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Maser is an acronym for Microwave Amplification by
Stimulated Emission of Radiation. A maser is similar to a laser, but
operating in the microwave region of the electromagnetic spectrum.
The first maser was built by Charles H. Townes, J. P. Gordon, and H. J. Zeiger at Columbia University in 1953.
The device used stimulated emission in a stream of energised
ammonia molecules to produce amplification of microwaves at a frequency of 24 gigahertz.
Townes later worked with Arthur L. Schawlow to describe the principle of the optical maser, or laser, which was invented in
1960 by Theodore H.
Maiman.
Masers are used as high precision frequency references, for example as an
atomic clock. They are also used as electronic amplifiers in radio telescopes.
For more information about frequency reference masers, see atomic
clock.
Telescopic masers use arrays of chromium atoms in an insulating aluminum oxide crystal as amplifiers, pumping the
energy in at a different radio frequency. That is, they use polished strips of synthetic ruby.
As the input signal comes in, a gold comb (gold is used because it cannot corrode and
change shape) distributes it along the strip of polished ruby. As the radio wave moves through the crystal, it knocks electrons
into different orbits. As the electrons wiggle into their new, lower orbits, closer to their atoms' nuclei, they add to the wave
that knocked them down. A radio wave is
made by a wiggling electric charge or magnetic field.
The comb-fingers are spaced so that the desired radio waves add together as they move down the crystal. This means that
unwanted radio waves don't add together, and are therefore filtered out.
Masers are cooled by liquid helium, which is at a temperature of only about 4
kelvins. This reduces the noise from electrons, nuclei, and other charged particles
that can be bounced around by the molecular motion of heat.
The electrons are pumped away from their nuclei by longer, ignorable radio waves put onto the ruby from a different comb.
Masers can also be found in nature. In interstellar space, water molecules in star-forming regions can suffer a population inversion and emit radiation at 22 GHz, creating the brightest spectral line in the radio universe. Some water masers also emit radiation from a vibrational mode at 96 GHz.
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