Wednesday, January 16, 2013

Siemens EPD-1D dosimeter teardown

Ebay can get you a lot of really exotic items really cheap. Period. This was no difference. While browsing through looking for a cheap (and yet still functional) NaI(Tl) scintillator, I came across an offer that looked like this:

(please excuse the violet background, I'm not the author of the picture...)

(© goodgadgets2u)

If you are still clueless about WTF is that, you are looking at the business end of a Siemens EPD-1D gamma dosimeter. Three PIN photodiodes (no idea on actual part number), each with a different level of shielding. The opaque one has a thick lead alloy shield around it (pictured above it), the right one seems to have a thin lead disc on top and the middle one is covered by thin aluminium foil supported by a polymer substrate.
The blue thing are three separate very sensitive FET amplifiers.

Instead of the sawed off guts, I got an entire unit (non-functional though).

© me (you can shamelessly steal these)

The unit did do something after being connected to power and restarted, but after performing a self-test and a few beeps, it always stopped reacting, while displaying an error.
So, autopsy time:

front cover, backside view
(note the shields for the diodes)
back cover, backside view

back cover, front side view

PCB, front
note the second piezzo buzzer (no idea why)
PCB, back
PCB, back (again)
(slight corrections so that everything is readable)
For some (to me unknown) reason, they decided to use a 4-bit MCU(the biggest IC on the picture), made by NEC in Ireland (Damn this thing is old). The small one labeled "X24C16S" is a 16K (2048 x 8 bits) serial E2PROM, most likely to store the dose measurements. (I might attempt to read the contents some day)
I have no datasheets on the other ICs though.

I'm just speculating, but the IC closest to the blue amp board is probably an ADC, the logo is Texas Instruments, but the search on their website returns nothing.
(Feel free to contact me if you know anything about any of these ICs)

Other than that, there are two crystals, one is for the CPU (1 MHz), the other is a real time clock (32.768 kHz). Note that the 1 MHz crystal has a ceramic package with a transparent window, so you can actually see the crystal itself (the thin rod in the middle).
That about wraps up the summary of interesting stuff  I can tell about this board. (for now, at least)

I'm still in the process of reverse engineering the PCB, since actually I need to finish building a capacitance meter first (SMD capacitors do not have the value displayed on them and the coloring is not standardised). I'm really only interested in the analog part of the detector, so I can rebuild it with a different MCU.
All will be posted in due time, so until then...

Sunday, August 7, 2011

Grind & polish your scintillator part 2

Been lazy and didn't bother to take any pictures during the procedure...sorry. Wasn't that much to see anyway - wet sanding with 600, 800 and finally 1200 grit sandpaper. Then notebook polishing again. Turns out it's not communist since it was made by a shareholder company. Someday in the beginning of the 90s. Still worked though.

Still far from perfect, but it's nice & transparent.
Note the TIR.
Here you can better see the surface texture.
Again, note the TIR.

This side is not that good, I might have to do a bit of work on it later. 
One of these sides is going to face the photomultiplier.

Turns out that Mirelon is good safe-packaging material. Normally it's used under floating floors
I have quite a bit of leftover, which is why I use it for almost anything...

note: I've found out, that the paper I used for polishing quickly clogs up and instead of polishing starts to create sort-of clumps that stick onto the scintillator. You have to use just one paper per side, use a clean one for the next side. Sometimes even that is too much.
Also, the PVT significantly charges with static electricity when rubbed on the paper. Tends to attract dust and holds the charge quite well. Also, it tends to charge you. 

Well, that's the scintillator. Wasn't that hard now, was it? BC-408 is quite easy to machine, you just have to be careful with the heating, it doesn't seem to mind wet and dry sanding. You certainly couldn't do that with an NaI(Tl) crystal since it's hygroscopic.

To come: I've managed to score 2 FEU-36 photomultiplier tubes for a ridiculously low price. It's an old Russian-made cesium-antimony photomultiplier with 13 dynodes. Also quite big, since it runs on between 1,24 to 1,6 kilovolts. Did I mention it was made in 1963?
Quite nice it comes with it's own datasheet which has !hand! written measurements. Also, it's completely in Russian. Yay for me...

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...

Friday, May 6, 2011


OK, it's alive! (at least it seems to be...)
But I'll still have to play with the color settings to make it more human-readible...