Operating Manual

4.6 Properties of radioactive sources
Activity (source strength)
The activity of a radioactive substance is given by the number of atoms of the substance
which disintegrate per second.
This is measured in Becquerels (Bq), 1 Becquerel corresponds to 1 disintegration per
second(1 Bq = 1/s).
Specific activity
The specific activity of a radioactive source is the activity of this substance per weight unit,
expressed in Bq/g.
Specific gamma-ray emission factor (k-factor)
The k-factor is the generally used unit for radiation output of a source and is defined
as the activity measured at a fixed distance. It indicates the specific gamma-emission
(gamma constant) measured at 1 metre distance.
The higher the k-factor, the smaller the source can be for a particular source strength.
A source of small dimensions will improve the sharpness of a radiograph.
Table 1-4 shows the various k-factors and half-life values.
Half-life of a radioactive source
Of an Iridium192 source with an activity of 40 GBq for example 10 GBq will remain after
two half-lives (148 days), 5 GBq after three half-lives (222 days) etc.
3736
4.4 Radioactive sources (isotopes)
Natural radioactive sources
The elements from this group which have been used for the purposes of industrial radio-
graphy are radium and mesothorium. These give a very hard radiation, making them par-
ticularly suitable for examining very thick objects.
A disadvantage of natural sources, next to their high cost, is that it is not possible to make
them in dimensions small enough for good quality images and still give sufficient activity.
Artificial radioactive sources
Artificial radioactive sources for NDT are obtained by irradiation in a nuclear reactor. Since
1947, it has been possible to produce radioactive isotopes this way in relatively large quan-
tities and in a reasonably pure state and particularly of sufficiently high concentration; the
latter being extremely important in NDT because the size of the source has to be as small
as possible. Among the many factors deciding a source suitability for non-destructive
testing are the wavelength and intensity of its radiation, its half-life and its specific radia-
tion. In fact, only a few of the many artificial radio-isotopes available have been found to
be suitable for industrial radiography.
4.5 Advantages and disadvantages
of artificial radioactive sources
Advantages
1. require no electric power supply; easy to use in the field
2. can be obtained in a range of source diameters, so that if necessary a very short
source-to-film distance with a small diameter source can be used, for example,
for pipes of small diameter
3. a wide variety of radiation hardnesses
4. higher radiation hardness (more penetration power) than those of conventional
X-ray equipment can be selected
Disadvantages
1. cannot be switched off
2. the energy level (radiation hardness) cannot be adjusted
3. the intensity cannot be adjusted
4. limited service life due to source deterioration (half-life)
5. less contrast than X-ray equipment
Isotope Half-life Specific gamma constant
or k-factor
Ytterbium169 31 days 0.05
Iridium192 74 days 0.13
Selenium75 120 days 0.054
Cobalt60 5.3 years 0.35
Caesium137 30 years 0.09
Table 1-4 Various k-factors and half-life values