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A Cs vapor is generated in the oven, the
atoms are collimated, and the beam of Cs atoms are directed to pass through a
strongly diverging field of the “A” magnet, the “state-selector” magnet. The force on an atom of magnetic moment i in a
magnetic field B is
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Fi
= -i (B)
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Therefore, the atoms
are deflected by amounts that depend on their magnetic moments, i.e., their
energy states. The atoms in the (3,0)
state are deflected in a different direction than those in the (4,0) state.
In this manner, the two types of atoms can be physically separated. The (3,0)
and (4,0) levels are the A and B levels, referring back to “Generalized
Atomic Resonator” earlier in this chapter,
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The state selecting magnet
"selects" one of the two atomic levels. The applied microwave at the atomic
resonance frequency causes a state change (a spin-flip; see “Hydrogen-Like
Atoms,” earlier in this chapter); the second magnet deflects those atoms to
the detector which have undergone the state change. The magnets' peak field is ~10 kgauss.
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The atom detector is a ribbon or wire
(e.g., W or Pt) at ~ 900°C. The Cs atoms are ionized, the ions are collected,
the current is amplified and fed back into feedback network. In this way, the
microwave frequency is locked to the frequency of maximum ion current, thus
the atomic transition frequency controls the microwave frequency, i.e., the
frequency of the crystal oscillator.
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Much less than 1% of the Cs atoms reach the
detector in conventional Cs standards (hence optical pumping's advantage -
see “Optically Pumped Cs Standard” later in this chapter.)
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F. G. Major, The
Quantum Beat - The Physical Principles of Atomic Clocks, Springer-Verlag,
1998.
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J. Vanier and C.
Audoin, The Quantum Physics of Atomic Frequency Standards, ISBN
0-85274-434-X, Adam Hilger, 1978.
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