I believe this is defined more by the time-base("atomic accuracy" as determined by atoms within an element shifting between discrete states of energy) than it is by the element per se. Clocks with atomic accuracy can utilize Rubidium, as stated by European Space Agency in its description of the Galileo satellite:
"The Galileo satellites will carry two types of clocks: rubidium atomic frequency standards and passive hydrogen masers. The stability of the rubidium clock is so good that it would lose only three seconds in one million years, while the passive hydrogen maser is even more stable and it would lose only one second in three million years. However this kind of stability is really needed, since an error of only a few nanoseconds (billionths of a second) on the Galileo measurements would produce a positioning error of metres which would not be acceptable.
An atomic clock works like a conventional clock but the time-base of the clock, instead of being an oscillating mass as in a pendulum clock, is based on the properties of atoms when transitioning between different energy states.
An atom, when excited by an external energy source, goes to a higher energy state. Then, from this state, it goes to a lower energy state. In this transition, the atom releases energy at a very precise frequency which is characteristic of the type of atom. This is like a signature for the type of material used. All that is needed for making a good clock is a way of detecting this frequency and using it as an input to a counter. This is the principle behind an atomic clock.
The transitions between energy states can take place by releasing or absorbing energy at optical or microwave frequencies. An atomic second corresponds to 9 192 631 700 counts of the frequency of the energy detected in the transition of the Cesium 133 isotope when exposed to suitable excitation."
"The Galileo satellites will carry two types of clocks: rubidium atomic frequency standards and passive hydrogen masers. The stability of the rubidium clock is so good that it would lose only three seconds in one million years, while the passive hydrogen maser is even more stable and it would lose only one second in three million years. However this kind of stability is really needed, since an error of only a few nanoseconds (billionths of a second) on the Galileo measurements would produce a positioning error of metres which would not be acceptable.
An atomic clock works like a conventional clock but the time-base of the clock, instead of being an oscillating mass as in a pendulum clock, is based on the properties of atoms when transitioning between different energy states.
An atom, when excited by an external energy source, goes to a higher energy state. Then, from this state, it goes to a lower energy state. In this transition, the atom releases energy at a very precise frequency which is characteristic of the type of atom. This is like a signature for the type of material used. All that is needed for making a good clock is a way of detecting this frequency and using it as an input to a counter. This is the principle behind an atomic clock.
The transitions between energy states can take place by releasing or absorbing energy at optical or microwave frequencies. An atomic second corresponds to 9 192 631 700 counts of the frequency of the energy detected in the transition of the Cesium 133 isotope when exposed to suitable excitation."