X-ray spectra
Using X-ray diffraction the spectrum
produced by an X-ray tube can be Intensity (I)
investigated. Such a spectrum is
shown in the diagram, in which the
two curves represent two different
accelerating voltages.
K
K
The X-ray spectrum can be
considered in two parts:
(a) a continuous background, and
(b) a series of sharp peaks.
high accelerating voltage
L
L
The background is due to radiation
emitted as the electrons are slowed
down by electromagnetic attraction
of the nuclei of the material. The
minimum wavelength and therefore
low accelerating voltage
the maximum energy and frequency
Wavelength ()
of an X-ray in this spectrum is
produced when an electron is
stopped by just one nucleus. The rest of the curve is produced by electrons losing only part
of their energy during collisions with many nuclei. The minimum wavelength m is given by
the equation:
Minimum X ray wavelength (m) = hc/eV
where V is the accelerating voltage and c the velocity of light: the smaller the value of V, the
greater the minimum wavelength will be and the smaller the maximum frequency.
The peaks on the spectrum are characteristic of the particular target material. Electrons from
the inner shells are removed completely by the bombarding electrons. An electron from an
outer energy level falls back to fill the vacancy emitting a photon of a definite wavelength,
thus giving a sharp peak on the spectrum.
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