Operating Manual

Table 2-2 shows the average HVT-values for steel, table 3-2 shows the values for lead.
For a heterogeneous beam the HVT is not constant; the second HVT is slightly larger
than the first. In general, in industry where relatively hard radiation is used, a fixed
“average” HVT is applied.
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2.8 Filtering (hardening)
All materials, for example a metal layer between the radiation source and the film, cause
absorption and filtering. The position of the metal layer plays an important role in the
effect it has. A metal layer in front of the object will “harden” the radiation because it fil-
ters out the soft radiation. The degree of hardening depends on the type and the thickness
of the material. This phenomenon is used to reduce excessive contrast (variation in den-
sity) when examining objects of which the thickness varies greatly.
A metal layer between the object and the film filters the soft scattered radiation that
occurs in the object, thereby increasing the contrast and consequently the image quality.
This method of filtering is for example applied in the use of Cobalt60 in combination with
exposure time reducing intensifying screens, which are sensitive to scattered radiation.
Lead, copper and steel are suitable filtering materials.
2.9 Half-value thickness
A convenient practical notion (number) of the linear absorption coefficient is the intro-
duction of the half-value thickness (HVT). It quantifies the penetrating power of radia-
tion for a particular type of material and is defined as the thickness of a particular mate-
rial necessary to reduce the intensity of a monochromatic beam of radiation by half, as
shown in figure 9-2. This HVT-value depends on the hardness of radiation.
Table 3-2. Half-value thickness for lead
Fig. 9-2. Illustration of half-value thickness
thickness
HVT HVT HVT HVT
intensity
Element/Isotope Symbol Average energy level Half-value thickness
in MeV in mm lead
Ceasium137 Cs137 0.66 8.4
Cobalt60 Co60 1.25 13
Iridium192 Ir192 0.45 2.8
Selenium75 Se75 0.32 2
Ytterbium169 Yb169 0.2 1
Thulium170 Tm170 0.072 0.6