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The core of the atomic clock is a microwave cavity containing the ionized gas, a tunable microwave radio oscillator, and a feedback loop which is used to adjust the oscillator to the exact frequency of the absorption characteristic defined by the behavior of the individual atoms.The microwave oscillator fills the chamber with a standing wave of radio waves. When the radio frequency matches the hyperfine transition frequency of caesium, electrons in the caesium atoms are able to absorb the radio waves and move between two energy states. Only those atoms that have changed states are allowed to impinge on a detector. When the incidence of detected atoms decreases because the frequency of the microwave oscillator has drifted from the true resonance frequency, the frequency of the oscillator is corrected.This adjustment process is where most of the work and complexity of the clock lies. The adjustment tries to correct for unwanted side-effects, such as frequencies from other electron transitions, temperature changes, and the "spreading" in frequencies caused by ensemble effects. One way of doing this is to sweep the microwave oscillator's frequency across a narrow range to generate a modulated signal at the detector. The detector's signal can then be demodulated to apply feedback to control long-term drift in the radio frequency. In this way, the quantum-mechanical properties of the atomic transition frequency of the caesium can be used to tune the microwave oscillator to the same frequency (except for a small amount of experimental error). When a clock is first turned on, it takes a while for the oscillator to stablize.