Hydrogen maser

The amount of energy needed to reverse the spin of the electron is equivalent to a photon at the frequency of 1.420405751768 GHz,[1] which corresponds to the 21 cm line in the hydrogen spectrum.

In both types, a small storage bottle of molecular hydrogen, H2, leaks a controlled amount of gas into a discharge bulb.

[3] The storage bulb is inside a microwave cavity made from a precisely machined copper or silver-plated ceramic cylinder.

[5] To decrease the influence of changing external magnetic fields on the transition line frequency and be compliant to electromagnetic interferences, the cavity is surrounded by several nested layers of shields.

With advanced microwave cavities made out of silver-plated ceramic, the gain factor can be much higher, thereby requiring less hydrogen atom density.