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CERN: monitoring superconductors via digital image analysis
CERN, located near Geneva, Switzerland, is building a new proton synchrotron, a circular accelerator for proton particle collisions. There are more than 1200 magnets in use for keeping the protons on course. The superconducting wire used for the magnet coils has an inner filament structure that is inspected via automated particle analysis based on metallographical cross sections. For the digital acquisition of the microscope images, an Olympus BX51M microscope is used. The digital camera mounted onto the microscope and the image analysis software are by Olympus Soft Imaging Solutions.
CERN (French Conseil Européen pour la Recherche Nucléaire) is located in Meyrin near Geneva in Switzerland and is the world's largest research institution for particle physics. There are currently 6500 visiting scientists from more than 80 countries using the CERN accelerator facilities for their experiments. CERN has 2600 regular employees that ensure everything runs smoothly. CERN was founded in 1954 by 12 European nations as a joint project and celebrated its 50th anniversary in 2004. There are 20 member countries of the CERN organization that cover the operating costs. CERN bylaws mandate that research done here may not serve military purposes and that all results must be made publicly available
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 Figure 1:
Those interested in researching into the smallest building blocks matter is made up of need the largest instruments. CERN, near Geneva, Switzerland is where the most powerful circular accelerator in the world is being built: the Large Hadron Collider (LHC) for proton collisions. It has a circumference of 26.7 kilometers. The subterranean tunnel of the accelerator is indicated by the large circle.
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In the very center of things, what keeps things together?
CERN's primary task is the construction and operation of particle accelerators and detectors for investigating the most minute structures of matter. In these accelerators, particles are accelerated almost to the speed of light and collided head-on. Due to the high amounts of energy thus released, new particles are born that are verified and investigated by the detectors. The results of these experiments help to answer fundamental questions, for instance what matter is made of and what forces hold it together. Or as the 18th century German author and scientist Johann Wolfgang von Goethe had Faust state, his world-renowned character, "That I may know, what holds the world together in its innermost regions."
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 Figure 2:
There are a total of 1232 superconducting guiding magnets placed along the LHC accelerator ring for keeping the protons on course. Each of these magnets is 14 meters long. The two beam pipes in the interior are surrounded by superconducting magnet coils. A special metal compound wire, which superconducts at the operating temperature of 1.9 Kelvin, is used for the coils. The electricity going through the coils is capable of generating the required magnetic field of 8.4 teslas. To the lower right, four coil segments are shown.
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LHC: unattained particle energies
Currently, there is a new accelerator under construction at CERN – the most powerful tool ever built for investigating particles of matter and their properties. The Large Hadron Collider (LHC) is a circular accelerator in which protons are accelerated to an energy of 7 Tera electron volts. The LHC, which is to go into operation in 2007, is presently being installed in a subterranean circular tunnel with a circumference of 26.7 km, which has already housed the former LEP accelerator. An extremely powerful magnetic field of 8.4 Tesla is necessary to make the protons follow the curved path of the accelerator. Superconducting guiding magnets are needed for such a powerful magnetic field. Altogether there are 1232 of these magnets installed along the accelerator ring. Each of them is about 14 meters in length and weighs 30 tons. In order to allow the high current of about 12,000 amperes to flow through the superconducting magnet coils the magnets are cooled down to 1.9 Kelvin with superfluid helium.
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220,000 kilometers of superconductors
For the fabrication of the LHC magnet coils about 220,000 kilometers of superconducting wire are needed. The wire, which has a diameter of approximately 1 millimeter, consists of about 8000 fine fibers (filaments) made of a niobium-titanium alloy that are embedded in a copper matrix (fig. 3). Numerous metallurgical processing steps are necessary, such as extrusion and drawing, to reduce the diameter of initially 25 centimeter niobium-titanium bars down to 6 micrometers, which is the final filament diameter.
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 Figure 3
This is what the metal compound wire looks like from the inside (cross section). About 8000 niobium-titanium filaments are embedded in copper. The distribution of the fibers and the shape of the cross sections of the fibers affects the superconducting properties. This makes it essential for CERN scientists to be able to automatically investigate and evaluate these wire cross sections using digital image analysis.
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A magnetic effect that is not desired
During this process, the initially round niobium-titanium bars can become strongly deformed. The dimensions, the shape and the distribution of the fine filaments within the copper matrix affect the physical properties of the finished wire, such as the superconductor magnetization. Quantitative optical metallography of wire cross sections allows to determine how the geometrical and physical properties are interdependent upon one another. To obtain statistically, reliable results many niobium-titanium filaments have to be evaluated. This is why the scientists record the light microscopical images of the cross sections digitally and use automated detection and analysis for the image processing.
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A digital look at wire cross sections...
Preparing the wire cross sections for investigation involves first embedding them in epoxy resin. This ensures that the cross sections are precisely perpendicular. Then the cross sections are ground down with wet 600 and 1000 grit silicium-carbide paper. Mechanical polishing is done with polycrystalline diamond-particle suspensions (9 μm und 3 μm) and a colloidal silicium-dioxide suspension (0.1 μm). Without any further chemical processing, the cross sections thus prepared are then placed under an Olympus BX51M reflected light microscope. A ColorView digital color camera by Soft Imaging System mounted onto the microscope transfers the microscope images directly to the computer.
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...on to automatic classification
Figure 4a shows a true-color acquisition of a cross section the way it looks when it is transferred by the camera to the analySIS image buffer. analySIS is the software environment installed on the computer and developed by Soft Imaging System for digital image analysis and digital image management. The software offers the functions necessary for acquiring, further processing, interactive/automatic analysis, archiving and documentation of the digital images and related investigation results. The software subjects the original image to a shading correction first. This means that the uneven illumination of the sample under the microscope is subtracted digitally. Then the true-color acquisition is converted to an 8-bit gray-value image and the niobium-titanium filaments are separated from the copper matrix based on defined thresholds – all by the software. The Region Of Interest, or ROI is the area which is to be investigated and is defined interactively (yellow line in fig. 4b). Then a digital particle analysis is conducted. The software detects the various filament cross sections as separate particles and measures shape and size automatically.
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 Figure 4:
The image analysis software subjects the true-color image of the wire cross section (a) to a shading correction and converts it to a gray-value image. The niobium-titanium fibers appear darker than the copper surrounding them. The image segment to be analyzed is defined via the mouse and has a yellow border (b). The software detects the niobium-titanium fibers within the area selected and measures the shape and size of the cross sections of the fibers.
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A round and less a round
A sample shape analysis: quantitatively investigating deformation means determining the shape factor of each particle. The shape factor describes just how round a particle is. Particles, ie, cross sections that are perfectly circular have a shape factor of 1. The less round a particle is, the lower its shape factor. The mathematic formula is: 4*π*A/U2, where A is the area; U is the circumference of the particle. All measurement results are automatically entered into a sheet. Based on shape factor, particles (ie, the filaments) are all classified, thus categorizing them into shape classes. The results of all measurements are then statistically evaluated – either within analySIS or data is exported into another application. One thing statistical evaluation allows is correlating the degree of filament deformation with the superconductor magnetization (measured separately).
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Class at a glance
If so desired, the software presents classification results graphically. The filaments measured within the ROI are highlighted in color (fig. 5). This enables researchers to see at one glance which filament belongs to what shape class and the distribution of the various shape classes within the ROI, thus indicating to what extent the wire being investigated is deformed. The least deformed filaments are black; those with the highest degree of deformation are red. The color highlighting does not affect image data in any way. The software displays the highlighting in the image overlay, which is like a digital transparency placed over the image.
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 Figure 5:
The niobium-titanium filaments may be more or less deformed due to the production process. The image analysis software sorts the filament cross sections into classes of shape for statistical evaluation. Filaments that are perfectly round have a shape factor of 1 and are put into class 5 in this example. The cross sections are highlighted in a color corresponding to the class they are in. This makes it easy to see the degree of deformation and distribution thereof onscreen. Statistical evaluation helps to understand the correlation between degree of deformation and the superconducting magnetization, measured separately.
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Estimating magnetization
Evaluation of the measurements can show how the filament deformation during the production process can affect the superconductor properties. To put it simply, the greater the filament shape deviates on average from the ideally round shape, the greater the undesired superconductor magnetization. Take the filaments in 5b, for example. These are more deformed than those in figure 5a. The average shape factor of the filament cross sections in figure 5b – which was determined via image analysis – is 0.41. And this value is less than the shape factor determined for figure 5a: 0.51. The related superconducting magnetization is greater, however. Once there are sufficient measurements available, it is possible for the researchers at CERN to estimate the magnetization from the average filament deformation in the wire cross sections.
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Authors
This text was made possible by the kind support of Christian Scheuerlein, European Organization for Nuclear Research (CERN), AT-MAS-SC (magnets and superconductors), Geneva, Switzerland
Image source
CERN. reflected light microscope acquisitions with a ColorView on an Olympus BX51M microscope.
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