Operating Manual

2.2 X-rays
The radiation which is emitted by an X-ray
tube is heterogeneous, that is, it contains
X-rays of a number of wavelengths, in the
form of a continuous spectrum with some
superimposed spectrum lines.
See fig. 1-2.
The shortest wavelength of the spectrum is
given by the Duane-Hunt formula:
In which :
λ = wavelength in nanometers (10
-9
m)
kV = voltage in kilovolts
The average shape of the X-ray spectrum is generally the same however not truely identical
for different X-ray sets; it depends chiefly on the energy range of the electrons striking the
X-ray tube target and, therefore, on the voltage waveform of the high-voltage generator.
A constant potential (CP) X-ray set will not have the same spectrum as a self-rectified set
operating at the same nominal kV and current. The spectrum shape also depends on the
inherent filtration in the X-ray tube window (glass, aluminium, steel or beryllium).
The energy imparted to an electron having a charge e, accelerated by an electrical poten-
tial V is (eV) so the energy of the electrons can be quoted in eV, keV, MeV. These same
units are used to denote the energy of an X-ray spectrum line.
The energy of a single wavelength is :
In which:
E = the energy in electronVolt (eV)
h = Planck’s constant
v = frequency
c = the velocity of electromagnetic radiation, such as light (300,000 km/s)
The heterogeneous X-rays emitted by an X-ray tube do not however have a single
wavelength, but a spectrum, so it would be misleading to describe the X-rays as (say)
120 keV X-rays. By convention therefore, the ‘e’- in keV- is omitted and the X-rays
described as 120 kV, which is the peak value of the spectrum.
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2.3 Gamma-rays ( γ -rays)
Radioactivity is the characteristic of certain elements to emit alpha (α), beta (β) or
gamma (γ) rays or a combination thereof. Alpha and beta rays consist of electrically char-
ged particles, whereas gamma rays are of an electromagnetic nature.
Gamma rays arise from the disintegration of atomic nuclei within some radioactive sub-
stances, also known as isotopes. The energy of gamma-radiation cannot be controlled; it
depends upon the nature of the radioactive substance. Nor is it possible to control its
intensity, since it is impossible to alter the rate of disintegration of a radioactive substance.
Unlike X-rays, generated to a continuous spectrum, Gamma-rays are emitted in an isola-
ted line spectrum, i.e. with one or more discrete energies of different intensities.
Figure 2-2 shows the energy spectrum lines for Selenium75, Cobalt60 and Iridium192.
In practical NDT applications, sources (radio active isotopes) are allocated an average
nominal energy value for calculation purposes, see section 5.4. Spectrum components
with the highest energy levels (keV values) influence radiographic quality the most.
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λ
min
=
1.234
Ε=h
.
v λ
.
v=c
kV
Fig. 1-2. X-ray spectrum – intensity/wavelength
distribution
The small peaks are the characteristic radiation of the
target material
wavelength
intensity
relative intensity
energy (keV)
Fig. 2-2. Energy spectrum (lines) for Se75, Ir192 and Co60