SNM-11/СНМ-11 Tube - Again ...

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Joe Gayo
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SNM-11/СНМ-11 Tube - Again ...

Post by Joe Gayo » Sun Feb 17, 2019 8:18 pm

The Russian tubes have been discussed numerous times, but I recently made some observations that are worth sharing.

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 (http://www.fusor.net/board/viewtopic.php?f=13&t=12474). The amplifier is a simple common source JFET amplifier using the MMBF4416A. A typical detection waveform can be found here (http://www.fusor.net/board/download/fil ... &mode=view).

I decided to bring the bias voltage down to just where the corona discharge begins ~670V and the detection results are far more satisfying.
IMG_1153.jpg
Are there any downsides to running the tube on the corona discharge threshold? The detection seems clean and registers 1-2 counts/min background and high levels while running the fusor. Un-moderated I still have some detection events. I will need to record the relative levels to make sure it's statistically significant and keep my distance so that my body doesn't serve as a moderator either.
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CHM-11p2.jpg
CHM-11p1.jpg

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Richard Hull
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Re: SNM-11/СНМ-11 Tube - Again ...

Post by Richard Hull » Mon Feb 18, 2019 6:48 am

I have long held the Russian corona tubes are OK. Their application by inexperienced users in implementation is typically very poor as the tubes seem to be a bear to get to work and biased properly to really detect neutrons.

The best work in biasing and recognizing the moment they are really working have mostly come from folks who have already conquered and achieved fusion. With a known source of neutrons they ,only now, with knowledge on their side, can look back at their first attempts to use the tube. In most all cases they chase the data sheet biasing and input circuit to heel with a good deal less bias than the spec sheet recommends. As Joe has just posted, and as others have posted in the past, this common look back is successful and enlightening.

I think Joe has found his sweet spot on his particular Russian corona tube and it is now detecting neutrons with it. This has only been reopened due to his continued curiosity and the desire to back-track and see if he could make the mystery of the Russian tube disappear. I feel it is only possible due to his real hands-on experience in seeing how easy it was to get his 3He tube working and what real neutron production looks like in his particular situation. It is only with this positive result could he hope to go back and poke at the Corona tube and make it work accordingly.

About the only thing I would have added to his seeming success is to get a blisteringly hot U specimen, (100k++ CPM) and set it next to the naked tube and see if his supposed working Corona setup leaps far above his background count. If it does, he is still counting non-neutron events. Hot U ore has many energetic gammas streaking out of it between 200kev to over 1 MEV and many very hard Beta rays over 1mev peak, due to many grown-in daughters. If the naked tube does climb, put it in the moderator, place the U rock near the moderator and see it the count goes back to background, if it is does, You are probably OK and it is only counting neutrons, for the most part.

Both improperly set up boron lined tubes and Corona tubes have this latter issue like the plague. There is a "sweet spot" and it must be intelligently found to be called a true, neutron-only, detector set up.

Richard Hull
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Re: SNM-11/СНМ-11 Tube - Again ...

Post by Lukas Springer » Mon Feb 18, 2019 10:11 am

I have two SNM-11 here and will get a good neutron source next thursday, do you want me to test anything specific?
Just below the corona ignition point is the proportional region, where you don't get any corona noise, but also lower gas multiplication and most importantly a less stable multiplication factor.

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Re: SNM-11/СНМ-11 Tube - Again ...

Post by Richard Hull » Mon Feb 18, 2019 8:45 pm

I have often wondered about the variability between individual corona tubes. Regardless, they can be made to work by those adroit enough to tackle the electronics and have a decent neutron source and a very hot U source. They do have a "sweet spot".

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
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Re: SNM-11/СНМ-11 Tube - Again ...

Post by Lukas Springer » Mon Feb 18, 2019 10:12 pm

I can compara my two SNM-11 to see if there's any difference in ignition voltage, pulse amplitude, etc.
What do you mean by "a very hot U source"? Very high input voltage to the fusor?

I have not seen any kind of sweet spot on my tubes, quite the opposite;
they seem to be very constant in their output amplitude once the corona has ignited.

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Re: SNM-11/СНМ-11 Tube - Again ...

Post by Rex Allers » Tue Feb 19, 2019 5:32 am

Answering for what I think Richard meant.

The "hot U source" would be uranium.

Ideally the neutron detector tube can be tuned to accurately respond to neutrons, but also not respond to gammas or x-rays, that is, act like a Geiger tube. The hot U source (a lump of hot ore, I presume was meant) would provide a source for testing and hoping to see no response.

A hot radium dial from an old clock or gauge might be another source.
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Re: SNM-11/СНМ-11 Tube - Again ...

Post by Peter Schmelcher » Tue Feb 19, 2019 9:06 am

I did lots of work many years ago with various Russian tubes. I knew a Russian Physics and he graciously translated a detector data sheet for me.

The SNM11 / CHM11 tube has a maximum do not exceed voltage of 1600V. The recommended operating voltage is 700V and when detecting thermal neutrons the efficiency is 15%.

I designed my own extremely low noise and linear preamp just to determine if the tube I had was bad and it was. Eventually I got a different tube and was able to test my neutron detection setup using cosmic ray generated neutrons. The preamp RMS noise is about 15kV and I posted details and many oscilloscope pics.

The figure of merit for a charge amplifier (or integrating amplifier) is the RMS noise voltage. This sets the error bars on any measurements you make. The Geiger He3 tube I have can also be used as a proportional tube which means that the incident kinetic energy of the neutron can also measured. I think of the physics like aero breaking of space vehicles. Decelerating friction through the pressurized gas creates a plasma and the amount of plasma created to stop the object is in proportion to the initial velocity of the neutron. I have always believed this is a great diagnostic tool because the addition or removal of a moderator can be easily observed on an oscilloscope and statistically measured.

-Peter

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Re: SNM-11/СНМ-11 Tube - Again ...

Post by Lukas Springer » Tue Feb 19, 2019 3:59 pm

They have some U ore samples with a couple hundred µSv/h, I hope that is enough to test gamma rejection.

I attached the (simplified) datasheet for the SNM-11, where it states that corona ignition voltage is no more than 700 V and working voltage is 1600 V.

What do you mean your preamp noise was 15 kV, Peter?

What "Geiger He3" are you talking about?
If you operate a He3 tube (or any neutron detector) in the geiger region you lose any chance for gamma discrimination.
You can't measure neutron energy with a proportional detector.
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Re: SNM-11/СНМ-11 Tube - Again ...

Post by Peter Schmelcher » Wed Feb 20, 2019 1:31 am

Increasing the bias voltage increases the gain of the tube. This change is not linear. At some higher bias voltage the atoms start to behave more like a ring of dominos, knock one over (ionize one atom) and they all fall down (which is Giger operation and I do not use the tube at that voltage).

Yes, the preamplifier output noise voltage is 15kV rms but this is referenced to the input signal, connect the tube bias voltage to ground and measure the preamp rms voltage output noise. Next you measure a known input signal but again at the preamplifier output. I used the He3 capture energy for thermal neutrons as a known input signal. I stuck my He3 tube into a test tube with water around five sides. Looking at a scope with infinite persistence turned on the strongest band is the He3 neutron capture energy. This calibrates the tube and preamplifier gain together. The catch is you need to know the tube is biased in the linear region. Commissioning my setup, I made many long measurements of cosmic ray liberated neutrons with small increasing adjustments of the tube bias voltage. I have a couple of small LND 25169 tubes with 10barr of He3 inside them. They have a reasonably wide linear plateau region where the tube gain changes only slightly with voltage.

I also have some larger 3 or 4 barr Russian tubes from fleabay. I posted rather cryptically about them for other forum members to purchase because they were such a good deal. For the life of me I can't find a link to that post.

There are many interesting books and papers on line if you do some searching. For example
http://www.fusor.net/board/viewtopic.ph ... 913#p74913
http://www.fusor.net/board/viewtopic.ph ... 577#p74577

Have fun
-Peter

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Re: SNM-11/СНМ-11 Tube - Again ...

Post by Richard Hull » Wed Feb 20, 2019 6:03 am

Discussions and exchanges on the Russian tubes are always good and I enjoy them. I just wish someone who has extensively used and biased these tubes would do a FAQ in this forum!! In this manner the fax would be a great original work by a credible worker and updates and responses could herd all the good data into one unit.

If someone bellies up to the bar....Might I suggest a title in the standard FAQ format.... FAQ - Russian neutron detection tubes.
This would be a place for Russian 3He and corona tubes. At the beginning of any responses or additional data one might identify the tube or tubes by number under discussion and what type they are. The value of the FAQ would not be all inclusive as most typical FAQs strive to be, but act as a collection point for the common experiences of users. A FAQ would avoid having to ferret out old good loose individual posting over this entire forum.

Once a FAQ is up, any old posters who dropped pearls of wisdom in the past might respond in the FAQ with links to there original postings.

Any takers?

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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