English

DTIC ADA239372: Proceedings of the Annual Precise Time and Time Interval (PTTI) Applications and Planning Meeting (22nd) Held in Vienna, Virginia on 4-6 Dec 1990

The first successful magnetic resonance experiments were those of Rabi and his associates with molecular beams in 1938. The initial experiments measured the interactions of nuclear magnetic moments with external magnetic fields, but radiofrequency spectra were soon observed that depended on internal interactions within molecules atoms. Some of the radiofrequency spectra lines were almost independent of external conditions and could therefore be used as the highly stable periodic component of an atomic clock. At present different atomic clocks can be chosen to fit the need. When high stability is not required, optically pumped rubidium cells can be chosen for their low cost, small size and light weight; such cells are stable to 10-11 over a one month period and more stable for shorter periods. When greater accuracy and stability is required, cesium beam tubes are usually used; they are accurate to better than 10-13 over extended time periods. For the highest stability requirements, as in long baseline radio astronomy and navigation in outer space, hydrogen masers are often required; they are stable to better than 10-15 over several hours. When optical frequencies need to be measured

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