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MASER, acronym for microwave
amplification by stimulated emission of radiation,
a device that amplifies or generates microwaves or radio waves. A maser
producing radiation in the optical region is called a laser. Principle of
Operation
As in lasers, amplification of
radiation in masers is obtained by stimulated emission. This occurs when a photon induces an
excited atom or molecule to fall to a lower energy state while emitting a
photon of the same frequency as the incoming photon. The emitted photon travels
in the same direction and in phase with the incoming photon, which is not
absorbed during the interaction. The amplitudes of the two waves add up, and
amplification of the incoming wave has taken place. Masers make use of those
transitions in molecules or crystals that correspond to the energies of
microwave or radio frequencies. Types of Masers The first maser
oscillator was developed by the American physicists Charles Hard Townes, James
P. Gordon and Herbert J. Zeiger in
1954, and made use of the frequency of the ammonia molecule. This frequency
corresponds to the energy of the photon emitted when the nitrogen atom moves
from one side to the other of the triangle formed by the three hydrogen atoms
in an ammonia molecule. The hydrogen maser makes use of the frequency
corresponding to that of the photon released when the spin of the proton in a
hydrogen atom flips over with respect to the spin of the atom’s electron. This
reaction is also known to occur in outer space, and several natural masers have
been discovered since 1965. Paramagnetic masers use energy transitions
corresponding to the orientations of the magnetic moments of paramagnetic ions
in crystalline substances placed in an external magnetic field. Different
frequencies can be obtained by varying the magnetic field. Applications
Because of the high stability of the generated frequencies, masers serve as time standards in atomic clocks. Masers are also used as low-noise radio frequency amplifiers in satellite communication and radioastronomy.
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