Cesium is an extremely reactive alkali metal with an exceptionally low melting point (about 28 °C) that can spontaneously ignite in air. It is not abundant in the crust and is extracted mainly from pollucite. Its proudest identity is the definition of the second itself: since 1967, one second in the International System of Units has been defined as 9,192,631,770 times the period of the radiation corresponding to the transition between the two hyperfine levels of the cesium-133 atom. The world's atomic clocks, GPS satellite timekeeping, grid synchronization, and financial transaction timestamps all rest on cesium atomic clocks, making it the time reference of modern navigation, communications, and financial infrastructure. Its second-largest industrial use is in oil-and-gas drilling fluids: cesium formate solutions are dense, low in viscosity, and good in lubricity, serving as completion and drilling fluids for high-temperature, high-pressure deep wells and offshore oilfields, where they maintain wellbore stability and protect reservoirs several kilometers underground. In photoelectric and vacuum electronics, cesium is used as a sensitizer for photocathodes and photomultiplier tubes; because it readily releases electrons under light, it is a key coating for night-vision devices, scintillation counters, spectrophotometers, and astronomical photodetectors. It is also used in atomic fountain clocks and as a fining agent in specialty glasses.
Cesium chloride (CsCl) is the most common cesium salt, a white crystal. Because its density-gradient solutions can separate DNA, viruses, and proteins of different densities, it has become a standard reagent in molecular biology for ultracentrifugal separation of nucleic acids and plasmids; it is also used in infrared optical windows and specialty crystal growth. Cesium hydroxide (CsOH) is one of the strongest known soluble alkalis and is highly corrosive; it is used mainly as an electrolyte in specialty alkaline batteries and as a strong base in organic synthesis, and is a feedstock for preparing other cesium compounds. Cesium carbonate (Cs2CO3) is a widely used weak base in organic synthesis, used in pharmaceutical synthesis and the production of liquid-crystal monomers, and also as a fining agent in specialty optical glasses and cathode-ray tubes. Cesium iodide (CsI) is an important scintillation crystal: thallium-doped CsI(Tl) has high light yield and can be machined into large sizes, and is widely used in X-ray and gamma-ray detectors, including security inspection machines, CT detectors, nuclear-medicine gamma cameras, and high-energy-physics calorimeters.
