measured resonance in the atom. NIST-7, the U.S. primary frequency standard, is a thermal, atomic-beam machine that uses optical pumping for atomic state preparation and detection, and digital frequency control. This technology enables the new evaluation techniques described here. All known systematic effects are determined by means of experiments not involving, or limited by, precision frequency measurements. This both speeds the evaluation and reduces the combined standard uncertainty. Its present value is 5-10(-13) for NIST-7.