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Registered Member #2893
Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242
OK change of plans. I'm not going to use that giant lead box, so all that work was for not. It's still a really cool thing though, so I'm certainly not going to get rid of it, maybe I can store the uranium (that I'm still hoping to buy) in it.
What made me change my mind? Well I bought an x-ray tube, and it came today. Look at it:
Heh, that certainly wasn't what I as expecting. If I wanted to I could make this thing handheld!
So now I'm back to square one. I don't think I'm going to cast a new box, because I have enough lead sheet to make a box for this thing 1cm thick. I'm kind of happy that I'm not using this big box. Why? It's heavy. Now that I'm not using it I can make this machine much more compact, and I have a lot of "weight space" to build the PSU, the battery, and the arduino controlling circuity.
Thanks goes to X-ray for providing this awesome tube. .8mm focal spot, 7ma @ 70kV. This is going to be one hell of an xray machine.
Just in case anyone was wondering, I copied and pasted all the r's w's and the q.
Registered Member #2893
Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242
Alright, no more procrastinating. This is what I've got so far....
The lead shield is 3mm thick, I need to make it 7mm thick. That should be plenty to block the 75kV rays. I also need to drill some holes in it and make a lid.
The tube is being held in a tupperware inside the shield. It'll be filled with olive oil and if I did my maths right it can store about 22kJ of heat before it gets 30 or so degrees hotter.
The CW is in a tupperware too and will be filled with oil, and it'll be powered via a ZVS at about 60khz. I need to make HV feedthroughs in both tupperwares, I think something involving plastic tubes and JB weld might work. The wire connecting the two containers needs to have very thick insulation, I'm thinking some projection TV wire and a bunch of electrical tape could work.
I need to get a 22V lipo, hopefully my other CW sells soon so I can buy it. Power will be adjustable from 17 to 21V via an LM317 + 2n3055 and I'm going to need to find a hefty TO3 heatsink as this thing will be dissipating lots of watts, though it won't be run continuously. The power will be applied to the circuit via a mechanical relay as there is a very little chance it'll fail closed circuit. Exposure time will be controlled by an arduino, which will also monitor the temperature of the 2n3055, the tube oil, and the battery voltage. Possibly I can make this thing radio controlled too, rather than a long wire as a trigger.
Heater voltage will be fixed at 2.6V, and that will allow 3mA through the tube if the graphs are right. Not sure how to go about this, maybe d batteries and a rheostat. I'm kind of afraid that an LM317 will die due to electrostatics from the CW.
I think the box is a little too small. Hopefully AC moore will let me return it w/o a receipt (lost it).
Registered Member #2893
Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242
Yay, AC moore let me exchange box. I got 2 smaller ones, plan on gluing them together. All the electronics will be in one box while the HV stuff will be in the other. That should be enough HV isolation...
I started to make the primary coil feedthroughs on the CW case, I used brass hardware because it's solderable. Testing indicates that it is oilproof, so yay!
Tube heater is going to be fixed at 2.8V to limit current at about 3mA \, and the heater will be powered by D cells. Maybe I'll have a voltmeter on the back that tells me the heater voltage, too low/hi I'll open up the box and adjust the stat.
I've been thinking about anode current monitoring and I realized that it can be measured simply by putting a 5mA meter in series with HV ground. So I need a 5V meter and a 5mA meter. Hmm.... where get?
As for the lead shield, so far it is 5mm. I was aiming for 7mm, but I found this transmission calculator and according to it:
5mm Pb has a transmittance of 6.59e-22 at 50kVp 5mm Pb has a transmittance of 1.26e-13 at 70kVp 5mm Pb has a transmittance of 1.72e-8 at 90kVp
I'll be 10 feet away from this thing while it's on (obviously not in the beam path) so if these numbers are correct then compton scattering is a bigger problem than direct exposure from the tube!
Proud Mary, you are the radiation expert, so does this seem correct to you?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Grenadier wrote ...
Yay, AC moore let me exchange box. I got 2 smaller ones, plan on gluing them together. All the electronics will be in one box while the HV stuff will be in the other. That should be enough HV isolation...
I started to make the primary coil feedthroughs on the CW case, I used brass hardware because it's solderable. Testing indicates that it is oilproof, so yay!
Tube heater is going to be fixed at 2.8V to limit current at about 3mA \, and the heater will be powered by D cells. Maybe I'll have a voltmeter on the back that tells me the heater voltage, too low/hi I'll open up the box and adjust the stat.
I've been thinking about anode current monitoring and I realized that it can be measured simply by putting a 5mA meter in series with HV ground. So I need a 5V meter and a 5mA meter. Hmm.... where get?
As for the lead shield, so far it is 5mm. I was aiming for 7mm, but I found this transmission calculator and according to it:
5mm Pb has a transmittance of 6.59e-22 at 50kVp 5mm Pb has a transmittance of 1.26e-13 at 70kVp 5mm Pb has a transmittance of 1.72e-8 at 90kVp
I'll be 10 feet away from this thing while it's on (obviously not in the beam path) so if these numbers are correct then compton scattering is a bigger problem than direct exposure from the tube!
Proud Mary, you are the radiation expert, so does this seem correct to you?
Well now, if we say your worst case is Va 90kV, Ia 7mA, with 5mm Pb shielding, the dose-rate centre-beam at 3000 mm will be of the order of 100 μSv/hr.
What sort of dose-rate is 100 μSv/hr? In radiogeology, it's a rule of thumb reckoning on the dose-rate you'd expect to get when standing on top of a planar pitchblende load, or the gamma reading you get when you put a GM tube right up to a chunk of best quality pitchblende.
It means that if you did a 10 second radiography exposure you'd receive 0.03 μSv, not a lot, it's true, but still a figure to be added to your cumulative life time dose.
Without the 5mm Pb shield, the dose rate rises to about 2 Sv/hr:
The Bells of Hell go ting-a-ling-a-ling For you but not for me: For me the angels sing-a-ling-a-ling, They've got the goods for me.
If we go for a more prudent 50kV/1mA, with your 5mm Pb shielding, then the dose-rate at 3000 mm will be about 3E-6 μSv/hr - below the theshold of detection for all normal radiological protection purposes.
There are far too many variables and unknowns for any sort of accuracy in these generalised calculations, so you should consider these figures as a very rough guide only, figures to be confirmed by real world dosimetry.
Registered Member #2893
Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242
Hmm... Well I'm not going to be center beam for obvious reasons, instead I'll be behind the machine and thus behind the angled tungsten anode. Therefore there should be little radiation there to begin with as the tungsten will block most of it. I suppose I could add one more mm of Pb, or better yet I could make a small Pb jacket for the tube, which would be light and easy to make. I could also step back another 8 or so feet.
I never intend to run the thing at 90kVp, that's just the worst case scenario where the regulator fails and the full 22.7V from the lipo goes into the CW. (pretty sure it'll give 90kV in that case). Also, the tube will probably 'splode if it runs for more than 5 or so minutes, so a failure would be self limiting. The MCU will measure the temp anyway and kill the CW's power if there is an issue. I plan on using 2 relays in series, so if one fails the thing can still be turned off. The more protection the better, and components are cheaper than hospital bills.
May I ask what formulas you used to calculate that PM?
Registered Member #2893
Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242
Alright, work has been a little slow but this is what I've got so far.
I scratched my head for hours and cursed at a bunch of adhesive bottles because I couldn't get anything to stick to the polypropylene Tupperware. About to give up I grabbed my hot glue gun and tried that for the hell of it and what do ya know, it glued the PP, and did a fine job of it too. Let the records show that hot glue is the only thing that bonds PP.
This is the HV feedthough I came up with. It consists of a rubber insulated wire inside an aquarium tube inside some random tube I found behind the sofa. A bic pen was also used. Here's a paint image that details what I did.
It seems to be putting out more than 1mA of current, and judging by how fast the meter was pegged I'd say it's about 1.7mA. I need a bigger meter.
I'm not sure if I want to leave the current limiting resistor in or take it out when I connect the tube up. I don't want to accidentally kill my 20mA diodes... nonetheless I need to order some parts, mainly the 5mA meter, the 5V meter and the 22v lipo. I plan on ordering that stuff tomorrow. I'm not sure yet if I want to monitor the anode voltage with a resistive voltage divider and yet another meter.
I don't want to use analog meters for the x-ray because well I won't be able to read them from 15 feet away. But digital ones might not like being connected to the CW. Even though their voltages are not going to be exceeded they might build up a static charge and bust...
Registered Member #1938
Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701
An exciting thread, looking forward to your first results!
Soon, probably in a month or two, oil will leak around the HV connector. Hot glue is dissolved, but even worse, you seem to have a few little pipes that will encourage the capillarity effect. Best thing would have been to use a simple metallic screw, uninsulated but strongly pressing the plastic wall. Then over it you could have applied epoxy or hot glue as an insulator. This would hold better, but... the plastic contains air, so in time between the pressing screw and the plastic wall a gap would develop. So the final solution would be a tight screw and a rubber fitting. Just my 2 cents.
Regarding the decisions on the measuring instrument: using digital will fail. On my low voltage variable supply (30V max) I use a cheap chinese voltmeter based on some unknown uC . A few spikes and its dead, despite the protection with neon lamp, zener diode or other filters. You should really get an analog meter.
For the readings you can always set the camera on the tripod and make a short film. Then turn the lights off and do whatever you want to experiment.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
radhoo wrote ...
Regarding the decisions on the measuring instrument: using digital will fail. On my low voltage variable supply (30V max) I use a cheap chinese voltmeter based on some unknown uC . A few spikes and its dead, despite the protection with neon lamp, zener diode or other filters. You should really get an analog meter.
For the readings you can always set the camera on the tripod and make a short film. Then turn the lights off and do whatever you want to experiment.
I agree with Radu 100% here, and use moving coil panel meters for measuring X-ray cathode and filament/heater current. I connect one side of the meter to Earth, and put a gas discharge arrestor across it to stop the meter becoming 'live' if it fails open circuit.
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Grenadier's big thread of Röntgen related shenanigans
If I wanted to I could make this thing handheld!
