SNM-11/СНМ-11 Tube - Again ...

This area is for discussions involving any fusion related radiation metrology issues. Neutrons are the key signature of fusion, but other radiations are of interest to the amateur fusioneer as well.
Lukas Springer
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Real name: Lukas Springer
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Re: SNM-11/СНМ-11 Tube - Again ...

Post by Lukas Springer » Wed Feb 20, 2019 10:09 am

I'd be interested in doing a FAQ, since this is kinda what I tried to do with this thread:
viewtopic.php?f=13&t=12474

I will collect more data tomorrow, write up something and post it here, but that might take a while, since I've got a lot of exams right now.

Peter, I know how a geiger tube works, I just don't know about any geiger neutron detector, since that would not make any sense, losing all ability to discriminate neutron / gamma events.

I still don't understand your "15 kV noise", since most detectors work with an order of magnitude less voltage and would be completley swamped out by the noise.

Peter Schmelcher
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Re: SNM-11/СНМ-11 Tube - Again ...

Post by Peter Schmelcher » Wed Feb 20, 2019 8:20 pm

I would guess all these Russian tubes were originally intended for Chernobyl. They got stuck in storage for 30 years before finally being considered surplus junk and we got them cheap.

Lukas I agree Geiger mode operation seems odd but who knows what was intended 30 years ago. If I was asked to design a preamplifier for intermittent monitoring in the core of the sarcophagus over a few hundred years I doubt any solid-state electronics would survive so Geiger mode operation, vacuum tubes, silver mica capacitors, ceramic standoffs, and ceramic insulated wire I would all consider reasonable.

State of the art charge preamplifiers are a big topic, 12kV rms of noise in 2010 was as good as it got for the folks at CERN. Don’t get bogged down with the jargon of charge amplifiers, if we were talking about preamplifiers for satellite dishes the units would be noise figure or noise temperature. All you want to do is make a measurement and know how accurate it is. When we talk of He3 capturing a neutron 764 keV of energy is released, the 15kV that I mentioned is my measurement error. If you asked me to measure a single He3 thermal neutron capture event I would report something like 760keV +- 15keV. If I plotted or averaged hundreds of these capture events hopefully I would report 764keV.

-Peter

Luke Weston
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Re: SNM-11/СНМ-11 Tube - Again ...

Post by Luke Weston » Thu Mar 21, 2019 2:57 am

"I've been frustrated by the signal to noise ratio of the SNM-11 tube at the recommended bias voltage of 1600V. The bias resistor is 50Meg and the signal is ac coupled and terminated as shown in the data sheet and here. The amplifier is a simple common source JFET amplifier using the MMBF4416A."

Okay, the Russian datasheet clearly shows the "load" resistor is 16k and 47M for the bias resistor.
And the coupling capacitor is 390... is that 390 pF? I don't know how to translate it but I assume it's pF.
I've been using 100pF because it's the closest HV ceramic I had in stock.

For the sake of consistently reproducing your experiment, could you share the specifics of the common-source JFET amplifier?
What drain bias resistance and what supply voltage? I'll try reproducing the effect at the low voltage you describe.

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