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Grenadier's big thread of RÃ¶ntgen related shenanigans

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Adam Munich

Mon Jun 07 2010, 01:41AM

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

So i decided not to quit on my ray machine.
Any input? Please have some input.

Wolfram

Mon Jun 07 2010, 10:24AM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

Was the head an S.S. White?

I'm almost certain the transformer is a 60Hz unit with a 110V primary, the reason you got such a high voltage from it when giving it 12V on the primary is that when the voltage on the primary was removed, the magnetic field in the core collapsed and induced a high voltage in both the primary and the secondary. If you had connected an oscilloscope across the primary when doing the test, I bet you would see spikes of a couple of hundred volts.

The way you are driving the transformer with a pulsed relay is not right, it will give you some high voltage, but at very limited power, and you might excite resonances in the transformer secondaries, giving you a much higher output voltage than expected, risking the transformer and making the output voltage hard to predict. The transformer is made to be driven by a 60Hz sine wave, and this is certainly the best way to do it.

The transformer has two secondaries that are wound in the opposite direction, and there should be a black wire from the inner end of each secondary. These two wires should be connected to each other and to the core at all times when you are operating the transformer, or else there is a very real risk of arc-over between the inner windings of the secondaries and the core. If you want to measure the current through the transformer (which is also the current flowing through the x-ray tube), you can connect an AC milliampere meter in series with one of these wires.

Here

is the schematic of a typical x-ray head, I hope this clears up any confusion

The transformer should be immersed in oil unless you are operating it at very low output voltages, I wouldn't risk running it at more than 10kVp per secondary without oil. You might be able to operate the tube at 60kVp in air before you start to get trouble with corona and arcing, but I'm a bit worried about the wooden support you're using for the tube. Wood isn't a very good insulator unless it has been thoroughly dried and laquered. Also, running the tube out of oil will limit the maximum power that you can put into it.

Doing these changes will greatly increase the the maximum radiation output of your setup. It will also increase the controllability of the system, both in the sense that you can predictably set the working voltage by giving the transformer a specific AC voltage on the input (a variac will be handy here) and in the sense that you can see what current it is running at. And if you want to run it at low power, that's no problem, just run the tube at a very low current.

Was there a ceramic radiation-shielding tube around the x-ray tube when you removed it from the head? It could be a good idea to put this back on the tube, to shield against off-axis radiation.

I should add the obligatory warning about ionizing radiation, but you seem to know what you're doing safery-wise, the lead shileding box that you've built is a very good protective measure, just don't assume that it will stop all radiation at all times. Normal geiger counters aren't the most suitable for measuring X-rays, but Proud Mary knows much more on this subject than me, and hopefully she can give you some tips.

Anders M.

Steve Conner

Mon Jun 07 2010, 10:51AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I'm puzzled. Why would you take apart a working X-ray head and use the tube and transformer to build another X-ray head?

Are you doing fine art radiography, so you had to take the shielding off to let the low-energy rays out? But then, why not just take the aluminium filter off the original head?

I agree by the way, the lead shielding box is pretty cool.

Proud Mary

Mon Jun 07 2010, 01:22PM

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

Anders M. wrote ...

Normal geiger counters aren't the most suitable for measuring X-rays, but Proud Mary knows much more on this subject than me, and hopefully she can give you some tips.

Thanks for the plug, Anders, but if you look at my Geigermania thread in the Projects section, you'll see that I'm only at the very beginning of any real understanding of gas amplification detectors, but I will say that GM tubes are wholly unsuitable for the measurement of X-rays, and nowhere worse than around 60-75keV, where the uncompensated energy response curves of typical tubes looks much like Mount Fuji.

Without measurement you have no science.

Adam Munich

Mon Jun 07 2010, 06:42PM

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

Well, of course the tube and transformer are immersed in cooking oil. I've had very good luck with canola oil. I've pushed that transformer to 200KV and it holds up fine. Canola oil has a D-strength of 12MV/M.

The wood is just to hold the tube. It's soaked through in oil.

The transformer is rigged to have the center tap grounded to the core.

I don't plan on any fine measurement with my GM tube, But it is a mica window model (tube, not pancake), and thus very sensitive to all kinds of radiation. It is mainly just going to be used to detect any changes over background.

IF YOU LOOKED AT PAGE 2 OF THE THREAD... you will see that i have a new circuit to drive it. Here is the diagram..

Wolfram

Mon Jun 07 2010, 09:52PM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

That looks like a much more reasonable circuit.

I last checked that thread right after you first joined here, so I missed the updates. No need to yell.

Are you sure about the 200kV figure? How did you measure this? Resistive dividers can be inaccurate even at 60Hz if they have significant stray capacitances, and the 1kV/mm formula for sparks between needle points can overestimate the voltage when yo go above a few tens of kilovolts. You mention the transformer is a 75kV unit, and I'd bet the core saturates at less than 90kV out.

Looking at the picture of the transformer again, I see that I misunderstood at first, the two black wires are of course the primary. The silver colored braided wire from one of the secondaries is the current measurement point, so if you connect a say 100 ohm resistor between this point and and the core, you can measure the current going through the tube, 100mV/mA obviously.

Also, once you have got it working properly, I would either just remove the bulb or at least connect a wire back to the control box from the point between the bulb and the transformer. You want to know the primary voltage, because that tells you the approximate secondary voltage. And you need to know the tube voltage to get consistent exposures.

Anders M.

Adam Munich

Mon Jun 07 2010, 10:07PM

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

Nah, if i run it like an induction coil i can get a 6" spark out of it. I'd say that's at least 200kv.

If I remove the bulb, i get way too much current flowing through the thing. It'll draw about 12A unballasted.

I'll definitely use the current measurement point now.

Since it is a 3 wire cord I'm using, I'll feed a line after the bulb through the ground wire. to measure it at the control box. (obviously I'm not plugging this cord into the mains)

Another problem i have is, the transformer has a 500ms voltage spike while the light bulb warms up and builds it's resistance. I was thinking of just turning the heater on a second late, but that HV needs to go somewhere... Any better ballast ideas than a 60w light bulb?

I could possibly keep the bulb warm and switch via relay to the transformer, however my extension cord has only 3 wires. I won't be able to measure the voltage and keep the bulb warm. I could put the ssr on the machine itself, but i don't want that failing in case a stray x-ray hits it.

Oh and yeah, it was an ss.white rebranded keystone

Wolfram

Mon Jun 07 2010, 10:35PM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

I would use a wirewound resistor instead of the bulb, a 100W or 200W one would be fine, though I'm still not convinced it's really neccesary. If the transformer draws a lot of current that's either saturation, which should only happen at close to full input voltage, or too high voltage on the filament of the x-ray tube which leads to it drawing more current than it should, and therefore also the transformer also drawing more current from mains. In the first case, just turn the variac back a little bit, and the no-load current should go down to almost zero, and in the latter case reduce the filament voltage. If you measured 12A with the transformer connected directly to mains without the variac and the secondary unloaded, it was also because of saturation. This

is a picture I found that illustrates saturation, but the numbers on the scale could be different in your case. These X-ray transformers are also often made to be run at reduced mains voltage from the control unit which explains why it would saturate when connected directly to mains. Do a quick test with the variac to see at which input voltage the primary current starts to get uncomfortable at.

A 6'' spark does not automatically indicate 200kV. See the pictures almost at the bottom of this

page for a 160kV X-ray supply making 12'' arcs.

Are you planning on using intensifying cassettes?

Anders M.

Adam Munich

Mon Jun 07 2010, 10:56PM

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

I still want a ballast, because I'd rather have a long exposure with a moderate amount of activity than a short exposure with alot of activity [more dangerous]. Plus, i don't want to accidentally blow up my tube.

With a 60w bulb and the thin sparks able to connect at Just over 6cm {wire electrodes, no balls} the transformer draws .3A, with 50V across the primary. There are no arcs formed, just sparks. This leads me to believe that the output is in the sub-ma range, which is what I'm looking for.

A cold 60w bulb has a resistance of 16 ohms, hot 240 ohms. I'd like the kvp to be higher, because even if the transformer is the 90kv model [wasn't labeled], 50v on the primary only means about 45kv on the secondary, which makes sense with the 6.2cm sparks on braided wire electrodes. That's a little low. I'd like up to 65kVp. (75 can't be achieved, because ballasting also lowers the voltage. Should've bought the 0-140V variac.) Anyone know where to find a cheap 0-250ohm 50w rheostat? I can't afford to pay $30 for one...
Also, the ballasted transformer draws .5A when the sparks can't connect.

I also really need a better way of measuring kV.

The tube isn't connected yet, because my 0-120v variac is still in the mail. I want to figure out the circuitry before i put in the tube.

Yes, i am going to use intensifying cassettes and kodak b/w paper film.

Adam Munich

Mon Jun 07 2010, 11:31PM

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

just tried a 100w bulb as the ballast, i get .5a draw, 11cm sparks on wire [80% humidity], 100v across the primary with loud skinny sparks (no arcs).

So... this should be 50 watts drawn by the transformer, and at .6mA maximum @75kV. Perfect!

With the variac, i can lower the input voltage, thus lowering the output. Not quite sure if it will lower mA. At a lower voltage, it could possibly draw more amps to keep the 50W draw.

edit:

hmm, it draws .8A with the secondary shorted. So .8ma is still acceptable.

edit2:

So, i tried it with an equivalent resistor. Being a fixed load, it allowed the transformer to draw 2.8A when the wires were close. not good. So i will have to use a lightbulb. However, i need to keep the bulb warm, otherwise i get a voltage spike during warm-up. I can't just leave the heater off, because that spike needs to go somewhere, so i will ditch thte ssr, and use a spdt relay. The relay when de-energized will keep the bulb warm, it'll then energize and switch the bulb through the transformer. Instead of a high voltage going low [bulb warms] i'll have a low voltage going high [bulb cools].

Bad drawing.
2j1rd4j

Mod edit: Oversized picture. You also win the OMG Worst Schematic Evur award.

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Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob