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Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Linas wrote ... i i have very big x-ray tube, and have 200Kv sstc, is it posible make good x ray machine?? and how tu know, how many pm or nm (nanometre)electromagnetic radiation ?
There is no reason in principle why an X-ray tube could not be driven by a Tesla coil. In the early days of X-radiography, induction coils were widely used to excite X-ray tubes, and AC machines are still in use today, especially for portable work where the weight and size of the machine must be kept to a minimum.
I would forsee your biggest problem being in the matching of the coil to the load. An indirectly heated X-ray tube of the type shown in your picture will draw a sizeable anode current [have a relatively low impedance] and detune your coil to the extent that little power is produced.
What is to be done? You may get best results by running your X-ray tube under starvation conditions. Run the heater from a variable current controlled source, so you are able to gradually turn up the heater current until emission just begins. You may then increase it (so increasing the tube anode current) until you have reached the maximum anode current your Tesla can provide when loaded.
You might make things easier for yourself by first setting up your rig with the coil output wired to the tube anode, and tuning the coil (but at much reduced power) to get maximum tesla output with the unheated tube as the (largely capacative) load. Once you have achieved this, you might then start tweaking up the heater current until the tube starts to draw current.
Thermionic devices don't like it if you apply too much juice before they are warmed up, so I'd suggest only increasing both heater and anode currents in small increments until conditions are stable and X-ray emission starts.
As for the wavelength, the greater the excitation voltage, the smaller will be the value of the minimum wavelength limit. This is known as the Duane Hunt Relationship.
If we imagine that you have matched your coil to the tube anode and your coil is actually able to deliver 100KV to the tube, then from the Duane Hunt Law we can show that the mimimum wavelength produced will be 0.0124 nm.
Finally, you will most likely get some cold cathode emission from the tube if you apply anode voltage with no heater current, so don't assume it is safe just because it isn't heated. The impedance of the unheated tube will be very much higher, so there will be less voltage drop across the tube, and hence more energetic X-rays, though a lot less of em!
Registered Member #1134
Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351
Nice tube! Personally I would avoid the tesla coil approach, since results can be unpredictable. A much better approach would be a ZVS driven flyback (b&w ac type) with an external multiplier. If you are unable to rotate the anode, you need to limit the anode current to under 5mA.
Looking at the tube, I'm guessing you could probably get away with running in air upto 60 kV, after that you will have to sink it in oil.
Registered Member #1134
Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351
Harry wrote ...
Some 'general purpose' beta/gamma GM tubes will fail to detect low-energy X-rays altogether, and none but those specially designed for the job should be relied upon for a meaningful indication of dose.
Low energy X-rays are the most hazardous to human health, so one can't afford to not detect them!
I checked my lowest energy x-ray source (10 keV) today, with a standard Civil Defense CD V700 geiger counter. Even with the counter set on the highest range it will happily peg the meter!
Whilst the reading from a common geiger counter cannot be relied upon, it will at least give a positive indication of the presence of x-rays.
CD V700 Geiger counters can be had off of eBay, at a very moderate cost, and I encourage anyone working with radiation, or suspecting that something may be emitting radiation, to purchase one.
Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4059
Harry wrote ...
Unless they are specifically designed for detecting the low energy X-rays likely to be produced here, GM tubes are not at all a good way to detect them.
X-ray sensitive GM tubes are constructed much like a longer version of an end-window alpha tube, but with a thicker mica window. A heavy fill gas like argon or krypton is usually used, and at a much higher pressure than in other types of GM tubes - typically 600 - 650 torr.
Some 'general purpose' beta/gamma GM tubes will fail to detect low-energy X-rays altogether, and none but those specially designed for the job should be relied upon for a meaningful indication of dose.
Low energy X-rays are the most hazardous to human health, so one can't afford to not detect them!
Hmm. Well, I discovered by accident that an off-the-shelf B&Q el cheapo CMOS camera makes a fairly nice spinthariscope, using Am-241 source and a thin mica window.
Might be worth trying this with a thin (beryllium?) window to exclude light, or (my own method) silver paint coated mica from a heatsink, cut down to minimal thickness. Pyrolytic graphite might work just as well being pure carbon, and is simpler to pare down (you can sand it!) :)
If anyone does get this working please let me know as my lab is currently in a mess.
regards, -A
"Bother" said Pooh, as he generated a black hole at the LHC...
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
For anyone who doesn't know of it already, the Rad Pro Calculator For Desk Top PCs (freeware) allows you to enter X-ray tube voltage, tube current, a variety of metal filters of graduated thickness, and a variety of shielding materials, to estimate the dose rate at a given distance.
It can do a lot of other interesting things besides.
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X-ray :D
