Click on the picture to
enlarge. |
The Tube used by Rontgen was
something like this. For a bigger image and more details, click on
the picture. |
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This represents the next step.
Instead of impacting on the end wall of the tube, the electrons now land
on a metal anode. |
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In order to accelerate the
electrons sufficiently to generate X-rays, a minimum of about 40,000v is
needed. Rontgen used an Induction coil, which takes its supply from a
battery. |
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Another way of generating a
high voltage is to use an electrostatic generator, like this Wimshurst
machine. This device can generate high voltages, but not much current. |
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In order to improve the
output, various metals were used as the anode. This tube had a Platinum
foil anode. Since this has a higher atomic number than the more commonly
used aluminum, it gave better results. |
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The tube extending from the
main body of the X-ray tube is used to control the gas pressure. Heating
the tube with a gas flame releases gas from the glass walls of the tube.
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The tube shown is an example
of about the most sophisticated Gas tube to be made. It had a tungsten
target, set in a copper anode. The "T" shaped structure is the
regulator. |
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This is a close-up of the
regulator. An iron wire wrapped around a bundle of asbestos releases a
small amount of gas when energized, giving some control of kV. |
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Modern diagnostic X-ray tube
use a rotating anode, in order to increase the output. About 99% of the
energy put into the tube is converted into heat at the anode. |
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The electron beam is
concentrated on an area of only a few mm2. so that if the anode does not
rotate, the tungsten will melt. The melting point is 3410ºC! |
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The repeated heating and
cooling causes a roughening of the anode surface, and a drop in output. |
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The heating causes the
tungsten to expand and contract, which can cause the anode to crack.
Cutting slots in the anode reduced the risk. |
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In order to increase the rate
at which heat is lost, the back of the anode is chemically blackened.
The front of the anode must be left untreated, as the electron beam
would strip off any coating. |
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The thermal capacity of the
anode can be increased by increasing the mass, but the added weight,
damages the bearings. The whole weight of the anode must be supported by
the bearings. |
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This tube has a composite
anode, made of Tungsten backed by a block of carbon. This gives high
thermal capacity for less weight than a plain tungsten disk. |
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About 20% of the electrons
striking the anode bounce off, to land again outside of the target area.
This causes extra-focal radiation, shown here. |
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The first X-ray Televisions
systems used Image Orthicon tubes, like the one shown here. This has a 3
inch target. |
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This is a photograph of an
original Potter-Bucky diaphragm, or Grid. The modern version is
flat rather than curved. |
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A lot of my time is spent in
teaching. Click on the picture for an example of the presentations that
I have available.
This example is on Electrical Safety.
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The other activity that
occupies a large proportion of my time is electrical safety checks.
There is now a requirement for an Earth Reference Terminal in every
medical location. I have a number of designs of ERTs
available.
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Email: info@xrayengineering.co.uk
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