Sunday, June 5, 2011

Grind & polish your scintillator, part one...

If you have no idea what a scintillator is, in short it produces short pulses of light when ionizing radiation passes through. That being said, it's quite obvious, that it's a key part in a scintillation counter/detector. (Note - it's NOT A GEIGER counter if it has no GM tube...)
Since I've always wanted to build my own radiation detector, I've been collecting information on them wherever I could. To build your own, you have several choices - 1) a GM tube, 2) an ionization chamber, 3) a scintillator.
GM tubes come in different shapes, sizes, materials, capabilities and prices. Old surplus soviet made tubes are dirt cheap, however they ignore most of the radiation passing through them, but have a sturdy construction. Unless it's a "pancake" probe, it's unlikely that it will have an end window, so it won't detect alpha radiation & will ignore low energy beta. Western made end window tubes do detect alphas, but are quite expensive and hard to find. Pancake probes go from $50 up on e-bay. Also, they are incredibly fragile, they need to have a mesh at the end window to protect it, even so it's still very easy to destroy it. They also do not like rapid pressure changes.

Ionization chambers differ in construction, I will not describe them here. hint - fluorescent tubes for lighting can be used in this way as well...

Now for scintillators - unlike the previous two, the detection medium can be both solid or liquid, meaning they have greater density, so they capture(and detect) more radiation. I may play with liquid scintillators in the forseable future, but not now. As for solid scintillators, they can be inorganic or organic. Since growing large inorganic crystals is expensive, so are the scintillators. As for gamma detection, they are far superior to organic ones, but in cost and versatility the organic ones win.
When searching, I found that a user Geoelectronics sells small quantities of plastic scintillators on e-bay at a very reasonable price. The one available was RP-408 made by Rexon. Aparently (judging from the emision spectrums), it is identical to BC-408 made by Bicron. It was for $8 a 1x1x1'' piece, so I ordered a 1x1x2'' piece. The picture (yes, I know...) showed neat looking, straight and polished pieces of it.

This is what I recived.

Not that I want to bad mouth someone, but if I can't cut straight with a hacksaw, I would at least clean it from metal shavings(not shown).

See the cuts?
(And yes, none of the sides is straight.)

So I looked for the most leveled side (took a while), and then just stuck a piece of 80 grit sandpaper onto a slab of glass (it's the most straight thing normal people can get their hands on) with sticky tape... (hey, it was at hand and did it's job)

After approx. 15 minutes of sanding. 

It's straight!!!

Now, how the hell do I level off the other side?! Sanding?


Yup, that is a normal drill press, with a slab of glass taped to it... No, you should definitely not do this with a normal drill press. The bearings are not built for being loaded on the other 2 axis. But this thing is just flimsy polyvinyltoluene with some anthracene in it, it should't hurt the poor CCD (Cheap Chinese Drill). At least not too much...

more machine abuse...

Now, one more thing CCDs do not like is prolonged run. Like more then 20 minutes... The motor gets too hot to touch, if I continue, the elamel on the wires starts to smell...

Where there's a will there's a way...

"That" is a universal grinder/sander/table reciprocating saw/fan... Made in Czech Republic some while ago, it doesn't seem to be around for sale anymore. Too bad. Wasn't even expensive back then.

Being a bit too hasty...

Since I'm using my own hands, it pays to be patient. Oh well, just turn the drill on and clean it off with a random piece of wood.

Some more machine abuse...

Note the paper, without it, the half an hour of sanding would be lost. Also, PVT is rather easily damaged*.

"machining" done!

* - See the beveled corner? I've managed to drop the damn thing when I was laping it oustide onto a terrazzo surface from approx. 20cm. Instantly, two corners were chiped.

Lapping done!

I've used 80 grit dry paper to level the sides, then 220 grit wet and last 600 grit wet.

Clear side - this is how it will look finished.
( I hope )   

After some polishing...

Not perfect, but it's a start...

 Because I'm a cheapskate, I didn't buy sanding paper with more grit and a real polishing solution. What did I polish it with then?

Comunist made school notebook!

Thats right, the coarse paper this notebook is made out of is perfect for the job. The bad thing is that 600 grit is apparently not enough to properly prepare the surface for polishing. Oh well, that's what part 2 will be for...

From left to right - 80, 220 and 600 grit paper.

I forgot to take a picture before I started, so these are after I was done with them. Note the darker stripes on the 80 grit paper - that is it's original colour. It's impossible to get the fine dust out of the paper, doing this outside or wearing a respirator is probably a good idea...

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