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.