Fig 1. Cesium.
Fig 2. Cesium fountain clock.
Cesium (Cs) is a soft, silvery, ductile, metallic element produced as a by-product of zinc refining and by reduction of cesium chloride (Fig 1). It is found mainly as the mineral pollucite. Cesium is one of the alkali metals, found in Group I of the periodic table. It is the most electropositive and alkaline of elements, turns to a liquid at only 28.5°C, and burns spontaneously in moist air. It is used in photoelectric cells, as a catalyst promoter, and to make special glass. The radioisotope cesium-137 can be employed in radiotherapy, but is now rarely used. The cesium (atomic) clock provides the standard measure of time: the electron resonance frequency of the cesium atom is exactly 9,192,631,770 cycles per second. Cesium has also been used in ion propulsion.
|relative atomic mass||132.905|
|first ionization energy||376 kJ/mol|
|atomic radius||265 pm|
|ionic radius||167 pm|
|melting point||28.5°C (83.3°F)|
|boiling point||690°C (1,274°F)|
Cesium fountain (atomic) clock
Cesium fountain clocks (Fig 2) derive their timing reference from the frequency of an electron spin-flip transition that occurs in a cesium 133 atom when probed by tuned microwaves.
In a vacuum chamber, six lasers slow the movements of gaseous cesium atoms, forming a small cloud (1). A change in the operating frequency of the upper and lower lasers launches the atomic clock, fountainlike, up through a magnetically shielded cavity (2). As gravity pulls the cloud back down through the cavity, the electrons in the atoms are bombarded by a microwave generator (3) whose emissions are set to the predetermined frequency of a piezoelectric-crystal oscillator. The microwaves flip the spins of the electrons, changing their quantum-mechanical energy states. After the cloud falls farther, a laser probe causes the cesium to fluoresce, revealing whether its electrons have flipped their spins, a reaction that is monitored by a detector (4). The detector's output signal is then used to make the slight correction needed to tune the microwave emitter to a precise resonant frequency that can serve as the time beat for a clock.