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Grenadier's big thread of RÃ¶ntgen related shenanigans

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Proud Mary

Tue Mar 22 2011, 11:15PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Grenadier wrote ...

I need to figure out how to mount the tube in that tupperware w/o using any glue. Maybe some sort of foamboard contraption could do that.

There are various types of metal pipe clips and clamps you could use if you pad them with glass cloth, and don't over-tighten them.

If you don't want to use metal at all, you could simply wrap the tube fairly loosely with coarse glass cloth, or rock wool, leaving a hole over the exit window, and agitate it so the dielectric oil penetrates all way through it.

You could carve a support yoke out of refractory materials like brick. Or you could cast supports out of Plaster of Paris, fire cement, or even concrete, again padding the contact surfaces with glass cloth or rock wool.

James

Tue Mar 22 2011, 11:28PM

Registered Member #3610 Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506

Grenadier wrote ...

Well the CW is certainly capable of making 1.5mA, and 30Gy/hr is certainly enough x-radiation for me. The tube will also be under oil to help suck away some heat.

I need to figure out how to mount the tube in that tupperware w/o using any glue. Maybe some sort of foamboard contraption could do that.

Get some styrene (sheets, not the foam), polycarbonate, acrylic, etc and fabricate a bracket for it that fits snugly in the housing. I get this sort of stuff from Tap Plastics as they have a bin of cutoffs they sell by the pound but you can get it other places too. The dental heads I've opened up had the tube bolted to a plastic mount by way of a bolt into a threaded hole in the anode stud and I suspect most are set up similarly.

If I were gonna try to mount a tube in my own housing, I think I'd get some plastic pipe and cut a disk out of plastic to stick over each end of the tube that fit into the pipe. Use threaded cleanout fittings for the ends and seal the wires through them, and drill & tap a small port through which to top off the oil. Wrap the outside of the pipe with lead sheet of sufficient thickness with a hole for the beam exit and then mount that whole assembly into a housing of some sort to protect the shielding.

It is WELL worth it to pick up an ion chamber survey meter if you are going to play with this stuff. Eberline makes some nice ones, as do others. Those cold war era Victoreen ones from the civil defense program are NOT sensitive enough. Probably cost you a couple hundred bucks for a decent used one but it's cheaper than cancer. Do NOT skimp on shielding or safety in general. Even more important than protecting yourself is ensuring that you don't expose anyone else to radiation. It is invisible, can scatter, and undeniably hazardous.

Adam Munich

Tue Mar 22 2011, 11:33PM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Well I don't have a couple hundred dollars to spend on a survey meter...

I do have 7mm of lead shielding, and I'll be standing quite a distance away from it, so that should be plenty to reduce the dose-rate to nearly nothing. I am going to get a dosimeter too.

As for the plastic pipe idea, I already made a lead shield that conforms to a piece of tupperware so I can't really do much but work with what I've got right now.

Adam Munich

Fri Mar 25 2011, 02:50AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Mostly mechanical stuff today, just built the tube holder.

Smile!

Eventually I'll replace the canola oil with something inorganic, but this works for now. The x-rays need to transverse 1cm of oil, and although that will attenuate the beam I don't think it'll have very much effect. My biggest concern is that the tube will flashover under oil, and that is why I left the limiting resistor in the CW. W/o it a flashover = dead diodes. In order to flash over the HV would need to transverse about 4cm of oil, and at 75kV that is unlikely to happen (i hope).

I've been thinking about the tube heater and I was considering a linear voltage regulator, a low dropout one. Running off of 2 alkaline cells there is no chance that the tube's heater can be overvolted if the Vreg fails, though I am concerned about the reliability of such a regulator when it comes to electrostatics. I'm not so sure I'll have any room for D cells, so I may have to use C cells.

I was looking at the discharge curves of Lipos and it appears that I'll need a 10s lipo to compensate for voltage drop. I plan on using an LM317 and a TIP122 as a pass transistor, and that should give me about 8A of regulated current. I'll need a beefy heatsink, but since it is not running continuous duty heating shouldn't be too big of a problem, even if something like 40W of heat are being made when the kVp is lowered to 50kV. Eventually I plan on going the switch mode route, but meh this is good enough for now.

There was something else I was going to say but I seem to have forgotten it. Oh well.

Edit:

Now I remember, I need to cut holes in the Pb sheild and make the anode's HV feed through. I'm thinking a bic pen and a zillion pounds of hot glue could work.

klugesmith

Fri Mar 25 2011, 04:17AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

I've been thinking about regulated battery-powered filament supplies.
If the battery is a nominal 3.7-volt Li-ion type, then there isn't much
voltage headroom. With just a rheostat to control the filament power,
anode current will be -very- sensitive to battery voltage.

Here's an undeveloped concept: a low-dropout Current regulator,
which is easy on the filament when switched on.
I think it'll be easy to make one for (say) 2 amps, with dropout
voltage less than 200 mV (100 mV for sense R and 100 mV for the pass transistor).

Anybody here already done that, with part numbers & stable feedback?

uzzors2k

Fri Mar 25 2011, 09:09AM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Klugesmith wrote ...

Anybody here already done that, with part numbers & stable feedback?

Yeah.

Scaling up to 2A shouldn't be hard, just make sure your op-amp can work down to ground if you choose a low sense resistor voltage.

Adam Munich

Fri Mar 25 2011, 12:22PM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

So I was thinking this could possibly work. The two diodes are used to create a reference voltage while the op amp & resistive divider regulate&adjust stuff.

Adam Munich

Sat Mar 26 2011, 04:42AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Today I built the high power linear regulator.

I used a 2SA1943 PNP transistor to pass most of the current and it works very well. The heatsink gets only a couple of degrees above ambient even when the regulator drops 10V @ 5.5A, so that's good.

The regulator has a relatively high dropout of 2.9V so the 10s lipo will still work for giving the CW 31V. Most of the battery's discharge curve will be at 35v so that leaves 33v for the CW when the regulator isn't uhh, "in use".

Inducktion

Sat Mar 26 2011, 06:56PM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

Grenadier wrote ...

Today I built the high power linear regulator.

Is that a CPU heatsink? How did you cut the holes for the screws? I still can't figure out how to do it.

Adam Munich

Mon Apr 04 2011, 12:53AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

So the past week has been nothing but fail when it comes to woodworking.

First I sanded the boxes, and they looked good: until I stained them. The stain went on uneven and wouldn't get dark. After trying again and again to darken it I then asked HVCOMM what finish would be best. I suggested acrylic but NOOOO! use shellac.

So I bought a can of spray on shellac and put it on: it ran. Then after it was dry I picked it up and dry after 24 hours my ass! Fast forward a week and 7 more tries and it still looks like ass. A week and $20 wasted. Gah I'm pissed.

I'm just going to get new boxes and try again; it'll be cheaper anyway.

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