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Commercial X-Ray |
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Beryllium is a high-performance material in commercial x-ray applications due
to its unusual combination of properties. Unlike most metals, beryllium has a
low mass absorption coefficient, which means that it is highly transmissive to
x-rays. Most metals are high absorbers of x-rays and other forms of radiation.
This makes beryllium ideal for use as an x-ray window.
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What is an X-Ray Window?
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If one thinks about the principle on which a glass window on a building
operates, it makes understanding how an x-ray window works much simpler. A
glass window on a building performs two basic functions: it acts as a barrier
between the environments inside and outside the building, while at the same
time allowing visible light to pass through it. A beryllium x-ray window is an
opaque piece of metal, often in the form of relatively thin-gauge beryllium
sheet or foil, which follows the same principle. It acts as a barrier between
the vacuum or inert gas environment inside an x-ray source tube or detector and
atmospheric conditions external to the device, while at the same time allowing
x-rays to pass through it. In the case of an x-ray source tube, beryllium
permits the x-rays generated within the tube to be transmitted externally so
that they can be used for their intended purpose. In the case of an x-ray
detector, the beryllium permits external x-rays to penetrate inside of the
instrument so that their presence can be recognized.
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Commercial X-Ray Window Applications
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Most commercial beryllium x-ray windows fall into one of two broad
classifications: medical and industrial/analytical. X-ray source tubes with
beryllium windows are used in a variety of medical diagnostic applications,
including bone densitometry, computed tomographic (CT) scanning, mammography,
and more. The use of beryllium windows in medical tubes has become more
prominent
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since the development of metal body tubes for these applications. On
the industrial/analytical front, beryllium windows are used in tubes for x-ray
diffraction, x-ray fluorescence, baggage inspection, in-situ analysis, and
more. The use of beryllium windows in industrial/analytical tubes and detectors
continues to be of interest for a wide range of non-destructive testing (NDT)
applications requiring imaging or analytical capabilities.
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Manufacture of Commercial X-Ray Windows
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Beryllium foil, in the shape of discs, rectangles, or arc segments, are joined
to the window subassemblies by epoxy bonding, Electron Beam Welding, brazing,
or diffusion bonding. Each technique has its own niche, whether it be the low
cost approach of epoxy bonding that can only be used in low temperature
applications without intense radiation, such as x-ray detectors, or diffusion
bonding, used in many x-ray source tubes as a high volume production method for
window subassemblies that must withstand high heat in the manufacturing
process, high radiation levels, and many cycles of high heat load during use.
Other processing may be done to the window, such as applying a metallic coating
such as Tungsten, Silver, or Rhodium, to act as a transmission window.
Protective coatings, such as BR-127 or aluminum, may be applied to the window
to protect it from harsh environments. The method of manufacture and any
additional processing depends on the end use, but as with all commercial
applications, reliability and flexibility of the supplier is critical to
keeping production running.
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