|NDT.net - February 2002, Vol. 7 No.02|
The radiological inspection has a tradition of more than 100 years and is predominantly based on the combination of radiation source and film. Industrial film radiography uses sources with a radiation energy in the range of 10 - 12000 keV. Special film systems were developed for industrial NDT, which permit an inspection with higher contrast and spatial resolution as compared to medical applications.
X-ray film systems are able to resolve fine structures down to 12 mm (at 100 keV). For a radiation energy > 80 keV films are combined with lead screens in order to intensify the action of radiation. The lead screens generate only a negligible increase of unsharpness as compared to fluorescence screens. Most NDT standards allow, and even require the application of lead or other metal screens only. NDT-films are capable to become exposed up to an optical density of five in an almost linear range. This makes industrial radiography more sensitive to thickness changes than medical applications. The long tradition of film radiography and its excellent properties results in a certain quality level for industrial radiology. This also includes practical advantages of using X-ray films wrapped in a dust and water prove envelope. Dropping down the film in its envelop into dust or mud does not harm the result of the exposure. However, it makes a difference if somebody drops down a 50.000 $ flat panel detector, which is usually not shockproof.
|Fig 1: Radiological detectors of the last 40 years.|
The detector development of the last 40 years is shown in Fig. 1. Promising new developments are technologies like computed radiography (CR) with imaging plates and flat panel detectors, which provoke an upheaval in industrial radiology. The progress is accelerated by the possibilities of the fast developing computer technology. The opportunity to combine digital radiographs with modern and affordable computer technology, corresponding to fig. 2, paves the way for new stationary and mobile intelligent methods. Furthermore, this new digital industrial radiology (DIR) also opens new application fields which are inaccessible with classical radiography.
Since 1980 the radioscopy with the system X-ray tube, manipulator and X-ray intensifier is increasingly covering all on site (not mobile) problems for polymer and light alloy testing. This system has the potential of real time testing corresponding to the TV-standard. The dynamic testing yields a higher probability of detection of all kinds of flaws including cracks. Modern automated flaw detection software makes this method increasingly attractive for automated serial inspection in production lines. Automated car wheel inspection is a typical example here.
Since 1998 new flat radiation detectors are available on the market. These devices may replace the heavy intensifier technology in future. They today already provide a higher spatial resolution and dynamic. The sensitivity is comparable, but the real time ability is limited. Presently, this changes completely the design of on site radioscopy and computed tomography (CT) devices. It can be observed that CT currently expands from a rather specialized method to a routine application.
|Fig 2: Overview about the methods of digital industrial radiology.|
Two types of flat panel detectors are now on the market: The first design is based (see fig. 3) on a photo diode matrix which is read out by thin film transistors (TFT). These components are manufactured of amorphous silicon and they are resistant against high energy radiation. The photo diodes are charged by light, which is generated by a scintillator converting the incoming X- or gamma rays. This scintillator could be a polycrystalline system that provides some additional unsharpness by light scattering or an directed crystalline system which acts like a face plate (fibers in light direction) with lower unsharpness due to reduced light scattering. The next generation of flat panels is based on a photo conductor like amorphous selenium or CdTe on a multi-micro electrode plate, which is read out by TFT's again. This generation provides the highest sharpness and has the potential for high resolution systems which could compete with NDT-film. Currently all available systems on the market reach a resolution of 120 - 140 mm. Weld inspection and fine casting testing requires at least 50 mm resolution. This could be obtained by magnifying technique with mini- or micro-focus tubes. Systems with 50 mm detector resolution are under development for medical applications like mammography.
|Fig 3: Scheme of a flat panel detector: The scintillator converts X- or gamma rays into light, which is detected by the photo diodes. They are read out by thin film transistors (TFT) on the basis of amorphous silicon, which is resistant against radiation.|
Flat panels are suitable for in-house and in-field applications. Nevertheless, in-field applications are limited by the rough environmental conditions in some areas. Digital data may be obtained either by film digitization or directly by the application of computed radiography (CR) using phosphor imaging plates (IP). The IP-reader is always separated from the inspection site. CR is based on flexible IP's which can be exposed and erased up to 1000 times (10000 in medicine). The exposed IP is read out by a laser scanner and needs no chemical processing. IP's are available for spatial resolutions of about 50µm up to 300µm. The latter ones are very fast and can be exposed more than 10 times faster than film. Using adequate IP's and scanners, the image quality is sufficiently for weld inspection up to corrosion detection.
The typical example for an intelligent method is the computed tomography (CT). Only the numeric back-projection of the measured projectional data yields the cross-sectional image information. As a consequence this method requires a computer. In principal CT is the best available method for 3D-analysis of any object as long as differences exist in the density and/or atomic number of the components to inspect. CT is fully developed for laboratory applications but not yet satisfactorily for field applications. The reasons for restrictions in mobile usage are the need for full access from all directions to the object and the requirement of several hundred projections, taken at different angles through the object. Access and measurement time are restricted in most cases.
A solution is provided by a mechanized X-ray inspection system, which is based on the combination of an X-ray line camera and X-ray tube, mounted on a manipulation system. It was constructed to perform a tomographic analysis of welded pipelines. Fig. 4 shows the prototype for inspection of pipe segments. The X-ray tube can be rotated synchronously with the line camera for acquisition of digital radiographs of girth welds. An additional manipulator axis for shifting the X-ray tube parallel to the pipe axis permits to perform the inspection under different angles. Specialized numeric routines were developed to reconstruct the 3D-image of the weld. This method is very sensitive to cracks and lack of fusion. The depths and shape of these defects can be reconstructed and measured. Fig. 5 shows the image of a reconstructed crack in an austenitic girth weld.
|Fig 4: View of the line scanner. X-ray tube and camera are mounted on a manipulater for mechanized inspection.||Fig 5: Profiles of a welded wall of an austenitic pipe after stress crack corrosion. The crack (black) is located near the weld (light area). A and B are reconstructed profiles at different positions.|
The special advantage of the line camera technique is the good collimation of radiation. This reduces the scattered radiation and yields a corresponding contrast enhancement. The system was successfully applied for inspection of water filled pipelines.
X-ray topography uses the scattered radiation only as shown in fig.6 with the typical measurement geometry. The primary radiation is absorbed by a beam stopper. The detector receives the scattered radiation in an defined angle depending on the sample position. Scanning the sample results in a 2D-image (topogram). In addition to the radiography with primary rays, topography provides information about textures, crystal structures, fiber debonding, fiber orientations, porosity and more.
|Fig 6: Principle of X-ray topography.|
As more and more the synchrotron radiation is available it is also included in the process of materials characterization. In this context new methods were developed, such as: X-ray microscopy, micro-CT, K-edge radiography, phase contrast radiography and others.
Public safety is another application field of digital industrial radiology. In addition to the classical radiological inspection, it becomes increasingly important to obtain information about spatial structure and chemical composition of the objects.
Commercially, the dual energy technique is well introduced in modern baggage inspection equipment. It provides an additional information about atomic numbers of the inspected material. Fig. 6 shows a dual X-ray tube set and a typical dual energy radiograph of a bag. This technique provides the operator with color coded images where the color indicates the presence of illegal drugs or explosives. Advanced versions of inspection units include additionally a spectrometer to measure the spectrum of the scattered radiation. This method is very sensitive and allows to detect and specify various chemical compounds in the object. These techniques are also suitable for the inspection of soldered pipes and mixed polymers with high contrast.
||Fig 6: Dual X-ray tube assembly for dual energy inspection of baggage and typical image of a bag. The atomic number of the components in the object are represented by the color usually.
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