A Flash x-ray machine.
By Leslie Wright
I was doing a little research on Flash X-ray tubes,and did some patent searches on them. These small tubes, (some are as short as 8 inches and 1 inch diameter) are capable of producing MULTI-MEGAWATT! pulses of x-rays, with energies ranging from 100keV to well into the MeV range!
Such tubes are used for imaging things like very thick cable joints (such as on pylons), and are used by the military to image Nuclear warheads! They are also used to capture short lived events, such as Sabot`s penetrating armour (since the x-ray flash is very short, a few nanoseconds).
I manage to acquire one of these tubes, as unused surplus, for £80. A little googling reveals these tubes, even when new, are far cheaper than other x-ray tubes, roughly £250!
Above: a commercial flash x-ray tube.
Below: There are just two electrodes in a flash x-ray tube. a small pointed anode, and a large cathode, with many small fine points on it. When a sufficiently high voltage, and high current pulse is applied to the anode, electrons (e-) are drawn off the fine points of the cathode by field emission, and accelerated toward the pointed anode. The resulting beam (r) is perfectly circular, and very intense, and emerges through a thin metal or glass end window.
To generate the high voltages, at enormously high currents, a Marx-Generator is employed. As shown in the diagram below, each of the capacitors, is charged up in parallel, and discharged in series, once all the spark gaps (SG) fire.
In the diagram shown below, there are only four stages, but many more stages ar possible. My Marx generator has 10 stages at 30kV per stage, giving me 300kV at upto 5 pulses per second! The Erected capacitance of the Marx generator is 470pF, and I find this is about right, in terms of current, if you wish to try to replicate the setup.
Note the final spark gap (SG*) This gap is very important to the succesful operation of flash x-ray tubes. During operation in air, a marx generator, will always produce some corona between each of the spark gaps. This provides a conductive channnel (albeit high resistance) across each of the gaps, which very loosely connects each capacitor in series, even before the gaps fire!, almost like a CW voltage multiplier. The result, is that a voltage of around 60kV is present on the output sphere of the Marx generator. If we were to connect our cold cathode tube, DIRECTLY to the generator, then the 60kV or so, would soon heat the internal components of the tube in a CW fashion, and this should be avoided.
The purpose therefore, of the final gap, is to ensure we have fast, very steep switching from 0V to 300kV. In my design this consists of two 100mm diameter aluminium spheres placed 35mm apart.
Above: My FXR setup. The output is simply directed to the spark gap, which is connected to the FXR tube anode. The tube is hard to see in the photograph, but is mounted vertically, by means of a sheet of plexiglas. There is no need to oil insulate the tube, since the tube conducts so fast that a leader cannot form, and cause flashover.
Above: A picture of the tube in its mount. Connection to the anode is by the thick black wire. This has a spring soldered onto the end, to contact the anode pin.
Above: A photograph of a fluoroscope screen, placed approximately 30cm under the tube. An old laptop hard-disk, was placed approximately 20cm from the tube window, between it, and the screen. The tiny white speckles, are actually x-ray photons, triggering elemets on the CCD array of the camera! The picture is of a SINGLE flash from the tube.
To take x-ray photgraphs, again, I use multigrade paper, and intensifying cassettes. Exposure times vary from subject to subject, but are generally between 30 and 90 flashes (I set the Marx generator to 1pps, so 30 to 90 seconds).
Above: An x-ray photograph of the same laptop hard-disk, printed positive. The exposure time was 60 seconds at 1pps. the tube to cassette distance was 30cm.
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Copyright © 2008, Leslie Wright, All Rights Reserved.