Operating Manual

2.4 Main properties of X-rays and γ -rays
X-rays and γ-rays have the following properties in common:
1. invisibility; they cannot be perceived by the senses
2. they travel in straight lines and at the speed of light
3. they cannot be deflected by means of a lens or prism, although their path can be bent
(diffracted) by a crystalline grid
4. they can pass through matter and are partly absorbed in transmission
5. they are ionising, that is, they liberate electrons in matter
6. they can impair or destroy living cells
2.5 Radiation energy-hardness
Radiation hardness (beam quality) depends on wavelength. Radiation is called hard
when its wavelength is small and soft when its wavelength is long. In industry the quality
of the X-ray tube ranges from very soft to ultra hard. The beam quality is related to a tube
voltage (kV) range, or keV for isotopes.
The first two columns of table 2-2 below indicate the relationship hardness/tube voltage
range applied in NDT. The third column gives the related qualification of the radiation
effect, i.e. half-value thickness (HVT), described in detail in section 2.9.
Table 2-2. Comparative values of radiation quality (hardness) against tube voltage.
2.6 Absorption and scattering
The reduction in radiation intensity on penetrating a material is determined by the
following reactions :
1. Photoelectric effect
2. Compton effect
3. Pair production
Which of these reactions will predominate depends on the energy of the incident
radiation and the material irradiated.
Photoelectric effect
When X-rays of relatively low
energy pass through a material
and a photon collides with an
atom of this material, the total
energy of this photon can be
used to eject an electron from the
inner shells of the atom, as figure
3-2 illustrates. This phenomenon
is called the photoelectric effect
and occurs in the object, in the
film and in any filters used.
Compton effect
With higher X-ray energies (100
keV to 10 MeV), the interaction
of photons with free or weakly
bonded electrons of the outer
atom layers causes part of the
energy to be transferred to these
electrons which are then
ejected,
as illustrated in figure 4-2. At the
same time the photons will be
deflected from the initial angle
of incidence and emerge from
the collision as radiation of redu-
ced energy, scattered in all direc-
tions including backward, known
as “backscatter”, see section 17.6.
In this energy band, the absorp-
tion of radiation is mainly due to
the Compton effect and less so to
the photoelectric effect.
22
Radiation quality Tube voltage Global half-value
Hardness thickness for steel (mm)
Very soft Less than 20 kV
Soft 20 – 60 kV
Fairly soft 60 – 150 kV 0.5-2
Hard 150 – 300 kV 2-7
Very hard 300 – 3000 kV 7-20
Ultra hard more than 3000 kV > 20
incident
X-rays
ejected
electron
Fig. 3-2. Photoelectric effect
X-ray
100keV - 10 MeV
ejected
electron
scattered
radiation
Fig. 4-2. Compton effect
23