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Let us see why the krypton isotope discharge lamp (developed by
Dr. Engelhard of the Physikalisch-Technische Bundesanstalt,
Germany) has come to supersede the Michelson cadmium lamp.
In practice there are two principal causes of imperfection in spectral
lines, namely (a) Doppler line broadening, (b) fine structure due
to the presence of odd mass-number isotopes. The first of these
is due to the Doppler frequency charge of radiation received from
atoms with random thermal velocities along the line of sight; the
krypton lamp is very good in this respect because it can be run
(by putting it in a special cryostat) at a temperature as low as
63° K whereas a cadmium lamp operates at about 580° K (307° C).
The use of an even isotope of krypton eliminates the second effect
Fig. 1. Mercury interference fringes.
