Everything about Doppler Broadening totally explained
In
atomic physics,
Doppler broadening is the broadening of
spectral lines due to the
Doppler effect in which the
thermal movement of
atoms or
molecules shifts the apparent frequency of each emitter. The many different velocities of the emitting gas result in many small shifts, the cumulative effect of which is to broaden the line. The resulting line profile is known as a
Doppler profile. The broadening is dependent only on the wavelength of the line, the mass of the emitting particle and the temperature, and can therefore be a very useful method for measuring the temperature of an emitting gas.
The
Doppler profile in wavelength is a
normal distribution with a standard deviation of
»
Nuclear technology
In a
nuclear reactor, this effect reduces the
power generated as the reactor temperature increases.
When a reactor gets hotter, the accelerated motion of the atoms in the fuel increases the probability of
neutron capture by
U-238 atoms. When the uranium is heated, its nuclei move more rapidly in random directions, and therefore see and generate a wider range of relative neutron speeds. U-238, which forms the bulk of the uranium in the reactor, is much more likely to absorb
fast neutrons.
(External Link
) This reduces the number of neutrons available to cause
U-235 fission, reducing the power output by the reactor.
In some reactor designs, such as the
pebble bed reactor, this natural
negative feedback places an inherent upper limit on the temperature at which the chain reaction can proceed. Such reactors are said to be "inherently safe" because a reactor failure can't generate a
criticality excursion. It is worth noting, however, that because of
decay heat emitted from the decay of fission products, a meltdown is still theoretically possible if the ability to cool the reactor is lost, and thus the reactor design must be designed to prevent
loss of coolant accident.
Astronomy
In
Astronomy, Doppler broadening is one of the explanations for the broadening of spectral lines, and as such gives an indication for the relative temperatures of observed material. There are, however many other factors which can broaden the lines as well. For example high surface gravity (a sign of small stars) leads to high pressure, which in turn leads to Stark-broadening (see
Spectral line).
Further Information
Get more info on 'Doppler Broadening'.
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