Operating Manual

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Two types of X-ray tubes exist:
• The closed X-ray tube, a sealed evacuated glass tube containing all components to
generate X-rays. No part in it can be replaced or repaired
• The open X-ray tube with a removable/replaceable anode/target and filament
with its own high vacuum system for an almost unlimited life.
A closed system is cheaper and maintenance-free, but has a shorter life time than an open
tube. An open tube, with an almost unlimited life-time, can operate at higher voltages and
currents. Replacement of target or filament once damaged requires less than half an hour;
a very acceptable down-time.
Using micro- or nanofocus X-ray tubes has the following advantages:
• Very small defects are discernible
• Low backscatter because a small part of the object is being irradiated
• High resolution.
Disadvantages are:
• Costly if (separate) high-vacuum equipment is required
• Time-consuming, as for each high resolution exposure only a small part of the
object is being irradiated.
Tube heads
There are two types of tube heads for small focus X-ray tubes.
Figure 2-17 illustrates the two types.
The transmission tube provides the highest magnification (smallest focus).
The directional tube, as common in standard X-ray tubes, provides the highest energy.
This figure also shows the magnetic lenses that create the essential focussing of the
electron beam.
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17.1.2 High resolution X-ray microscopy
Magnification factors
For a number of years magnification factors up to 25 were sufficient. The maximum magni-
fication factor was determined by the smallest possible focal spot size. As illustrated in figure
1-17 larger magnification factors create unsharp images without providing more informa-
tion. Moreover the intensity of the output is limited by the heat dissipation of the target
anode. For some time this was a physical barrier. With the introduction of microfocus and
more recently nanofocus X-ray tubes, new techniques have been developed for inspection of
low absorbing objects like electronics, applying large magnification factors with still high
resolution.
Because of the need to inspect parts with ever decreasing dimensions, such as electronic
components and their joints or other products with extreme quality requirements and pro-
duct process control, ever growing image magnification factors were necessary, ending up
in so called “X-ray microscopy”.
The need for geometric magnification factors of up to 2,000 are no exception. This was the
incentive to develop X-ray tubes with extreme small spot sizes. Because there is a physical
limit to the minimum focal spot size (limited heat dissipation and output) other measures
(tricks) can be taken: a combination of software, lenses and cameras to further zoom in,
even up to 25,000 . It should be realised that magnification only make sense if the initial image
quality is sufficient, a poor image just creates bigger pixels or results in a vague image.
Microfocus and nanofocus X-ray tubes
Over the years industry developed X-ray systems with ever decreasing focal spot sizes to
meet the need for large magnification factors . At present focal spot sizes expressed in a few
hundreds of nanometres (nm) are on the market.
By conformation, manufacturers of X-ray tubes classify their tubes dependent on focus size
in a few categories:
• macrofocus with spot sizes > 100 microns (0.1 mm)
• microfocus with spot sizes ranging from 1 micron up to 10 microns
• nanofocus with spot sizes far below 1 micron.
Sizes of down to 0.25 micron (250 nm) do exist.
The Nanofocus** (see acknowledgements in chapter 20) X-ray system is just one
example of such a system.
The output of nanofocus X-ray tubes is proportionally lower than for tubes with larger
focal spot sizes. Heat dissipation of the target anode generating the X-ray beam puts a limit
to the output. The smaller the target the lower its output. Over-heating destroys the anode
by burn-in.
Magnetic
lens
Target
X-ray beam
Window
X-ray beam
Fig. 2-17. Transmission- and directional small focus tube heads
Target