Operating Manual

2.7 Penetrating power
The penetrating power of X-radiation increases with the energy (hardness).
The relationship of energy and penetrating power is complex as a result of the various
mechanisms that cause radiation absorption. When monochromatic ( homogeneous -
single wave length) radiation with an intensity I
o
passes through matter, the relative
intensity reduction ΔI/I
o
is proportional to the thickness Δt. The total linear absorption
coefficient (μ) consisting of the three components described in section 2.6 is defined by
the following formula:
Expressed differently:
In which:
I
o
= intensity at material entry t = thickness
I = intensity at material exit e = logarithm: 2.718
μ = total absorption coefficient
Figure 7-2 shows the resulting radiation
intensity (logarithmic) as a function of
increased material thickness, for soft and
hard homogeneous radiation.
When radiation is heterogeneous the
graphs are not straight, see figure 7-2, but
slightly curved as in figure 8-2.
The slope of the curves becomes gradually
shallower (because of selective absorption
of the softer radiation) until it reaches the
so-called “point-of-homogeneity”.
After passing this point the coefficient of
absorption remains virtually unchanged, as
if the radiation had become homogeneous.
The position of the point of homogeneity
varies with the nature of the material irradi-
ated. The graph shows that with increasing
material thickness, softer radiation is filte-
red out, more than hard radiation.
This effect is called “hardening”.
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Pair production
The formation of ion pairs, see
figure 5-2, only occurs at very
high energy levels (above 1 MeV).
High-energy photons can cause
an interaction with the nucleus
of the atom involved in the colli-
sion. The energy of the photon is
here converted tot an electron(e-)
and a positron (e+).
Total absorption/attenuation
The total linear absorption or
attenuation of X-rays is a combi-
nation of the three absorption
processes described above, in
which the primary X-ray energy
changes to a lower form of ener-
gy. Secondary X-ray energy arri-
ses of a different wavelength and
a different direction of travel.
Some of this secondary (scatte-
red) radiation does not contribu-
te to radiographic image forming
and may cause loss of image
quality through blurring or fog.
The contribution of the various
causes of X-ray absorption to the
total linear absorption coeffi-
cient (μ) for steel plotted against
radiation energy, are shown in
figure 6-2.
X-ray
> 1 MeV
ejected
electron
ejected
positron
Fig. 5-2. Pair production
Fig. 6-2 Absorption coefficient for steel plotted against radiation energy
PE = Photoelectric effect
C = Compton effect
PP = Pair production
= μ
.
Δt
I = I
o
.
e
-μt
ΔI
I
o
hard radiation,
high tube voltage
hard radiation
soft radiation
points of homogeneity
soft radiation,
low tube voltage
penetrated material thickness
intensityintesity
penetrated material thickness
Fig. 7-2. Intensity of homogeneous
radiation as function of increasing
thickness
Fig. 8-2. Intensity of heterogeneous
radiation as function of increasing
thickness hard radiation