A hydrogen maser, also known as hydrogen frequency standard, is a specific type of

maser A maser (, an acronym for microwave amplification by stimulated emission of radiation) is a device that produces coherent electromagnetic waves through amplification by stimulated emission. The first maser was built by Charles H. Townes, Ja ...

that uses the intrinsic properties of the

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...

atom to serve as a precision frequency reference. Both the proton and electron of a hydrogen atom have spins. The atom has a higher energy if both are spinning in the same direction, and a lower energy if they spin in opposite directions. The amount of energy needed to reverse the spin of the electron is equivalent to a photon at the frequency of , which corresponds to the 21 cm line in the hydrogen spectrum. Hydrogen masers are very complex devices and sell for as much as . There are two types to be distinguished: active and passive. In both types, a small storage bottle of

molecular hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, ...

, , leaks a controlled amount of gas into a discharge bulb. The molecules are dissociated in the discharge bulb into individual hydrogen atoms by an electric arc. This atomic hydrogen passes through a

collimator A collimator is a device which narrows a beam of particles or waves. To narrow can mean either to cause the directions of motion to become more aligned in a specific direction (i.e., make collimated light or parallel rays), or to cause the spati ...

then a magnetic state selector and into a storage bulb. The storage bulb is roughly high and in diameter and made of

quartz Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical ...

internally coated with PTFE. Adsorption onto, chemical interaction with, and perturbation of atomic state by the bulb surface is much reduced. Consistent interactions with the bulb increase the quality of the oscillation. A durable PTFE and bulb coating technology allows for over 20-year lifetime. The storage bulb is in turn inside a

microwave cavity A microwave cavity or ''radio frequency (RF) cavity'' is a special type of resonator, consisting of a closed (or largely closed) metal structure that confines electromagnetic fields in the microwave region of the spectrum. The structure is eithe ...

made from a precisely machined copper or silver-plated ceramic cylinder. This cavity is tuned to the resonance frequency of the atoms. A weak static magnetic field is applied parallel to the cavity axis by a solenoid to lift the degeneracy of the magnetic Zeeman sublevels. 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. In the active hydrogen maser, the cavity oscillates by itself. This requires a higher hydrogen atom density and a higher

quality factor In physics and engineering, the quality factor or ''Q'' factor is a dimensionless parameter that describes how underdamped an oscillator or resonator is. It is defined as the ratio of the initial energy stored in the resonator to the energy lo ...

for the cavity. However, with advanced microwave cavities made out of silver-plated ceramic, the gain factor can be much higher, thereby requiring less hydrogen atom density. The active maser is more complex and more expensive but has better short-term and long-term frequency stabilities. In the passive hydrogen maser, the cavity is fed from an external frequency. The external frequency is tuned to produce a maximum output in the cavity. This allows the use of lower hydrogen atom density and lower cavity quality factor, which reduces the cost. Image:Hydrogen maser.jpg, Exposed hydrogen maser File:ESA Galileo Passive Hydrogen Maser.jpg, Passive hydrogen maser used as a clock in Galileo satellite navigation system.

References

Bibliography

* * {{Cite journal, author1=Kleppner, D. , author2=Goldenberg, H.M. , author3=Ramsey, N.F. , date=1962, title=The Atomic Hydrogen Maser, journal=Physical Review, volume=126, pages=603, bibcode=1962PhRv..126..603K, doi=10.1103/PhysRev.126.603 Hydrogen technologies Laser applications Electronics standards Atomic clocks

See also

References

Bibliography