Show pageBacklinksExport to PDFBack to top This page is read only. You can view the source, but not change it. Ask your administrator if you think this is wrong. ====== Caesium-133 ====== Caesium-133 is the only stable isotope of [[caesium]]. The SI base unit of time, the second, is defined by a specific caesium-133 transition. Since 1967, the official definition of a second is: The second, symbol s, is defined by taking the fixed numerical value of the caesium frequency ΔνCs, the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom, to be 9192631770 when expressed in the unit Hz, which is equal to s−1. ---- caesium-133.txt Last modified: 2025/05/13 02:15by 127.0.0.1