X-Ray Anode

Various things to make charged particles fly

X-Ray Anode

Postby Tyler Christensen » Tue Jun 14, 2011 8:56 pm

Not sure if this is really the right posting category, didn't see anything chemistry (maybe something should be added).

Been working with an x-ray source, and recently we've been making various anodes to calibrate some instruments. Tried coating an aluminum anode in sodium chloride today by dissolving it in water, putting a drop on the tip of the anode, then heat drying the anode. This stuck fairly well, and then it was run in the source for a few minutes at 6kV 1mA beam current, and came out blown off in the center, and purple where it remained. Just out of curiosity, we were wondering what it is but couldn't seem to figure out. Only remotely reasonable purple substance of aluminum or sodium we seemed to come up with would be sodium permanganate, but that doesn't make much sense really, possibly some manganese in the aluminum alloy but I wouldn't think it would this easily react with the salt coating. Any thoughts what this is or what happened to it (pics attached)? This was scientific grade sodium chloride, not table salt.

Also, any tips on anode coating with salt so that it won't burn off or become chemically altered? We also used milk of manganese on another anode and interestingly that one worked great and didn't seem to burn off.
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Re: X-Ray Anode

Postby Doug Coulter » Wed Jun 15, 2011 9:47 am

This is an OK place to put this - we don't have a chemistry section because I'm afraid it would devolve into too much info about one of my hobbies - high explosives, and get us in trouble. :roll:
I have the licenses and approvals from BATF but...I'd like to keep them too.

Though an electro-chemistry thing might be good as we do a lot of that too, for physics. Plating and electro polishing are two things we do a lot of here, I'll see where that would fit.

Some things will just never fit into a simple category, and this might be one of those.

And an X ray tube is, after all, an electron accelerator. Looks like one with good focus too - I'd like to see the other end of that to see the design, as a side issue.

Lessee -- some of this would be guessing, so take it with a grain of salt, OK?
Yes, some Al has Mn and Si and other junk (Cu) in the alloy, but I sort of doubt that's the purple. More likely it's an interference color from the different index of refraction of salt vs 1.0 and a particular layer thickness showing that. You might have created a vapor deposition device by accident. Likely.

6 watts in that tiny focus is one heck of an energy density to handle -- John F was getting a silicon substrate to glowing heat with a beam of only 15w and liquid nitrogen cooling on the backside in his recent movie here, which I can't find now - he might have removed it. Salt has lousy conductivity (both kinds) and that's going to be a problem. If you have a thick layer, it's worse for a couple reasons. One is that it's going to act like a little capacitor, charge up, then arc through -- and salt goes blooie. The other is that it's going to melt/vaporize and same result. A little more info on why you're using it might suggest a better compound to work with there. You might have to go to "thin target" in the sense that some electrons will make it through to the substrate and make X rays from that too - but all spread out in energy if the salt is any thickness (energy straggling in the electrons that do make it through) -- it's a hard problem! You might reduce this problem by going to a pulsed mode so it can re-freeze and lose its surface charge more gracefully. Again, a guess, but one based on a little bit of experience (but not tons).

You might be better off with a deliberate vapor depostion of salt, real thin, and perhaps on a different substrate, depending on which other X ray lines would or wouldn't pollute what you're trying to measure here. And/or a different target composition than NaCl. There are some compounds of either Na or Cl that might work better, depending one what you're after with this.

You might also, with that nice focus, put the salt into a cavity with a tiny hole to let the beam in, which would let you hammer the salt in bulk, but keep most of it inside the cavity. The fact that it's going to get real hot might make for line-spread (doppler) but...I don't know enough about what you're doing to say. If the cavity was something that let out the X rays well (like Be) then it might work out.

Remember, a 5w resistor is big and even in air it gets pretty hot at 5w -- here, well, it's not as good a thermal situation. Can you observe the target when in use and see if you don't have some incandescence on it? You should see that at the melting point of salt (about 800c). At that temperature, it's going to surely react with the Al and any alloying components. Salt is only held together by bonds of a few eV...and both sodium and chlorine will react nicely with Al at temperature.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: X-Ray Anode

Postby johnf » Thu Jun 16, 2011 2:41 am

Hello guys

Tyler
not good using a Halogen salt. You might find that the purple colour is a tungsten salt on the anode --ie halogen lamp cycle , but in this case the filament is not running at high enough temp to complete the cycle to get the tungsten back on the filament.
Sodium metal would have been better but you will need a cooled anode so as not to get the sodium vapour pressure up.
Sorry not been posting lately, Dads crook its taking quite a bit of my time

No Doug I didn't pull down the Utube Vid, maybe they did as (was it Jerry ) suggested they might for using audio clips from lesser known movies

all for now, outa here, got a Sked to take on 3760kHz
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Re: X-Ray Anode

Postby Doug Coulter » Thu Jun 16, 2011 12:32 pm

For all I know that vid might still be up -- here, I just couldn't find it. Maybe it fell prey to the "water cooler auto delete" -- if so, we ought to relink it someplace safer, it was good. Funny too.

Duh, I didn't think about the well known tungsten-halogen cycle! You're very likely right about that one, and the deposited tungsten chloride would mess up the results too - I suspect the low Z anode was used for a reason. There is no question that 6kv electrons would disassociate NaCl (or any other compound for that matter). They even take pyrex apart pretty quick.

I used to listen on 80 meters religiously when I was a SWL, using an old ARC-5 mil surplus receiver. Fun times. I'll have to fire up my Ten Tec 320 again for fun.

Tyler, John's almost surely right about that - he's got a lot more experience in this sort of thing than the rest of us combined. More info on what it is you're trying to do would be nice to tap into all this expertise we have here.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: X-Ray Anode

Postby Tyler Christensen » Thu Jun 16, 2011 1:02 pm

With regards to more details on the source and its geometry to get a very focused beam, I attached an external image of the source end of the system. It's all pumping down with no plans for short term removal so I won't have a picture of the inside where the filament mounts. Also attached an image of the beamline and the whole setup.

The sodium salt is being used as a low energy x-ray emission material, along with many other materials. Need quite a few materials to get a really reliable calibration on a spectrometer attached to the system (the purpose of the whole setup being calibration). Some other materials include titanium, aluminum, copper, boron-nitride, carbon, manganese, iron, and chromium. Attached a graph of the some readings so far.

The chamber of salt is an interesting idea, although I'm not sure how practical that would be geometrically for this source given the limited anode space inside (it can't get much bigger than it already is and there isn't any significant depth to make a cavity).

There is a window viewing the anode, although I hadn't looked in it much during operation. Checked yesterday on a lower power sodium chloride run (0.1mA) and it was fluorescing with a blue beam, but not appearing to thermally glow. The second anode in the last picture is sodium chloride after a few minutes at 0.1mA 6kV, turned yellow instead of purple this time.
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Re: X-Ray Anode

Postby Joe Jarski » Thu Jun 16, 2011 7:09 pm

John's video is here. It was hiding in the "It almost worked" section.
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