Operating Manual

Therefore, an exposure chart for each individual X-ray set should be drawn up. This is an

excellent way to become familiar with the equipment, while time and money put into this

work will be amply repaid at a later stage.

Exposure charts for gamma-ray examination are drawn up in a similar way as described

above. Figure 5-9 shows one for a Cobalt60 source. A specially designed slide-rule can also

be used, since there is no need to consider individual radiation spectra as for X-ray tubes.

Figure 6-9 shows a similar exposure chart for an Iridium192 source.

9.4 The exposure chart

The exposure chart should be drawn on uni-directional logarithmic paper. The material

thickness (in mm) is plotted on the horizontal linear axis and the exposure value (in

mA.min) on the vertical logarithmic axis. For a given kilovoltage (for example 150 kV), we

can, using the previously described intermediate kV-thickness chart, determine that for an

exposure dose of 8 mA.min a density of 2 can be obtained at a thickness of 4.5 mm and for

an exposure dose of 200 mA.min, at a thickness of 15.2 mm.

These thicknesses, and the corresponding exposures, are then plotted on the graph paper

to give points A and B, see figure 4-9. Drawing a straight line linking points A and B, the

150 kV line is obtained. In a similar way the lines for other kV-values can be drawn in the

diagram, eventually resulting in the complete exposure chart of figure 4-9.

9.5 Use of the exposure chart

While it may be possible to gradually build up a store of information which can be consul-

ted in day-to-day work, it is better to make use of good exposure charts. This system has

many advantages to offer, particularly when it comes to choosing the most suitable wor-

king method. Apart from saving time, it gives a guarantee of efficiency and moreover does

away with, or reduces to an acceptable extent, the need for trial exposures on jobs which

are a little outside the normal routine.

Different X-ray tubes can in practice give quite different results, even though they may be

of the same type. Even a different cable length between the control panel and the tube may

be of influence.

Fig. 4-9. Exposure chart for a 200 kV X-ray set

Fig. 5-9. Exposure chart for examination of steel with a Cobalt60 source

Fig. 6-9. Exposure chart for examination of steel with an Iridium192 source

FFD = focus-to-film distance

density=2

Cobalt60

Film type C3

(Agfa D4)

Steel screens

Density 2.5

Iridium192

Film type C5

(Agfa D7)

Lead screens 0.027 mm

Density 2.5

mm steel