Operating Manual

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6.2 Radiation filters
When a metal plate, usually lead or copper, is placed between the tube window and the
object, radiation “hardening” occurs leading to a lower image contrast.
This may be counter-balanced by a metal filter placed immediately behind the object (i.e.
between object and film). This filter will cause the (softer) scattered radiation passing
through the object to be absorbed by the filter to a greater extent than the primary (har-
der) radiation. This also improves the image quality.
If the edges of an object being radiographed are not close to the film (as in the case of a
cylindrical body in figure 3-6) considerable scatter of the primary radiation can occur,
leading to fogging. This scatter can be prevented by positioning sheets of lead foil between
the object and the film as illustrated in this figure.
Reducing the contrast by filtration is also desirable when a radiographic image of an object
of widely varying thicknesses has to be obtained on a single film see section 18.2.
Typical filter thicknesses are :
0.1 – 0.25 mm lead for 300 kV X-rays
0.25 – 1.0 mm lead for 400 kV X-rays
6.3 Intensifying screens
The radiographic image is formed by only approximately 1 % of the amount of radiation
energy exposed at the film. The rest passes through the film and is consequently not used.
To utilise more of the available radiation energy, the film is sandwiched between two inten-
sifying screens. Different types of material are being used for this purpose.
Lead screens
Under the impact of X-rays and gamma-rays, lead screens emit electrons to which the film
is sensitive. In industrial radiography this effect is made use of: the film is placed between
two layers of lead to achieve the intensifying effect and intensity improvement of approxi-
mately factor 4 can be realised. This method of intensification is used within the energy
range of 80 keV to 420 keV, and applies equally to X-ray or gamma-radiation, such as pro-
duced by Iridium192.
Intensifying screens are made up of two homogeneous sheets of lead foil (stuck on to a thin
base such as a sheet of paper or cardboard) between which the film is placed: the so called
front and back screens.
The thickness of the front screen (source side) must match the hardness of the radiation
being used, so that it will pass the primary radiation while stopping as much as possible of
the secondary radiation (which has a longer wavelength and is consequently less penetrating).
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Scattered radiation also occurs in radiographic examination of cylindrical objects,
as shown in figure 3-6.
The effects of scattered radiation can be further reduced by :
• limiting the size of the radiation beam to a minimum with a diaphragm in front
of the tube window
• using a cone to localise the beam, a so called collimator
• the use of masks: lead strips around the edges of the object.
1
S
X-rays
Fig. 3-6.
Scattered radiation in radiogra-
phy of cylindrical objects.
Scattered radiation from object 1
causes a spurious band at B,
object 2 at A etc, unless lead strips
are used as shown in the lower
part of this figure
film
film
lead strips
A B C D
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