A
Hydrogen maser, also known as hydrogen
frequency standardA frequency standard is a stable oscillator used for frequency calibration or reference. A frequency standard generates a fundamental frequency with a high degree of accuracy and precision. Harmonics of this fundamental frequency are used to provide reference points.Frequency standards in a network...
, is a specific type of
maserA maser is a device that produces coherent electromagnetic waves through amplification due to stimulated emission. Historically the term came from the acronym "Microwave Amplification by Stimulated Emission of Radiation", although modern masers emit over a broad portion of the electromagnetic...
that uses the intrinsic properties of the
hydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H
2...
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 1,420,405,752 Hz.
A
Hydrogen maser, also known as hydrogen
frequency standardA frequency standard is a stable oscillator used for frequency calibration or reference. A frequency standard generates a fundamental frequency with a high degree of accuracy and precision. Harmonics of this fundamental frequency are used to provide reference points.Frequency standards in a network...
, is a specific type of
maserA maser is a device that produces coherent electromagnetic waves through amplification due to stimulated emission. Historically the term came from the acronym "Microwave Amplification by Stimulated Emission of Radiation", although modern masers emit over a broad portion of the electromagnetic...
that uses the intrinsic properties of the
hydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H
2...
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 1,420,405,752 Hz. This 1420 MHz frequency is important in radio astronomy because it corresponds to the 21 cm line of interstellar hydrogen.
Hydrogen masers are very complex devices and sell for as much as US$235,000. They are made in two types, active and passive.
In both types, a small storage bottle of molecular hydrogen leaks a controlled amount of gas into a discharge bulb. Molecular hydrogen consists of pairs of atoms bound together. The molecules are disassociated in the discharge bulb into individual hydrogen atoms by an arc. This atomic hydrogen passes through a
collimatorIn optics, a collimator may consist of a curved mirror or lens with some type of light source and/or an image at its focus. This can be used to replicate a target at infinity without parallax...
and a magnetic state selector. The atoms are thereby selected for the desired state and passed on to a storage bulb. The storage bulb is roughly 20 cm high and 10 cm in diameter and made of
quartzQuartz is the second most abundant mineral in the Earth's continental crust . It is made up of a continuous framework of SiO
4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO
2.There are many different varieties of...
. Its inside is coated with
teflonIn chemistry, polytetrafluoroethylene is a synthetic fluoropolymer of tetrafluoroethylene which finds numerous applications. PTFE is most well known by the DuPont brand name Teflon....
. This coating slows the recombination of the hydrogen atoms into hydrogen molecules. The storage bulb is in turn inside a
microwave cavityA microwave cavity is a closed metal structure that confines electromagnetic fields in the microwave region of the spectrum. Such cavities act as resonant circuits with extremely low loss at their frequency of operation...
made from a precisely machined copper cylinder. This cavity is tuned to the 1420 MHz resonance frequency of the atoms.
In the active hydrogen maser, the cavity oscillates by itself. This requires a higher hydrogen atom density and a higher
quality factorIn physics and engineering the quality factor or Q factor is a dimensionless parameter that describes how under-damped an oscillator or resonator is, or equivalently, characterizes a resonator's bandwidth relative to its center frequency....
for the cavity. The active maser is more complex and more expensive but has better long-term frequency stability. E.g., the CH1 75 model made by PTF weighs 90 kg and consumes about 100 watts of power. Its long-term frequency accuracy is about ±5×10
-16 over five years. When used as a clock, it will gain or lose a second in about 63 million years.
In the passive hydrogen maser, the cavity is fed from an external 1420 MHz 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. The CH1 76 model also made by PTF weighs 55 kg and consumes about 90 watts. Its long-term frequency accuracy is much worse than the active maser at about ±1.5×10
-12 per year.