How we made our pinhole camera screen

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How we made our pinhole camera screen

Postby Doug Coulter » Mon Sep 01, 2014 4:01 pm

Just to start the ball rolling here. We've used a charged-particle pinhole camera as a diagnostic for the fusor (and we're lucky we didn't melt it back then). It looks like this in operation:
PHR2.jpg
Pinhole camera, probably showing energetic electrons mostly.

At least back when we weren't making much fusion and/or putting in a ton of power - the fusor today would evaporate both the lead front and burn out the screen. Yes, I'm looking into other ways to make a more robust screen, among other things. This one lacked even an aluminum flash layer but worked anyway.

Chemicals needed:
ZnS:Ag, obtained in this case from Eljen. ZnS doped with silver gives that nice blue light, and is primarily (but not entirely) sensitive to charged particles, rather than gamma rays. In our case, that's what's wanted, we already know where the gammas come from, and were interested in other things.
Sodium Silicate. Potassium silicate would have been better, but is hard to find/get. We got ours at McMaster-Carr.
Distilled water.
HCl or acetic acid (about 20% in the latter case, from photo shortstop). Actually, almost any acid that won't take the ZnS apart can be used. This hardens the sodium silicate. IIRC we used HCl here, and not much of that.

We made a 2" screen by the following method.

Take a 250 ml beaker and fill it up to about 200 ml with water. Suspend about 100-200mg (forgive me if this is wrong, I'm going by memory, but will be making another soon and will make corrections as required) of the extremely fine mesh ZnS in it.
Add about half an eyedropper of Sodium Silicate to the mix. Stir well, we're looking for a uniformly cloudy white liquid here. You want the finest mesh ZnS you can get. And the sodium silicate should be a minor component of the mix - less is more here, we just want enough to barely glue the ZnS to the glass. We don't want the whole beaker of stuff to get gelled or anything.

Drop in the 2" piece of quartz or pyrex. The latter is better as it's a little electrically conductive.

Allow to settle. If t his doesn't take about overnight, then your ZnS isn't fine enough mesh.

Add about half a dropper full of the acid. Stir very gently. Let the mix sit a day or so.

Take out your screen (pour off the liquid gently and hook it out with a bent wire or the like). Let it dry, then bake it in an oven, starting low (below boiling) and working up to around 300f for about an hour.

You're done - you've made a CRT screen! Be careful, it's easy to scratch. You can clean off the backside after you mount it. In our case we used a piece of copper tubing, 2" ID by 2" long for the camera body. We had to turn out the ID just a little to make a step to seat the glass in - the glass was a little over spec, the tubing a little under, it worked out perfectly. We then soldered a 50 mil thick piece of lead over the other end, with a 30 mil hole in it as our "lens". We brazed on a piece of #10 wire and an adapter mount for our wiggle stick so we could move the thing around from outside the vacuum and note any interesting changes with position. That version eventually failed due to solder melting, and we are darned lucky none of that got into the turbo vacuum pump. Next time, all high temperature stuff.

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The difference between sodium silicate and potassium is that the sodium silicate will "brown" much easier, so the pros used potassium silicate back in the day for longer life of TVs. I got most of this from the RCA tube design manual. Not the one on how to design with tubes, the one on how to design the tubes themselves. Warning, this link is 147 megs.


Note that throughout this, less is more. If the coating is too thick, it will make a crappy screen and it'll be non-uniform. If you use what looks like "enough" ZnS, it's much too much, we want mg/sq here at most and it's pretty dense stuff.
Ditto the binder, in this case sodium silicate. We're not trying to make rocks, jello, or anything like that - just barely enough to bind a thin layer of ZnS to the glass.
If that settling doesn't take essentially overnight, you didn't have fine enough ZnS to make a good looking screen, and the sensitivity of the result will be "lumpy" at best - our first was a failure due to too much ZnS.
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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Doug Coulter
 
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