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Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Xray wrote ...
I have a couple of very small tubes (about the size of a large walnut) that have a portion of the thick glass ground down to a thin glass window. The thin glass may not be as transparent to X-rays as beryllium is, but it should work just fine for X-raying flowers and seashells, etc. Let me know if you want one of them and maybe we can work out a trade.
15 keV would be a good place to start for someone interested in the low energy end of the spectrum, because some ordinary off-the-shelf detection and measurement equipment and techniques can still be used here. For example, mica end-window GM tubes and home-made Al window ionisation chambers can still be expected to produce a signal.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1715
Proud Mary wrote ...
Xray wrote ...
I have a couple of very small tubes (about the size of a large walnut) that have a portion of the thick glass ground down to a thin glass window. The thin glass may not be as transparent to X-rays as beryllium is, but it should work just fine for X-raying flowers and seashells, etc. Let me know if you want one of them and maybe we can work out a trade.
15 keV would be a good place to start for someone interested in the low energy end of the spectrum, because some ordinary off-the-shelf detection and measurement equipment and techniques can still be used here. For example, mica end-window GM tubes and home-made Al window ionisation chambers can still be expected to produce a signal.
When I get back to the x-ray hobby, am planning to continue using my nominal 15 kV NST, before firing up an XRT bought from x-ray. Can attest that NST and an unthinned-glass dental tube, and no oil, can produce nice shadows of thin aluminum soda cans, and little plastic hotel-shampoo bottles. (The shampoo attenuates these x-rays more than it attenuates light.) Anode heat capacity is a _critical_ limit, especially since I think the screen luminance per HV watt goes down much faster than the anode voltage. Look up the meaning of "heat unit" as found in Coolidge tube datasheets. My next version will have a heatsink and/or oil around the anode terminal of the tube, even though HV insulation is not a problem at these voltages.
Oh, and I'm not going to skip the home-made ionization chamber. Starting with an off-the-shelf picoammeter will save some soldering time. 9V batteries in series can provide more than enough bias voltage. My metal-windowed Geiger counter responds strongly even at half of the NST voltage, so by silence it indicates when shielding & distance are adequate for safety. (Or that the X-ray source or Geiger counter are off, or broken.)
Registered Member #3429
Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288
Klugesmith wrote ...
My next version will have a heatsink and/or oil around the anode terminal of the tube, even though HV insulation is not a problem at these voltages.
When I read that you may use a heatsink on your X-ray tube, it reminded me of an old dental tube head that was made by Weber X-ray which used Sulfur Hexafluoride (SF6) gas as the hv insulation and heat sink. It was the ONLY dental tube head (to my knowledge) that used SF6 instead of oil. The X-ray tube had a ribbed aluminum heatsink clamped to the anode shank, and was fed hv from a split transformer that supplied 35KV AC to the anode, and 35KV AC (opposite phase) to the cathode. I believe that the gas was under light pressure (maybe 2 or 3 atmospheres). I've had a couple of those heads come into my shop many years ago, but I did not have the proper equipment that was necessary to replace the lost gas after opening the head to repair it. But I got to see how that head was made.
I don't know how difficult or practical it would be to get SF6 gas for hobbyist use, but it would be an interesting project if someone were to make an X-ray head that was insulated with SF6! I know that SF6 is a very dense "greenhouse gas", so there may be very strict EPA rules and regulations that apply to its sale, storage, and use. Electrical power distribution companies use it in many of their hv circuit breakers and other hv components, but I believe that it's being phased out because of its negative impact on the environment.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Klugesmith wrote ...
Proud Mary wrote ...
Xray wrote ...
I have a couple of very small tubes (about the size of a large walnut) that have a portion of the thick glass ground down to a thin glass window. The thin glass may not be as transparent to X-rays as beryllium is, but it should work just fine for X-raying flowers and seashells, etc. Let me know if you want one of them and maybe we can work out a trade.
15 keV would be a good place to start for someone interested in the low energy end of the spectrum, because some ordinary off-the-shelf detection and measurement equipment and techniques can still be used here. For example, mica end-window GM tubes and home-made Al window ionisation chambers can still be expected to produce a signal.
When I get back to the x-ray hobby, am planning to continue using my nominal 15 kV NST, before firing up an XRT bought from x-ray. Can attest that NST and an unthinned-glass dental tube, and no oil, can produce nice shadows of thin aluminum soda cans, and little plastic hotel-shampoo bottles. (The shampoo attenuates these x-rays more than it attenuates light.) Anode heat capacity is a _critical_ limit, especially since I think the screen luminance per HV watt goes down much faster than the anode voltage. Look up the meaning of "heat unit" as found in Coolidge tube datasheets. My next version will have a heatsink and/or oil around the anode terminal of the tube, even though HV insulation is not a problem at these voltages.
Oh, and I'm not going to skip the home-made ionization chamber. Starting with an off-the-shelf picoammeter will save some soldering time. 9V batteries in series can provide more than enough bias voltage. My metal-windowed Geiger counter responds strongly even at half of the NST voltage, so by silence it indicates when shielding & distance are adequate for safety. (Or that the X-ray source or Geiger counter are off, or broken.)
I'm sure a good many hastily assembled X-ray experiments dissapoint because too much attention has been paid to ever higher voltage, without thought to the consequences of high PSU output impedance, with the result that the anode voltage falls flat on its face as soon as the tube starts to draw current. Much better to see what can be done with a steady but modest supply like your 15 kV NST where you know what you're getting, and effective shielding is easy to implement, not of unmanageable weight, and is inexpensive.
Whilst only a small minority of photons of 15 keV will be produced at 15 kVp, most of those that are can reach out ten feet through the air to strike the clueless and the negligent:
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1715
Xray wrote ... I don't know how difficult or practical it would be to get SF6 gas for hobbyist use, but it would be an interesting project if someone were to make an X-ray head that was insulated with SF6! I know that SF6 is a very dense "greenhouse gas", so there may be very strict EPA rules and regulations that apply to its sale, storage, and use. Electrical power distribution companies use it in many of their hv circuit breakers and other hv components, but I believe that it's being phased out because of its negative impact on the environment.
Keep one's eyes open at hamfests! The cylinder in picture weighed about a pound more than the other one from same vendor, and sloshed when tilted.
Yes, SF6 is about as potent a greenhouse gas as anything, when its atmospheric lifetime is figured in. A commonly cited value is 24000 times more than same weight of CO2. By that reckoning, global releases of SF6 (about 5e3 tons/year) will cause about 1/250 as much anthropogenic global warming as our 30e9 tons of CO2. They haven't found a substitute, so mitigation efforts (as with freons today) are aimed at reducing accidental and deliberate venting.
Registered Member #2893
Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242
Proud Mary wrote ...
I'm sure a good many hastily assembled X-ray experiments dissapoint because too much attention has been paid to ever higher voltage, without thought to the consequences of high PSU output impedance, with the result that the anode voltage falls flat on its face as soon as the tube starts to draw current.
Heh... happens with my CW in this thing. That is the reason I'm setting the heater to a fixed voltage, so the only adjustments would be exposure time and kVp. Current doesn't [didn't] seem to vary that much when I changed the voltage, maybe a difference of 300uA between 70kV and 45kV.
I've installed the CEI tube in the head, just need to wait for some parts in the mail now to fix the microelectronics. Got _lots_ of 5V and 18V TVS diodes.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Grenadier wrote ...
Proud Mary wrote ...
I'm sure a good many hastily assembled X-ray experiments dissapoint because too much attention has been paid to ever higher voltage, without thought to the consequences of high PSU output impedance, with the result that the anode voltage falls flat on its face as soon as the tube starts to draw current.
Heh... happens with my CW in this thing. That is the reason I'm setting the heater to a fixed voltage, so the only adjustments would be exposure time and kVp. Current doesn't [didn't] seem to vary that much when I changed the voltage, maybe a difference of 300uA between 70kV and 45kV.
The problem with this approach is that you may find the anode heat capacity exceeded on higher settings of kVp, and have no way to bring it down. At the least this will quickly wreck your focal spot with pitting and fissures, as well as depositing a brown haze of conductive tungsten on the inside of the glass at a higher than normal rate.
Registered Member #2893
Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242
Hrm true. According to the charts at 150w the anode will reach 6kj in 1 min. So I'll limit the max power to 115w.
But now cei has me confused. It says tp use the grid as Hv ground and connect it to the heater with "15 / 25 kohms" or the focal spot will be affected. Not sure how to go about this when Hv ground == heater ground.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1715
Grenadier wrote ...
Hrm true. According to the charts at 150w the anode will reach 6kj in 1 min. So I'll limit the max power to 115w.
But now cei has me confused. It says tp use the grid as Hv ground and connect it to the heater with "15 / 25 kohms" or the focal spot will be affected. Not sure how to go about this when Hv ground == heater ground.
< ocx 70-g.
1. What are the cooling conditions (e.g. natural convection in oil?) for which the heat capacity chart applies? With 6 kJ the anode is going to be damn hot -- the limit is from thermal stress around the glass/metal seal. You could easily estimate the copper mass and temperature rise.
2. Look at the connection schematic in the datasheet. If you want to use the third wire (grid/focus) as CEI intended, your battery-powered heater supply must float, with a resistor of about 20K between one side and the grid. Negative side of HV supply goes to the grid. When there is no anode current, grid and heater are at the same potential. At an instant when there is 10 mA of HV current, the 10 mA flows through the 20K resistor and makes the grid 200 V negative with respect to the heater. This tends to reduce (regulate) the HV current, as well as focus the spot.
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Grenadier's big thread of Röntgen related shenanigans
