Codes for the inspection of welds and castings specify the maximum allowed radiation intensity, based on the type of material and the thickness of the object. Exposure charts are necessary to etablish the correct exposure value. A universal exposure graph or slide-rule can be used for radioactive sources, as these have a fixed natural radiation spectrum.
Fig. 1-9. Step wedge
The radiation spectrum of X-ray tubes varies with each tube, even if they are of the same type. This problem is easily solved by using a universal exposure chart for the specific type of tube, and then individualise it for each tube, the so-called “curve fitting”. The adaptation is normally limited to a zero-point correction, based on a few measured values obtained by trial. Sometimes the gradient of the exposure graph needs to be adjusted as well.
An exposure chart is produced by making a series of radiographs of a step wedge as illustrated in figure 1-9.
The radiation intensity level of most X-ray equipment is expressed by the amperage of the current through the X-ray tube, measured in milliampères (mA).
The exposure (radiation dose) is specified as the product of radiation intensity and exposure duration in mA.min. (intensity x time).
The exposure chart shows the relationship between the thickness of the object (in mm) and the exposure value (for X-ray tubes in kV and mA.min; for sources in GBq/h).
The exposure chart is applied for:
a given density, for example: 2 or 2.5
a given film-screen combination, for example D7 with lead screens
a given type of material, for example steel
The chart depends amongst others on:
type of X-ray equipment or radioactive source
source-to-film distance, usually 800 mm
development conditions, for example: automatic, 8 minutes at 28°C.
Type of X-ray equipment
Among the factors to be taken into account are: the voltage (in kV), whether alternating or direct current, the limits of voltage adjustment and the current through the tube (in mA). It follows that the exposure chart is unique for a particular X-ray set.
The radioactive source
Radiation intensity and half-life-time of the source have to be taken into account.
The exposure chart for an X-ray set is produced for a specified source-to-film distance. If another distance is used, corrections will be necessary, using the inverse square law.
When drawing up the exposure chart, intensifying screens used must be recorded and the same type of screens used again when making radiographs.
Type of film
The type of film must be indicated on the exposure chart, since the various types of industrial X-ray films are substantially different in sensitivity (speed).
An exposure chart must be as accurate as possible. Densities indicated are to be measured by a densitometer, see section 9.2. The radiographs that form the basis for the chart must have been made under controlled and reproducible conditions, whereby quality monitoring tools such as PMC strips as described in section 10.6 are used.
Developer formula, processing temperature and developing time all influence the final result. The exposure chart produced will be related to a particular well-defined developing process