Russian SNM-11

Linear and non linear

Russian SNM-11

Postby Steven Sesselmann » Fri Mar 21, 2014 6:00 pm

Hi Guys,

I am trying to breathe life into a Russian SNM-11 neutron tube and it refuses to play ball with me, maybe because I don't speak russian to it. I followed the wiring scheme with a 47 M Ohm load resistor, and a CR coupling with a time constant of around 6-10 µs, and was expecting to see a pulse at least once a minute, but nothing!

Running it at around 1700 V and looking for a pulse on my scope, the baseline is fairly noisy at around 5-10 mv ripple, I imaghine it must be the 47 M Ohm resistor which is noisy, as the voltage supply usually has less ripple than that.

Do you guys have any magic spells ?

Steven
http://www.beeresearch.com.au....take a difficult problem and find a simple solution!
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Re: Russian SNM-11

Postby chrismb » Fri Mar 21, 2014 6:14 pm

You need a high impedance preamp to help discriminate the pulses. One every few minutes, to background for the boron tubes. The best way to plan what you need to do is to watch the pulse signal on an oscilloscope and decide how to window for the pulses you are typically seeing from it. Depending on the circuit, the typical pulse is likely only a couple of times higher than the corona current.
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Re: Russian SNM-11

Postby Doug Coulter » Sat Mar 22, 2014 1:09 pm

There's a whole thread on these here: viewtopic.php?f=11&t=430&hilit=snm (IIRC, we have a few threads on these guys)

Jon Howard and I worked on a couple while he was visiting here as well. It truly helps to have a source of neutrons you know has decent flux to test things with so you can tell the corona noise from the cosmics from neutrons, to the extent possible.
We had "ok" (not great) results with the fine wine preamp topology, which has about 60k ohm input impedance (which the tube kind of sees via the coupling cap, and that low is, well, kinda too low but it worked anyway).

We had issues with them going into relaxation oscillation, lousy quantum efficiency, and yes, noise, even when loaded with a 10m scope probe after AC coupling (which acts like reducing the 50 meg series R too, as far as the AC impedance goes). While I have a good number of these, none are currently in use here - almost anything else works better. The corona noise is normal - and that's what it mostly is, not the series feed R. And you have to get into the corona region (the thing is pretty sensitive to the voltage supply, unlike the claims) and in more or less in the right part of that. We used a variable DC regulator driving a CCFL with diodes and filters for our HV, it was pretty quiet and decently regulated.

They need this high impedance or there's too much energy available to go from corona -> arc. That wrecks the tubes at some point. This includes any energy stored in the coupling cap if the output side is tied to ground with low impedance.

The word is, you CAN make these work, but being both small and with very lousy quantum efficiency - it's not easy. It's truly a huge help to have a known-hot neutron generator for testing. They work well enough on a fusor, but are useless for anything with few neutrons present, as cosmics will then dominate the count rate - and you won't know where to set your threshold - cosmics are all over the place in pulse size, where most of the neutrons are "somewhere in the middle of that". The tiny size of most of these means even cosmic ray hits are rare enough that you might not catch them easily on the 'scope.
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: Russian SNM-11

Postby Steven Sesselmann » Mon Mar 24, 2014 1:38 am

Thanks Chris and Doug,

As I suspected, the juice may not be worth the squeeze, everytime I think I saw a pulse on this thing, it turns out to be someone switching on a light bulb or the refrigerator moror starting up, I don't have much confidense in this tube. I bought some russian He3 corona tubes as well, but they have not arrived yet, I hope they are kinder to me.

As for the preamp, I am sure that can be dealt with as long as I can see a pulse on the scope.

I shall report back when I get frustrated over the He3 tubes.

Steven
http://www.beeresearch.com.au....take a difficult problem and find a simple solution!
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Re: Russian SNM-11

Postby chrismb » Mon Mar 24, 2014 10:19 am

I used a comparator and converted the signal to a digital output. I used very tight criteria to avoid false signals. You have to be very wary to ensure optimised EMC protections because amplifying the corona current does, indeed, naturally lend itself to picking up RF interference. I also included a 'disregard' when a + and - signal were seen near simultaneously by having an additional positive trigger which inhibited the count, as RFI tends to give a + & - signal, rather than just a -ve going signal. I did all that because I put the pre-amp at the end of a lead to the tube rather than have it right next to the tube (so as to get the tube right into the middle of a moderator). Somewhat a risky strategy, but I tested it and it worked fine.
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