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Grenadier's big thread of RÃ¶ntgen related shenanigans

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Adam Munich

Sat Jun 12 2010, 10:28PM

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

A) Wow, i really hope my keep-lightbulb-warm circuit will work now.

B) I read about seasoning and thought it would "cover itself", because i was going to start using the tube at a low voltage, and gradually build up my courage to bring it up. :) Now i know to start even lower. This could take a long time, and it should, because this tube hasn't been used in 10 years. I know it's vacuum is good though, because i tested it at 24v, 20ma as a thermionic diode.

More tube info;
Dunlee mfct 197?
Seems to be brosilicate glass.
Very minor browning.
Very tiny pits on anode, with a minor crack in an H shape.
75kVp max
2mA max
Heater white hot at 3.5v

Also... does filament polarity matter? I'm not sure if the electrostatic lens is hooked up to a wire, and if it is which one.

Adam Munich

Sun Jun 13 2010, 12:14AM

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

New schematic. I had to shorten the length of wire because the longest cord i have is 250ft. So now, a lead apron is not a must, but i'm going to wear one. As i said before, "real" lead aprons are pathetic, they only give .5mm protection. I'm going to make my own from some 1mm sheet. I'll see if i can get some from local roofing companies. I really don't want to pay $80 for 6sqft. online.

IntraWinding

Sun Jun 13 2010, 12:21AM

Registered Member #2261 Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581

Could you dig a hole in the ground for your experiments so you get hundreds of feet of earth protecting you, and avoid lead poisoning too?

Proud Mary

Sun Jun 13 2010, 12:36AM

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

Grenadier wrote ...

Heater white hot at 3.5v

You should plan to control the anode current by adjusting the heater voltage, for which you need a fairly 'stiff' EHT supply. It's no good if the anode voltage begins to fall when you increase the current due to an inadequate supply. I would suggest you start at 1.5V on the heater or filament , and gradually increase it until the tube's 2mA max is reached. Excess heater voltage is probably the quickest way of destroying a tube. I use a single Hawker Cyclon 2V 8AH lead acid cell (which has the same proportions as a standard "D" cell, but is rather larger) which is easy to 'float' at -20kv to -40kV in grounded anode configurations, compared with the problems of having a transformer which can hold of such voltages on its LT windings. Other advantages of using a single 2V cell is that the heater voltage cannot rise no matter what happens elsewhere in the circuit, and that no matter what may happen the production of X-rays will shut down in a few hours when the battery is discharged.

Once you have the apparatus running, you should make an X-ray pinhole camera in order to visualize the anode focal spot to see that all is right with the focusing cup potential.

Making all these adjustments and preliminary tests without compromising the shielding is time consuming.

Adam Munich

Sun Jun 13 2010, 01:30AM

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

@ IntraWinding
No. I could, but this box weighs 100lbs. If i put it in a hole, it's never coming out. Besides, i'll put my house in between me and the box too. Even though it's not, Sheetrock is better than nothing.

@ proudmary

I'm using a 3.7v lithium cell with a rheostat to control the heater current/temperature. I planned on adjusting that by a dial on the machine which i'll put a calibrated label on. Then i get well out of the way and the 0-120v is fed into the machine. This will warm up the lightbulb for a second, then a relay will switch feeding the current through the lightbulb through the transformer. By this method, the transformer is incapable of supplying more than .8ma, even when shorted. This way, i cant overheat the tube by accident. A longer exposure is better anyway; it gives more photographic control. After the exposure, i'll flip a switch or maybe two cutting off the mains and another turning off the arduino/12v supply, making sure it can't possibly expose while i'm adjusting it.

I'm building this thing with safety in mind, not accuracy. If the kVp drops with higher currents, so be it. I'll develop the photo and if it looks like crap, i'll turn up the variac.

X-ray pinhole camera? Explain please. I was just going to make images directly, just by placing an object in front of the intensifying screen. If it was blurry, then i probably have the heater wires backward and reverse them. I'm well aware how x-rays are produced/scattered/absorbed with the bremsstrahlung and compton scattering etc. However i'm still not too clear on how the focusing cup works. Does it lose focus at lower than intended kVps? My tube is of the 3 wire type.

FYI, it's going to be a self-rectifying circuit.

Proud Mary

Sun Jun 13 2010, 08:26AM

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

Grenadier wrote ...

X-ray pinhole camera? Explain please.

X-rays behave like visible light when they transit a pinhole.

If you make the smallest, most accurate pinhole you can in a lead or tungsten sheet, centre it in the beam half way between the anode and your film, and ensure that it is exactly at right angles to the beam with a plumb bob and triangle, then an image of the focal spot on the anode will be obtained, whose width can be measured directly.

You will also see directly what effect, if any, the pitting and cracking of the target has had.

Pinhole cameras are one of the key tools in X-ray astronomy. Arrays of pinhole cameras have been fitted to space craft to produce X-ray images of the universe.

Adam Munich

Sun Jun 13 2010, 01:34PM

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

I'll have to try it. It probably requires a long exposure.

Adam Munich

Mon Jun 14 2010, 01:58AM

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

Assuming worst case scenario of 100kv...

HVL of Pb for 100kv is .27mm
Thinnest part of box is 3.6mm
3.6/.27 = 13.3333...
so;
100% penetration, halved 13 times = 0.00162760417%

.0016% ...That gives me some confidence :)

So, with a value of...
1sievert/min inside box * .0016 = 160mrem/min directly in front of the thing. Not safe, but manageable.

So if i'm 200ft away, w/o shielding, inverse square law dictates that it'll be .004millirem/min. or .24mr/hr. That's not negligable, but it's safe because this thing will only be on for fractions of a minute once every 20 minutes or so.

.004mr/m = 0.00006 mr/s
0.00006 mr/s * 10s = 0.0006 mr/longexposure without apron.
so 20 exposures = 0.012millirem

Hmm, lets add the 1mm lead apron, or roughly 3 halving thicknesses.
.0006mr/exp /3 = 0.0002mrem/exposure with apron

If i make 20 exposures every day for an entire year (ridiculous) that would work out to be 1.46 millirem a year + background. Since a smoker gets roughly 270mrem a year, i think i designed a pretty damn safe machine.

This is all in the beam path. I'll be behind the tube, so values should be even lower.

Please correct me if i screwed up somewhere. That's alot of math at 10pm.

Wolfram

Mon Jun 14 2010, 06:06AM

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

And even that is assuming all of the energy is at 100keV, when in reality, very little energy will be at the peak voltage. I think the average photon energy will be around a third of the peak photon energy, but to actually calculate the amount of radiation that gets through the shielding is not as simple. You can always do like you did and assume all of the photons are at the peak voltage, then you'll know that you get a very conservative rating. This is the way RadPro does it too.

Where did you get the 1Sv/min figure from?

Anders M.

Adam Munich

Mon Jun 14 2010, 02:52PM

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

Well, since proud mary's estimate was .5sv at a meter, the distance from the tube to the far wall of the box is about .5m So 4sv/min. But i will only be running the tube at 1/4 max power, 1sv/min. It was really a complete guess. :) And you're right, it will be at 50kv most of the time whose HVL is .06mm. I'll never take it past 70kvp anyway.

Also, the actual thinnest part of the box is the gaps between the lip and he lid, i put 1mm lead tape there. I'll put on another 1mm. I designed it so the gaps face upward, so any stray rays can only hit a bird or some mosquitoes. (i don't live in appt, and it'll be outside)

The box's gaps are completely sealed withe the so-called welds. Pic on other thread. I couldn't do that for the lid because it wouldn't fit on if i did.

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