Alpha Probe, simple and cheap

  • Sonarflash
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  • Blind since 1966 - 51+ years with insight.
5 years 9 months ago #3280 by Sonarflash
Sorry that there are no pictures here.
I had a couple of spare Sovtube SBT-11A. This is a small, rectangular mica window GM tube. It works fine on 400 VDC. Instead of cutting into a project box to try and make a fancy mount, I just wrapped wire leads around the pins without soldering and used vinyl electrical tape to hold everything together. First, I had a rectangle of SS mesh from Atomic Dave. I folded edges to fit the SBT-11A rectangular face (to protect the mica window). Then, I wrapped the perimeter with vinyl electrical tape. Next, I wrapped a solid 16 or 18 gauge wire around the set of three anode pins which are located at one end. These were shorted together, and to this wire, I twisted on a 10 megohm resistor. Next, I twisted a second solid wire lead to the middle pair of ground pins ((these are nearest the block end), shorting them together. Layered a few pieces of electrical tape over these connections so the tape ends stretched over the sides but not over the SS mesh window cover. Then, I hooked on a GC cable that had the proper connector for plugging into my GK+ detector. My unit is set at 400 Volts. Any coax or wire pair should do, depending on your equipment. I've even hooked this up to a GK4B that isn't in a box. The SBT-11A is about 60% as sensitive as a larger, much more expensive LND7317. Background readings seem around that level with my equipment.
The Russian (Soviet era) SBT-11A has three (anode wires, about 14 or 16 gauge diameter) parallel bar anodes running the length of the rectangular window. The ground inside is a curiously folded arrangement of metal sheets. The three anode bars can be hooked up individually or common, as with the two rectangular ground sheets. Would be interesting to know why they designed the tube this way. Perhaps just for sensitivity? Or tracking alpha particles coming in at angles? Sort of an internal coincidence detection array?
Although this tape wrapped probe doesn't look fancy, it is tiny and very sensitive to alpha. I've tested it out with radioactive rocks and used it in thrift stores to check crystal glass. At $29.95 USD, it isn't expensive, and Sovtube still lists them.
Oh, and be careful with twisting wires onto the anode pins. While attempting to crimp them tight, I managed to rupture the mica window on a tube. That's how I got to investigate the innards.

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