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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.

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In recent years, microfocal systems have become more commercially available as an NDT tool.

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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.

 

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(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.

(a)
(b)
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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.

 

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