Operating Manual

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5.2 High voltage generators
Conventional (trans)portable X-ray equipment for use up to approximately 300 kV are pro-
vided with step-up HT transformers, rectifiers and smoothing capacitors. The X-ray tube
and the circuitry of this equipment are usually placed in an insulated tank. In most cases
these tank type sets use oil for insulation and cooling and weigh approximately 60 kg.
Gas is used when weight is important; the set than weighs approximately 30 kg.
Figure 3-5 shows an integrated
(all-in-one) tank set for 300 kV
with an asymmetric window. At
voltages over 300 kV housing eve-
rything in one tank becomes very
difficult because the high voltage
insulation would be inadequate.
Figure 4-5 shows a direct current
X-ray tube with a symmetric window.
Equipment up to 450 kV operating
on direct current is connected to a
separate high tension (HT) supply
unit by means of HT leads. As a result this equipment is bigger and heavier than “all-in-one”
tank sets and mostly meant for stationary or semi-ambulant use.
The 300 kV “all-in-one” tank set and the 450 kV direct current X-ray tube only are of
roughly the same dimensions.
Most tank sets are connected to a mains power supply with a frequency of 50 or 60 Hz. At
this frequency the supply voltage can be transformed upward.
This is followed by rectifying, which occurs in various forms. With some sets the X-ray tube
itself functions as rectifier, so called single-phase rectifying. If there is no smoothing
applied, considerable changes in voltage per cycle of alternating current will occur.
This periodic and greatly varying high voltage affects the intensity and spectrum of the
radiation generated, see section 4.3.
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Special types of X-ray tubes
Unipolar X-ray tubes with a long hollow anode, as shown in fig. 1c-5, are generally known
as “rod anode tube” and can be inserted into pipes or vessels. These tubes produce an
annular (panoramic) beam over 360º, so allowing a complete circumferential weld to be
radiographed in one exposure.
Figure 2-5 shows the conical anode
of a (360º) panoramic tube, which
allows a circumferential weld to be
radiographed centrally, hence uni-
formly, from within. With this
anode the axis of the electron
beam must strike the top of the
cone in such a way that the centre
of the generated X-ray beam is
perpendicular to the longitudinal
axis of the tube.
Note: Anodes shaped so that the cen-
tre of the generated X-ray beam is
not perpendicular (oblique) to the
centre line of the tube (which was
acceptable in the past), are no longer
allowed when work is to be perfor-
med to official standards.Tubes that
produce a real perpendicular beam
are known as "true panoramics"
There are also panoramic tubes in which the electron beam is focused over an extended
length by means of a magnetic lens or an electrostatic lens (Wehnelt-cylinder) to produce
a very small focal spot size. These sets are called microfocus rod anode tubes with which a
very small focal spot size, of less than 10 micrometers, can be achieved. Since the anode
can be damaged relatively easy through overheating the anode is usually interchangeable.
This requires a separate vacuum unit in order to restore the vacuum after replacement. The
advantage of this construction is that with different types of anodes, different radiation
patterns can be obtained for special applications. The maximum energy level is usually
below 150 kV.
However, there are 150 kV microfocus tubes with a fixed anode for enlarging or scanning
purposes, see section 17.1. With these tubes the tube current has to be kept low, because of
heat dissipation limitations of the non-interchangeable anode.
Some X-ray tubes used in the radiography of plastics and aluminium are equipped with a
beryllium window to allow the softer radiation generated at the lower tube voltages of
5 to 45 kV, to pass.
Fig. 2-5. Anode configuration for an annular panoramic tube
Fig. 4-5. Direct current X-ray tube for 450 kV with a symmetric window
X-ray
electron beam
cathode
X-ray
anode
filament
Window
Window
Fig. 3-5. “All-in-one“ 300 kV tank set with an asymmetric window