Microradiography
as Strong NDT Tool
by
Bahman Zoofan*
|
The magnifying capabilities of conventional
X-rays tubes are limited due to focal spot sizes in the range
of millimeters. Magnification of more than two to three times
can result in unsharp, fuzzy images. Microfocus tubes were developed
with focal spot sizes measuring tens of micrometers; this was
the birth of microradiography. Even though today's techniques
are reasonably applied to predominantly thin or low absorbent
material, drastic advances in digital radiography are expected.
Dietmar Henning
Contributing Editor |
INTRODUCTION
It has been reported that when
Wilhelm Roentgen discovered X-rays, it took only one month to
determine most of their properties. From an early time, radiographers
have been interested in the resolution of the radiographic image. Due
to the relatively large focal size of conventional radiographic
systems, geometrical unsharpness significantly limits the image
resolution. An analogy for this effect is the size of a light source
and the sharpness of the resultant shadow. In the very early years of
radiographic development with X-rays, some literature referred to the
technique as shadowgraphy, to describe image formation by
electromagnetic radiation.
In an attempt to produce magnified images with X-rays, two English
engineers, C.T. Heycock and F.H. Neville, tried to generate magnified
images by passing the beam of X-rays through a lead collimator with a
fine hole (Poen, 1959).
The setup that they used, which is shown schematically in Figure 1,
is limited in its effectiveness as it only allows a small portion of
the X-ray beams to pass through and it is not very practical to make
fine round holes in thick lead.
In
recent years, microfocal systems have become more commercially
available as an NDT tool.
The need to observe fine details in different applications, such as
microjoining, electronics, materials testing, biology and medicine,
led radiographers to the development of techniques that optically enlarge
high resolution radiographs. Figure 2 shows an example of a small electronic
part taken with a conventional tube without any magnification by R.C.
McMaster around 1960 at the Ohio State University. He used a very large
source to film distance to make the radiograph as sharp as possible.
The radiograph was enlarged by printing it on photographic paper with
dimensions of 203 by 254 mm (8 by 10 in.). Using this technique, he
was able to detect fine porosity in soldered joints, which would be
very difficult to resolve with a conventional image. This work illustrates
the benefits that can be achieved by enlarging high resolution images
and thus set the direction for the development of microradiography.

Figure 1 - An early attempt
to make magnified radiographs.
|
(a) |
 |
|
(b) |
 |
Figure 2 - Radiograph of an electronic
part taken with a conventional X-ray tube: (a) with no magnification;
(b) printed magnified on photographic paper and showing fine porosity
in the joints.
The need for high resolution X-ray imaging techniques posed
challenges for NDT engineers and pushed the design and manufacture of
affordable microfocus tubes. It is likely that technical progress in
this field was helped by the manufacture of small powerful vacuum
systems and reliable electronic control units.
To see the effect of the source size on the resolution of X-ray
images, we should look at the basic concept of geometric unsharpness
in radiography.
Considering the geometric unsharpness Ug in a radiographic image
with an effective source size F, source to detector distance a and
source to object distance b then:
| (1) |
|
Defining the image magnification M, like any other imaging
technique as:
| (2) |
|
Then the geometric unsharpness (Equation 1) can be written as:
| (3) |
|
This simple relation shows that in order to enlarge the
radiographic images with enough sharpness, the source size should be
in the range of a few micrometers. A simple classification of X-ray
tubes considers the focal spot sizes between 2.5 to 5 mm (0.1 to 0.2
in.) as large, less than 2.5 mm (0.1 in.) as small and less than 100
µm (3.9 x 10-3 in.) as fine (recently tubes with 0.5
µm [2 x 10-5 in.] focal spot size have been advertised).
Figure 3 shows the effect of the source size on the geometric unsharpness
of the radiographic image. Figure 3a shows a magnified image with conventional
radiography using a large focal spot size. The geometric unsharpness
in this case is unreasonably large and completely destroys the clarity
and fine detail within the image. The recommended technique for reducing
the unsharpness with a large focal spot radiographic source is to place
the object and the detector (film in this case) in close contact as
shown in Figure 3b. Figure 3c illustrates how a fine source can be used
for projection magnification and still result in negligible unsharpness
in the produced radiograph.

Figure 3 - Relation between geometric unsharpness
and source size: (a) conventional radiography with large focal
size, providing an enlarged image with large geometric unsharpness
(Ug 1); (b) conventional radiography with large focal size,
providing a sharp image with small geometric unsharpness (Ug
2); (c) microradiography, providing enlarged image with small
geometric unsharpness (Ug 3); SOD = source to object distance,
SFD = source to film distance.
Figure 4 - The relation
between focal spot size and geometric unsharpness for different
projection magnifications.
Figure 4 shows the geometric unsharpness versus focal spot size for
different magnifications (based on Equation 3). From this figure, it
is clear that large magnifications are only practical when fine focal
spot sizes are used. To obtain an image with a geometric unsharpness
of less than 0.2 mm (7.9 x 10-3 in.) an X-ray source with
a focal size smaller than 100 µm (3.9 x 10-3 in.)
should be used. The selection of the optimum projection magnification
for a specific source size depends on different conditions and needs
more discussion.
The use of a microfocal system has been traced back to 1960 (Fontijn
and Boogerd, 1969), but the applications were limited to some research
centers (Boogerd et al., 1976; Fontijn, 1975; Parish and Cason, 1977).
In recent years, microfocal systems have become more commercially
available as an NDT tool. In general, the capability and real
potential of microradiography is not well known by many industrial
radiographers who are used to using conventional radiography. This is
partly because few microradiography publications are available and
most codes and radiographic standards recommend that the geometric
unsharpness should be kept as small as possible by positioning the
sample as close as possible to the detector or film.
Therefore, the concept of projection magnification, in which the object
is far from the detector, is unfamiliar to many professional radiographers.
Figure 5 shows examples of sharp, enlarged radiographs taken with a
microfocus tube.
The other advantage of microradiography is that by keeping the specimen
a long way away from the detector, the scattered radiation generated
inside the object does not reach the detector. This reduction in the
scattered radiation can improve the clarity of the image dramatically
when compared to conventional radiography. Figure 6 shows an enlarged
real time microradiographic image of a US dime with 4x magnification.
The high contrast and clarity of the image reveal the fine surface contours
on both sides of the coin.
The high resolution images obtained by microradiography with a minimal
amount of scattered noise can be further magnified optically to improve
the detectability of details, as shown in Figure 7. This microradiograph
shows fine shells used for geological research.

Figure 5 - Applied microradiography: (a)
a solder joint in a small copper heat exchanger indicating excessive
porosity in real time microradiography with 36x magnification;
(b) a computer chip with microjoint wires (each individual wire
is 25 µm [9.8 x 10-4 in.] in diameter).

Figure 6 - Real time microradiograph of
a US dime with 4x magnification taken with a 5 µm (2 x 10-4
in.) focal size microfocus.

Figure 7 - Film microradiograph
of fine shells: (a) with 16x magnification; (b) optically magnified.
The shells were taken from samples of mud under a lake to study
its geological history.
DIFFERENT FEATURES OF MICROFOCAL TUBES
Hairpin Filament
One of the main differences between a conventional tube and a microfocus
tube is the design of the filament (cathode) inside the tube (Figure
8). In order to have a fine focal spot in the tube, the beam of electrons
that hits the target should be as fine as possible while keeping an
acceptable energy level. To achieve a fine beam of electrons, filaments
are bent into a hairpin shape (Figure 8c). The material of the cathode
is usually tungsten, with wire thickness of about 0.2 mm (7.9 x 10-3
in.). Other types of materials are sometimes used for special applications.
Demountable Tube Head
A consequence of using a hairpin filament in microfocus tubes to
produce a fine electron beam is the short life, which requires the
cathode to be replaced after only limited hours of operation. The
microfocus tube heads are therefore designed for plug-in replacement.
Since the seal and vacuum are broken during replacement, it is also
necessary to reestablish vacuum before reenergizing the tube.
Typically, microfocal systems have a small and efficient vacuum system
consisting of a rotary pump in conjunction with a turbomolecular pump.
The volume of the tube head is an important factor in the design of
microfocal systems and can be tuned for a normal working environment
so that evacuation only takes a few minutes. Another important factor
in the design of a microfocus tube head compared to a conventional one
is having a short distance between the target and the tube window.
This distance should be in the range of a few millimeters, compared to
several centimeters for a conventional one. This distance is important
because the object should be close to the tube window to achieve a
large projection magnification. As is apparent, all these factors in
designing a tube head for a microfocal system present many engineering
challenges.
Electron Beam Focusing
After the operational vacuum is reached inside the tube, the electron
beam can be accelerated by the applied high voltage at the anode that
helps the electrons pass through an opening and emerge as a fine
accelerated beam toward the rotary target.
The focusing unit is an electromagnetic, asymmetrical electron
lens. The centering unit contains the electromagnetic equipment for
correcting any deviation of the electron beams with respect to the
tube axis.
Smart Control Unit
It can be imagined that a microfocus tube with so many features needs
a smarter controller when compared to a conventional unit. A control
unit typically indicates the status of the vacuum, automatic warmup,
filament condition, setup for kilovoltages and applied tube current
and exposure time.
The control unit and associated computer programs also control the
output power and automatically expand the focal spot size if the
applied power is increased beyond its setting to prevent burning the
target. The control unit continuously indicates whether the X-ray tube
is in the microfocal mode or not. Usually fiber optics are used for
fast and reliable communication between the control unit and the
generator.
Microfocal systems for Failure Analysis
The combination of a microfocal tube with a real time imaging system
and precise mechanical manipulation equipment with multiple axes to
accurately position objects in front of the X-ray tube is a powerful
NDT tool, particularly for failure analysis purposes. Figure 9 shows
the results of a test on a safety switch typically used to prevent overcharging
of an electronic device.
Figure 8 - Cathode design in conventional
radiography and microradiography: (a) the tube gun for a conventional
ceramic X-ray unit with dual cathodes; (b) a plug-in cathode for
a microfocus tube; (c) a typical hairpin filament arrangement
for a microfocus tube.
|
(a) |
 |
(b) |
 |
Figure 9 - Safety switch: (a) microradiograph
showing traces of sparks and depositions on the contacts; (b)
actual size.

Figure 10 - Phase contrast
image of a honeycomb structure showing the outline of an area
not having enough glue between its layers. Image was taken with
a 5 µm (2 x 10-4 in.) microfocal system on high resolution
film with optimized object to film distance.
Phase Contrast Imaging
One of the interesting features of microfocal systems is their ability
to create phase contrast images. This new and promising imaging technique
needs fine focus tubes or X-ray radiation with high spatial coherence.
This imaging technique can be used to give enhanced images of low absorbent
materials, which inherently produce low contrast radiographs (Zoofan
et al., 2005). The technique is based on small, refracted angles and
phase shifts in the X-rays after passing through an object that has
a density gradient. Figure 10 shows an example of a phase contrast image
taken by a 5 µm (2 x 10-4 in.) microfocal system.
Some limitations of microfocal systems should be mentioned here as
well. One of the first limitations is their inherent low output
wattage. The applied current in a microfocal system is in the range of
microamperes, compared to milliamperes in conventional tubes. The fact
that microdiscontinuities are more critical for fine structures with
thinner thicknesses makes microfocal systems more practical for
testing of thin, low absorbent materials. Electronic components,
biological samples, plastics and parts with moderate thicknesses of
aluminum and titanium are great candidates for microradiography. The
second limitation of microfocus tubes is their high price compared to
conventional units. The higher price range often prevents their use in
companies with limited capital resources. Maintenance and costly
repair can be considered another limitation of these systems. The last
major limitation of a microfocal system is the lack of practical and
widely accepted techniques for the measurement of exact focal spot
size. Customers generally have to accept the claim of the tube
manufactures when they refer to the focal spot size of their
microfocal systems.
Due to the fact that microradiography is a relatively new imaging
technique, there is no related code or standard available. The finest
1 T hole in thin penetrameters can be magnified and made visible by
this technique. Even test patterns with 10 lp/mm (254 lp/in.) are not
fine enough to check the performance of a system. The best way to
evaluate the resolution of a microfocal system is by using fine
resolution patterns with details in the range of a few micrometers,
used for scanning microscopy.
Despite its limitations, microradiography is becoming a powerful
NDT tool and is continuing to be innovatively applied in areas where
traditional radiography cannot meet the needs of industry, academia
and research.
REFERENCES
Boogerd, W.J., P. van Zuylen and L.A. Fontijn, "An Unconventional 150
kV Microfocus X-ray Equipment for NDT Purposes," British Journal of
NDT, Vol. 18, November 1976, pp. 175-178.
Fontijn, L.A., "Mini-magnetic Lenses for Microfocus X-ray
Applications," Journal of Vacuum Science and Technology, No.
12, 1975, pp. 1359-1362.
Fontijn, L.A. and W.J. Boogerd, "New Demountable 0.1 mm X-ray Tube,"
Metal Construction, Vol. 1, No. 12, 1969, p. 585.
Parish, R.W. and D.W.J. Cason, "High Definition Radiography of Cast
Turbine Blades As a Method of Detecting and Evaluating the Incidence
of Microporosity," NDT International, August 1977, pp. 181-185.
Poen, O.S., Microprojection with X-rays, Dordrecht, The
Netherlands, Martin Nijhoff, 1959.
Zoofan, B., S.I. Rokhlin and G.S. Frankel, "Application of Phase
Contrast Microradiography in NDT," Materials Evaluation, Vol.
63, 2005, pp. 1122-1127.
* The Ohio State University, 190 Edison, 1248
Arthur Adams Dr., Columbus, OH 43221; (614) 292 3012; e-mail
<zoofan.1@osu.edu>.
Characterization Sciences Group, Pacific Northwest
National laboratory, PO Box 999, MSIN: K5-26, Richland, WA 99352.
Copyright © 2005 by the American Society for Nondestructive Testing, Inc. All
rights reserved.