Operating Manual

5
NDT equipment
39
5.1 X-ray equipment
X-ray sets are generally divided in three voltage categories, namely:
1. Up to 320 kV, mainly for use on intermittent, ambulatory work. Tubes are generally of
the unipolar alternating current type. Higher voltages are hardly possible with this
type of equipment because of insulation problems.
2. Up to 450 kV, mainly for use on continuous, stationary or semi-ambulatory work,
because of their dimensions, limited manageability and weight.
Tubes are of the bipolar direct current type.
3. Up to 16 MeV, so called Megavolt equipment.
Virtually exclusively applied to stationary work.
The first two categories are suitable for radiography on most common objects. Objects of
extreme thickness, however, require an energy even higher than 450 kV. In this case
Megavolt equipment is used, if alternative sources such as Cobalt60 prove unsuitable.
It will normally involve stationary installations of large dimensions and high weight. Lately,
portable versions have become available meant for ambulatory use.
Types of X-ray tubes
Depending on the shape of the anode, X-ray tubes produce :
a. a beam of radiation in one direction (directional tube)
b. an annular beam (panoramic tube)
X-ray tubes are either unipolar or bipolar.
Bipolar tubes
Figure 1a-5 shows a bipolar tube. The bipolar tube has the advantage that the potential dif-
ference with respect to earth on both the anode and the cathode is equal to one-half of the
tube voltage, which is a great help from the point-of-view of insulation. The exit window
is necessarily situated in the middle of the tube. Bipolar tubes usually operate on direct cur-
rent and are generally air, oil or water cooled. They are designed to operate at voltages of
100 to 450 kV and a tube current of up to 20 mA.
Unipolar tubes
In these (shorter) tubes, as shown in figure 1b-5, the anode is held at earth potential and
the cathode only has a potential difference to earth. This makes anode cooling a simpler
operation. It also means that for low/medium kilo-voltage sets, up to approx. 300 kV as often
used in ambulant applications, a single simpler high voltage supply source will suffice.
The radiation window is placed asymmetric which can be advantageous in practice.
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Fig. 1-5. – X-ray tubes
A = position of target
Panoramic
X-ray beam
Directional
X-ray beam
1a-5 Bipolar tube
1b-5 Unipolar tube
Directional
X-ray beam
Directional
X-ray beam
1c-5 Hollow anode tube giving annular (panoramic) beam
Focus
Focus