Operating Manual

5.4 Radioactive sources
Table 1-5 shows various radioactive sources for industrial NDT. The most commonly used
ones are Cobalt, Iridium and increasingly Selenium. Selenium is very attractive while it
permits lighter containers than Iridium. Due to its average energy level it often is a good
alternative for an X-ray tube, also attractive while no electricity is needed.
Average energy level (nominal value)
The spectrum of a source has one or more energy lines, as shown in figure 2-2. For sources
with multiple energy lines an average energy level is assumed, the so-called nominal value.
On the basis of these spectra data it is clear that Co60, Cs137
and Ir192 sources produce high-energy radiation and are the-
refore well suited to irradiate thick materials.
Yb169, on the other hand, is a source that produces relatively
soft radiation and is of a very small size (0.5 mm), which makes
it particularly suitable for radiographic examination of circum-
ferential welds in pipes of a small diameter and thin wall thick-
ness, with the source centrally positioned so that the weld can
be exposed uniformly in one exposure, as shown in figure 8-5.
4544
The main properties of a linear accelerator are:
1. very high output of radiation
2. very small focal spot dimensions (<2 mm)
3. considerable weight (approx. 1200 kg for an 8 MeV stationary installation)
Figure 7-5 shows an 8 MeV linac in a radiation bunker examining a pump housing.
Fig.7-5. Linac and pump house in a radiation bunker
Table 1-5. Radioactive sources used in industrial radiography, in sequence of nominal (average) energy level
Table 2-5. Radiation spectra and nominal values
Element Symbol Mass Specific gamma Average energy
Number constant level
k-factor in MeV
Cobalt60 Co 60 0.35 1.25
Caesium137 Cs 137 0.09 0.66
Iridium192 Ir 192 0.13 0.45
Selenium75 Se 75 0.054 0.32
Ytterbium169 Yb 169 0.05 0.2
Thulium170 Tm 170 0.001 0.072
Source Number of Main energy Nominal value
spectrum lines levels in MeV in MeV
Cobalt60 2 1.17 and 1.34 MeV 1.25 MeV.
Caesium137 1 0.66 MeV 0.66 MeV
Iridium192 >10 0.3; 0.31; 0.32; 0.47 en 0.6 MeV 0.45 MeV.
Selenium75 >4 120, 140 and 400 keV 320 keV.
Ytterbium169 >6 0.06 and 0.2 MeV 200 keV.
Thulium170 2 52 and 84 keV 72 keV.
source
film
Fig. 8-5. Ytterbium169 source in
central position for exposure of
circumferential weld
Fig. 6-5. Linear electron accelerator (linac)
X-rays
target
wave guide
electron gun
vacuum pump
magnetron
focus coils