Production and Properties of X-Rays
X-rays are produced when fast moving electrons hit a piece of metal (called the target).
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Electrons are thermionically emitted by the filament (cathode).
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The accelerating voltage is about 100kV.
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Less than 1% of the kinetic energy of the electrons is converted into X-rays so the
anode (target) must be cooled during operation.
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X-rays are not deflected by electric or magnetic fields but can be diffracted suggesting
that they have wave-like properties.
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X-rays are electro-magnetic radiations having wavelengths in the range
10-11m to 10-8m.
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X-rays cause certain substances to fluoresce, they affect photographic emulsions and
can ionise atoms. These three properties can be used to detect X-rays.
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The intensity of the beam of X-rays (Wm-2) depends on the number of electrons hitting
the target per unit time. This depends on the temperature of the filament.
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The penetrating power of the beam of X-rays depends on the kinetic energy of the
electrons. This depends on the accelerating voltage.
Research task.
Look for more notes and examples on the uses of X-rays in everyday life. Also try to find
information about the dangers and production of X-rays. (1 week)
X-Ray Spectra
A typical X-ray spectrum is shown below.
The graph shows the relative intensity of X-rays emitted at different wavelengths. It can be
divided into two parts, a continuous spectrum (the curve) and a line spectrum (the peaks).
Continuous Spectrum
Line or Characteristic Spectrum
An electron in the beam can have a collision with an electron in an atom of the target metal. If
an electron in a low energy level is excited to a higher energy level, an X-ray quantum can be
emitted when an electron falls to fill the "space" in the lower energy level.
The wavelengths at which the peaks in the spectrum occur thus depend on the material of the
target.
These "lines" in the spectrum are named after the energy level to which an electron falls, as
shown below.
The minimum wavelength, λmin, is given by:
where
h = Planck’s constant
c = speed of light
e = electronic charge
V = accelerating voltage across tube
Nuclear explanation of production of X-rays.
This is easiest to see using the simple Bohr
model of the atom. In such a model, shells
of electrons surround the nucleus of the
atom containing the protons and neutrons.
The innermost shell, called the K- shell, is
surrounded by the L and M - shells. When
the energy of the electrons accelerated
toward the target becomes high enough to
dislodge K- shell electrons, electrons from
the L - and M - shells move in to take the
place of those dislodged.
Each of these electronic transitions produces an X-ray with a wavelength that depends on the
exact structure of the atom being bombarded. A transition from the L - shell to the K- shell
produces a Kα X-ray, while the transition from an M - shell to the K- shell produces a Kβ Xray.
These characteristic X-rays have a much higher intensity than
those produced by the continuous spectra, with Kα X-rays
having higher intensity than Kβ X-rays. The important point
here is that the wavelength of these characteristic X-rays is
different for each atom in the periodic table (of course only
those elements with higher atomic number have L- and M shell electrons that can undergo transitions to produce Xrays). A filter is generally used to filter out the lower
intensity Kβ X-rays.
For commonly used target materials in X-ray tubes, the X-rays have the following well-known
experimentally determined wavelengths:
Element
Kα Wavelength (λ) Å
Mo
0.7107
Cu
1.5418
Co
1.7902
Fe
1.9373
Cr
2.2909
Web links.
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http://www.saburchill.com/physics/chapters2/0087.html
http://www.saburchill.com/physics/chapters2/0088.html
http://www.tulane.edu/~sanelson/geol211/x-ray.htm
X-ray spectra of periodic table: http://ie.lbl.gov/xray/mainpage.htm