In a thermal atomic beam a transition of the atoms into an excited state is induced by micro wave excitation.
These caesium frequency standards measure the frequency of the quantum mechanical transition, which is fixed by the definition of the unit second. The second is realized by a time and frequency standard, which is an atomic clock.īesides different types of caesium frequency standards as high precision standards also rubidium frequency standards and hydrogen masers have been developed. It is defined by taking the fixed numerical value of the caesium frequency Δ ν Cs, the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom, to be 9 192 631 770 when expressed in the unit Hz, which is equal to s -1. The second, symbol s, is the SI unit of time. Since by the redefinition of the base units at 26 th meeting of the General Conference on Weights and Measures the second has been defined as:
The hertz (Hz) is the unit of frequency (number of periodic events per second), where 1 Hz = 1 s -1. It is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom. The SI unit of time, the second (s), was defined 1967 by the 13 th General Conference on Weights and Measures as follows: Mass, Determination of concentration, Densityįrequency Standards Realization of the units of time and frequency
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