Transcript Production of X-rays

Production of X-rays (1)
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X-rays are produced when rapidly moving
electrons that have been accelerated
through a potential difference of order 1
kV to 1 MV strikes a metal target.
Evacuated
glass tube
Target
Filament
Production of X-rays (2)
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Electrons from a hot element are
accelerated onto a target anode.
When the electrons are suddenly
decelerated on impact, some of the
kinetic energy is converted into EM
energy, as X-rays.
Less than 1 % of the energy supplied is
converted into X-radiation during this
process. The rest is converted into the
internal energy of the target.
Properties of X-rays
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X-rays travel in straight lines.
X-rays cannot be deflected by electric
field or magnetic field.
X-rays have a high penetrating power.
Photographic film is blackened by X-rays.
Fluorescent materials glow when X-rays
are directed at them.
Photoelectric emission can be produced
by X-rays.
Ionization of a gas results when an X-ray
beam is passed through it.
X-ray Spectra (1)
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Using crystal as a wavelength selector, the
intensity of different wavelengths of X-rays can
be measured.
X-ray Spectra (2)
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The graph shows the following features.
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A continuous background of X-radiation in
which the intensity varies smoothly with
wavelength. The background intensity reaches
a maximum value as the wavelength
increases, then the intensity falls at greater
wavelengths.
Minimum wavelength which depends on the
tube voltage. The higher the voltage the
smaller the value of the minimum wavelength.
Sharp peaks of intensity occur at wavelengths
unaffected by change of tube voltage.
Minimum wavelength in the X-ray
Spectra
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When an electron hits the target its entire
kinetic energy is converted into a photon.
The work done on each electron when it
is accelerated onto the anode is eV.
Hence hf = eV and the maximum frequency
f max
eV

h
Therefore,
min
hc

eV
Characteristic X-ray Spectra
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Different target materials give different
wavelengths for the peaks in the X-ray
spectra.
The peaks are due to electrons knock out
inner-shell electrons from target atoms.
When these inner-shell vacancies are
refilled by free electrons, X-ray photons
are emitted.
The peaks for any target element define
its characteristic X-ray spectrum.
Uses of X-rays
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In medicine
To diagnose illness and for
treatment.
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In industry
To locate cracks in
metals.
X-ray crystallography
To explore the structure
of materials.