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Registered Member #1829
Joined: Sun Nov 30 2008, 01:06AM
Location: Raleigh N.C.
Posts: 74
Hello all, I've just started playing around with my vacuum apparatus and I want to make sure I'm safe here. I've done some research and I think I'm ok but I'm also paranoid about ionizing radiation and want to be doubly sure. Here's my knowledge so far: It's possible to create x rays with as little as 120 volts. However, such low energy electrons would need a very hard vacuum and very close cathode / target spacing to create x rays before losing their energy. So far I'm only using a water aspirator and one half of my NST (~10Kv peak) so I'm pretty sure I'm ok. Does this seem correct? Eventually, though, I'll want to move to a rotary pump and maybe a voltage multiplier as my budget allows. Is there a general pressure/voltage curve for x rays or anywhere I could do some more research?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
It is true that X-rays are produced at low voltages, but their energy is so low that they are stopped and absorbed before they can get anywhere.
When you are talking about glass X-ray tubes, then 15kV is roughly the lowest anode voltage that will produce rays capable of penetrating the glass.
You may succeed in producing some X-rays with a very finely tuned, new, two-stage rotary pump with fresh oil in it, but for most purposes it is necessary to use a mercury or oil diffusion pump to get a sufficiently high vacuum for X-ray emission.
So-called cold cathode X-ray tubes (ones without a heater or filament) of the type used in the very early days of X-radiography operates at a higher pressure than modern types, as residual gas ions were actually necessary for them to work properly.
The easiest point of entry for the amateur X-radiographer is to use EHT television shunt triodes such as the European PD500, or the American 6BK4B as X-ray tubes by putting 50kV - 60kV on the anode. Strong X-rays are emitted, but are not as well focused as those from a "real" X-ray tube with a proper target. Nonetheless, X-ray photos of surprisingly good quality can be produced in this way.
Take great care. Remember that low-energy X-rays are much the most dangerous, because they are readily absorbed by your body.
You can detect X-rays with a G-M tube, but not measure them, for which an ionization chamber is required. You can make and calibrate one yourself from a diecast metal box containingg two isolated PCB plates.
Registered Member #1829
Joined: Sun Nov 30 2008, 01:06AM
Location: Raleigh N.C.
Posts: 74
Thanks for the reply. I'm actually trying to avoid accidental irradiation at the moment. My current experiments are a homemade thermionic valve and a cathode ray tube neither of which should need more than 15Kv. If I get a better vacuum I guess I'll get a GM tube with it just to be safe. I'm not much interested in radiography but I do seem to recall some articles online about homemade bubble or cloud chambers. I might make one of those one day and see if I can hit them with some x rays.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
The ionization chamber is the only effective way of measuring X-rays so far as the amateur is concerned. A G-M tube can usually - but not always - detect X-rays, but can not give any information about their energy. Therefore they are as good as useless to the serious experimenter.
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Xrays?
