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CESIUM ATOMIC CLOCKS  Cesium Atoms at Work "...till like a clock worn out with eating time."John Dryden (1631-1701) The 1955 Cesium Atomic Clock at the National Physical Laboratory, UK. It kept time to asecond in 300 years. A "cesium(-beam) atomic clock" (or "cesium-beam frequencystandard") is a device that uses as a reference the exact frequency ofthe microwave spectral line emitted by atoms of the metallic elementcesium, in particular its isotope of atomic weight 133 ("Cs-133"). Theintegral of frequency is time, so this frequency, 9,192,631,770 hertz(Hz = cycles/second), provides the fundamental unit of time, which maythus be measured by cesium clocks. Today, cesium clocks measure frequency with an accuracy of from 2 to 3parts in 10 to the 14th, i.e. 0.00000000000002 Hz; this corresponds to a timemeasurement accuracy of 2 nanoseconds per day or one second in 1,400,000years. It is the most accurate realization of a unit that mankind hasyet achieved. A cesium clock operates by exposing cesium atoms tomicrowaves until they vibrate at one of their resonant frequencies andthen counting the corresponding cycles as a measure of time. Thefrequency involved is that of the energy absorbed from the incidentphotons when they excite the outermost electron in a cesium atom to jump("transition") from a lower to a higher orbit. According to quantumtheory, atoms can only exist in certain discrete ("quantized") energystates depending on what orbits about their nuclei are occupied by theirelectrons. Different transitions are possible; those in question referto a change in the electron and nuclear spin ("hyperfine") energy levelof the lowest set of orbits called the "ground state." Cesium is thebest choice of atom for such a measurement because all of its 55electrons but the outermost are confined to orbits in stable shells ofelectromagnetic force. Thus, the outermost electron is not disturbedmuch by the others. The cesium atoms are kept in a very good vacuum ofabout 10 trillionths of an atmosphere so that the cesium atoms arelittle affected by other particles. All this means that they radiate ina narrow spectral line whose wavelength or frequency can be accuratelydetermined. Kinds of Cesium Clocks Cesium clocks are of two general kinds: a "laboratory (or primary)standard" about as large as a railroad flatcar and a "commercial (orsecondary) standard" about as large as a suitcase. Only a fewlaboratory standards exist; they are used at research labs for frequencymeasurements of the highest accuracy. Examples are the NIST-7 standardat the National Institute of Standards and Technology (NIST)in Boulder, CO and the atomic fountains at NIST, Physikalisch-Technische Bundesanstalt (PTB), Germany, the Paris Observatory, France, and USNO.Commercial standards, being industrially produced, are cheaper, butstill provide state-of-the-art measurement of precise time and timeinterval. A timing center maintaining an ensemble of such clocks canaverage their readings to produce a "mean timescale" for scientific andpublic use.  The U.S. Naval Observatory operates about 70 such cesiumclocks, as well as other precision clocks like hydrogen masers, in 18vaults whose temperature and, usually, humidity are closely controlledin order to minimize perturbations by their environment. The timemeasurements are made by devices called time-interval counters thatcompare each clock's time against that of one "Master Clock," whosefrequency is steered to match its time to the average of the otherclocks. This time is the Observatory's measure of the atomic timecalled Coordinated Universal Time (UTC). Some cesium clocks aretransported to remote locations in order to synchronize other clocks. USNO Cesium Clocks  Most of the Observatory's cesium clocks are model HP5071A, made byAgilent Technologies, Inc. of Santa Clara, California. With an improved cesium tube and new microprocessor- controlled servo loops, the 5071A vastly outperforms the earlier 5061 cesium frequency standards. The Naval Observatory 5071A's feature HP's optional high-performance cesiumbeam tube, with accuracy 1 part in 10E12, frequency stability 8 parts in 10 to the 14th,and a time domain stability of < 2 parts in 10 to the 14th with an averaging time of 5 days. Other companies that producecesium clocks include Datum, Inc. of Beverly, MAand Frequency Electronics, Inc. of Uniondale, NY. Principles of the Cesium Clock  In a cesium clock like these, liquid cesium is heated to a gaseousstate in an oven. A hole in the oven allows the atoms to escape at highspeed. These particles pass between two electromagnets whose fieldcauses the atoms to separate into two beams, depending on which spinenergy state they are in. Those in the lower energy state pass throughthe ends of a U-shaped cavity in which they are irradiated by microwavesof 3.26-cm wavelength. The absorption of these microwaves excitetransitions of many of the atoms from the lower to the higher energystate. The beam continues through another pair of electromagnets, whosefield again divides up the beam. Those atoms in the higher energy statestrike a hot wire, which ionizes them. Thereafter, a mass spectrometerselects only the cesium atoms from any impurities and directs them ontoan electron multiplier. The frequency of the microwaves is adjusteduntil the electron multiplier output current is maximized, constitutingthe measurement of the atoms' resonance frequency. This frequency iselectronically divided down and used in a feedback control circuit("servo-loop") to keep a quartz crystal oscillator locked to a frequencyof 5 megahertz (MHz), which is the actual output of the clock, alongwith a one-pulse-per-second signal. The entire apparatus is shieldedfrom external magnetic fields. The first method for accurately measuring hyperfine frequencies bymolecular beam resonance was developed by I.I. Rabi and his associatesin 1937 at Columbia University. The first molecular clock, usingammonia gas, was built by H. Lyons at the National Bureau of Standardsin 1949. The first atomic clock, a cesium-beam frequency standard, wasbuilt starting in 1949 and was first operated in 1951, resulting in thefirst direct measurements of cesium hyperfine frequencies. The clock,called NBS-1, was the first in a series that is now up to 7 (NBS is nowthe National Institute of Standards and Technology, so their lateststandard is called NIST-7). Between 1953 and 1955, L. Essen and J.V.L. Parryof the National Physical Laboratory (NPL) in Teddington, England built a cesium atomicclock. These atomic clocks were later refined by others, notably N.F.Ramsey and J.R. Zacharias, and are the primary standards referred to above.The first trapped-ion standard was developed at the NIST in 1985. The first atomic fountain clock was built in 1995 at theParis Observatory, France.The first atomic clock timescale was established in 1955 at NPL, though itdiffered signicantly from the astronomical ephemeris timescale establishedby Dr. William Markowitz, head of Time Service at the U.S.Naval Observatory. Markowitz and Essen collaborated on the determination ofa best value for the cesium hyperfine frequency, and in 1958 they reported avalue of 9,192,631,770 hertz (cycles/second). The Second In 1967, the 13th General Conference onWeights and Measures first defined the International System (SI) unit oftime, the second, in terms of atomic time rather than the motion of theEarth. Specifically, a second was defined as the duration of 9,192,631,770 cycles of microwave light absorbed or emitted bythe hyperfine transition of cesium-133 atoms in their ground stateundisturbed by external fields.The first commercial atomic frequency standards were built by R. Daly ofNational Company and J. Holloway of Varian Associates. Their cesium tubedesign was incorporated by L. Cutler and his coworkers at Hewlett-Packard (now Agilent Technologies), Inc.in what would become the largest selling series of commercial standards. Recent improvements in cesium clocktechnology include replacement of the state-selection magnets with laserbeams, which can select and detect the required transition with greater efficiency and less motion of, and hence less noise from, the radiating atoms.Back to Time Service Home Page Dr. Lee A. Breakiron (breakiron.lee@.usno.navy.mil) |
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