cesium-133

radioisotope

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June 24, 2026

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Cesium-133 is a specific form, or isotope, of the chemical element cesium. While most cesium is stable, some isotopes are radioactive. Cesium-133 is not radioactive and is crucial for defining the standard unit of time, the second.12

The frequency of the radiation associated with the transitions between the energy states of cesium-133 atoms is precisely 9,192,631,770 hertz. This incredibly stable and consistent frequency is the basis for atomic clocks, making them the most accurate timekeeping devices available. These clocks are so precise that they are used to establish the international time standard.34

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atomic clock
…the ground state of the cesium-133 atom.”
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…the ground state of the cesium-133 atom.”
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quantum mechanics
Cesium clock
…the ground state of the cesium-133 atom. Prior to 1967, the second was defined in terms of the motion of Earth. The latter, however, is not nearly as stable as the cesium clock. Specifically, the fractional variation of Earth’s rotation period is a few hundred times larger than that of…
Read More
…the ground state of the cesium-133 atom. Prior to 1967, the second was defined in terms of the motion of Earth. The latter, however, is not nearly as stable as the cesium clock. Specifically, the fractional variation of Earth’s rotation period is a few hundred times larger than that of…
Read More