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The meaning of the preceding definition is as follows. The caesium atom has a ground state electron state with configuration [Xe] 6s 1 and, consequently, atomic term symbol 2 S 1/2. This means that there is one unpaired electron and the total electron spin of the atom is 1/2. Moreover, the nucleus of caesium-133 has a nuclear spin equal to 7/2.
The BIPM restated its definition at its 26th conference in 2018: "[The second] is defined by taking the fixed numerical value of the caesium frequency Δν 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." [86]
The second, symbol s, is defined by taking the fixed numerical value of the caesium frequency, Δν Cs, the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom, [9] to be 9 192 631 770 Hz, which is equal to s −1.
One second is now defined to be exactly 9 192 631 770 cycles of the hyperfine structure transition frequency of caesium-133 atoms. On October 21, 1983, the 17th CGPM defined the meter as the length of the path travelled by light in a vacuum during a time interval of 1 / 299,792,458 of a second .
The SI second is defined as a certain number of unperturbed ground-state hyperfine transitions of the caesium-133 atom. Caesium standards are therefore regarded as primary time and frequency standards. Caesium clocks include the NIST-F1 clock, developed in 1999, and the NIST-F2 clock, developed in 2013. [67] [68] Caesium has several properties ...
Caesium-134 is found in spent nuclear fuel but is not produced by nuclear weapon explosions, as it is only formed by neutron capture on stable Cs-133, which is only produced by beta decay of Xe-133 with a half-life of 3 days. Cs-134 has a half-life of 2 years and may be a major source of gamma radiation in the first 20 years after discharge ...
The exact modern SI definition is "[The second] is defined by taking the fixed numerical value of the cesium frequency, Δν Cs, the unperturbed ground-state hyperfine transition frequency of the cesium 133 atom, to be 9 192 631 770 when expressed in the unit Hz, which is equal to s −1." [1]
In other words, 13.6 eV is the energy input required for the electron to no longer be bound to the atom. The exact definition of one second of time since 1997 has been 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 at rest at a ...