Operating Manual

Examples :
15 mm steel: 100 + 15 x 8 = 220 kV
12 mm aluminium: 50 + 12 x 2 = 74 kV
10 mm plastics: 20 + 10 x 0.2 = 22 kV
In the range 200-400 kV, only a significant change in voltage, say 30-40 kV, will cause a
noticeable difference in defect discernibility.
Selection of gamma source
As it is not possible to vary the radiation energy emitted by a gamma-ray source, it is
necessary to indicate a range of thickness which may be satisfactorily examined with
each type of radio-isotope.
The upper limit is decided by the source strengths commercially available and the maxi-
mum tolerable exposure time: the lower limit is determined by the decrease in contrast
and the related reduced image quality.
The lower limit, therefore, depends on the required degree of defect discernibility.
When this is insufficient in comparison to what is achievable by the use of X-ray equip-
ment, another type of isotope providing a reduced energy radiation could be selected.
Table 3-11 shows the thickness range usually recommended for various gamma sources.
The table applies to steel. If, for reasons of convenience, gamma rays are used on thin
specimens which could also be X-rayed, it should be understood that the resulting radio-
graphs will be of poorer quality compared to X-radiographs.
103102
11.3 Other considerations with regard
to the source-to-film distance
Inverse square law
As explained in the previous section, the effect
of U
g
can be reduced by increasing the focus-to-
film distance F.
One of the properties of electromagnetic radia-
tion is that its intensity is inversely proportional
to the square of the distance, better known as
the “inverse square law”. Both X - and Gamma
radiation follows that law.
The intensity of radiation per unit area of film is
inversely proportional to the square of the sour-
ce-to-film distance (s-f-d).
As figure 6-11 shows, at a distance 2F from the
source, a beam of rays will cover an area (b)
four times greater than area (a) at distance F.
Consequently, the intensity per unit of surface area for (b) will be only 1/4 of the value
for area (a). This means that, all other parameters being equal, at distance 2F exposure
time must be multiplied by four to obtain the same film density.
This principle obviously has its (economical and practical) limitations, beyond which a
further increase in s-f-d is just not feasible.
Selection of radiation energy (kV)
Once the appropriate source-to-film distance is chosen, the correct kilo voltage can be
determined from an exposure chart (see chapter 9).
The importance of choosing the exact kilo voltage varies considerably with the kilo vol-
tage range being considered. For X-rays below 150 kV the choice is reasonably critical
and gets more critical still at lower kilo voltages.
The kilo voltage to be applied is specified in (EN) standards, see chapter 20.
Table 2-11 gives useful empirical rule-of-thumb values for radiographs of aluminium,
steel or plastic objects.
Fig. 6-11. Inverse square law for distances
Table 3-11.Thickness ranges in mm for examining steel with the usual types of gamma sources.
Note: Standard sensitivity permits a slightly poorer image quality than high sensitivity.
Thus a larger thickness range can be inspected coping with the quality requirements.
Table 2-11. Rule-of-thumb values for the selection of kilo voltage
Material kV-value
Steel 100 kV + 8 kV/mm
Aluminium 50 kV + 2 kV/mm
Plastics 20 kV + 0.2 kV/mm
Source type Standard sensitivity High sensitivity
technique in mm technique in mm
Co60 30 - 200 60 - 150
Ir192 10 - 80 20 - 70
Se70 5 - 40 10 - 30
Yb169 1 - 15 3- 10
Tm170 1- 10 4 - 8