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KF5JRV > TECH 29.04.16 12:33l 49 Lines 2684 Bytes #999 (0) @ WW
BID : 2206_KF5JRV
Read: GUEST
Subj: NIST Atomic Clock
Path: IZ3LSV<IW0QNL<JH4XSY<JE7YGF<XE1FH<HG8LXL<N0KFQ<KF5JRV
Sent: 160429/1124Z 2206@KF5JRV.#NWAR.AR.USA.NA BPQK1.4.65
The Time and Frequency Division, part of the NIST Physical Measurement
Laboratory, maintains the standard for frequency and time interval for the
United States, provides official time to the United States, and carries out
a broad program of research and service activities in time and frequency
metrology.
NIST-F1 and NIST-F2 are called fountain clocks because the cesium atoms are
tossed in the air and fall back down inside a vertical tube during a key
routine repeated thousands of times an hour.
A gas of cesium atoms is introduced into the clock's vacuum chamber and six
infrared laser beams gently push about 10 million atoms into a ball. In this
process, the lasers cool the atoms to temperatures near absolute zero and
slow them down significantly, to enable precise measurements of their natural
vibrations.
Two vertical laser beams produced by the six lasers are used to gently toss
the atom balls upward through the flight chamber (the "fountain" action), and
then all of the lasers are turned off. This little push is just enough to
loft the ball about 1.3 meters high through a microwave-filled cavity. Gravity
brings the ball back down through the microwave cavity.
During the trip, some atomic states of the atoms are altered, while others
remain the same, as they interact with a microwave signal from a maser. When
the trip is finished, another laser is pointed at the atoms. Some atoms—those
whose energy states were altered by the microwave signal—emit light, or
fluorescence. The resulting photons, the tiny packets of light emitted, are
measured by a detector.
This process is repeated while the microwave signal in the cavity is tuned to
different frequencies. Eventually, a microwave frequency is found that alters
the states of most of the cesium atoms. This frequency is the natural
resonance frequency of the cesium atom (9,192,631,770 Hz), or the frequency
used to define the second.
NIST-F2 was designed using lessons learned from NIST-F1. The key advance is
that the vertical flight tube is now chilled inside a container of liquid
nitrogen, at minus 193 ºC, or minus 316 ºF. This cycled cooling dramatically
lowers the background radiation and thus reduces some of the very small
measurement errors that must be corrected in NIST-F1.
NIST-F2 also has other novel features. Some critical components shrink by 1
centimeter in length each time the clock is cycled to very cold temperatures,
so the clock design allows for shrinking and expanding. In addition, the
system of six laser beams used to cool the cesium atoms is arranged in
a new way. Additional design enhancements to NIST-F2 are planned for the
future.
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