Is this neutron counter dead?

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Michael Kaufmann
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Is this neutron counter dead?

Post by Michael Kaufmann »

i got a very old soviet helium3 neutron corona counter chm-18-1, hooked it to a hv source in this way and attached scope input to the R2. got 20 µA tube current at 3600 v (2660 v on the tube) which seems totally normal, according to the datasheet, around 8 µA at 2400 v.
BUT - look at this monstrous output pulse

IMG_20211127_230446.JPG

108 volts just can't be right : ) and there are many pulses with 50-80 v amplitude like every 5-10 seconds, which also cannot be correct. if I reduce the supply voltage, the amplitude of the pulses will also decrease significantly, but not their count. all this happens without moderator/neutron source.
the tube doesn't react at all to a moderate radium source.

so how exactly do they die? what happens at the end of their life?
Last edited by Michael Kaufmann on Sat Nov 27, 2021 1:33 pm, edited 1 time in total.
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Dennis P Brown
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Re: Is this neutron counter dead?

Post by Dennis P Brown »

Not at all knowledgeable of He-3 detectors but certainly age could be an issue. No idea how old your Russian tube is but if surplus, could be an issue. For instance, since He-3 has a half life of 12 years, if the detector is 20+ years old then it would be highly reduced in performance. Others here should be far more knowledgeable.
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Scott Moroch
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Re: Is this neutron counter dead?

Post by Scott Moroch »

Dennis, I believe you are thinking of H-3. He-3 is stable, so the age of the detector (assuming no leaks), is not an issue.

As a very simple, first test, I would perform a continuity test between the anode and the cathode of the detector. If the anode is shorted to the tube wall, then it is dead.

I will leave others who have more experience with corona tubes to comment further.

-Scott
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Michael Kaufmann
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Re: Is this neutron counter dead?

Post by Michael Kaufmann »

@Scott

i can't quote myself (LOL), but this is my "continuity test" - a much better one btw:

>>>got 20 µA tube current at 3600 v (2660 v on the tube)
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Richard Hull
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Re: Is this neutron counter dead?

Post by Richard Hull »

The tube is proportional and demands a very special charge sensitive preamplifier. You may have destroyed it by over volting it. Such a pulse is indicative of a GM tube response. The tube is indeed a 3He tube and a proper neutron pulse detection should not be able to ever be seen on a low impedance o'scope without the special preamp. Do you have a neutron source??? A good neutron tube in a proper setup should never, ever give an output pulse due to intense x-rays of a very hot and intense Alpha, Beta or gamma radiation source!

You will need a tremendous amount of research into neutron detection and a 3He tube will not detect any normal radiation no matter how intense that you might normally encounter. Information is in the FAQs. the 3He tube is not a normal radiation detector.

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Liam David
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Re: Is this neutron counter dead?

Post by Liam David »

My primary detector is a CHM-18 tube, but I have never run it higher than ~1500V and typically use it at ~1300-1400V. You might expect to see ~12cpm background with this tube when moderated, so I am not all that surprised by the frequency. The amplitude is almost certainly the result of having the voltage too high.
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Michael Kaufmann
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Re: Is this neutron counter dead?

Post by Michael Kaufmann »

@Richard

this is not a proportional tube, and I don't have a neutron source yet - at this moment I was planning to observe background thermal neutrons. but the darn thing is constantly giving me 1-10 cps - I bought it for $25, so maybe it was dead for a long time already.

@Liam

yea - your detector is another story - not supposed to work in corona mode, hence lower voltage. i am getting ~10 cpm only at the largest pulses, but if i also count them in the 10-50 v range - i am getting up to ~10 cps from time to time.
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Richard Hull
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Re: Is this neutron counter dead?

Post by Richard Hull »

At the high voltage it is acting as a GM tube and sensitive to anything that happens along, even ripple spikes. You are horribly abusing the tube if not ruining it at such voltages.
Many GM tubes operate at 400-900 volts and give negative detection spikes equal to between 1/5 to 1/10 or more of the applied voltage (50-90 volts). Giant negative pulses are indicative of GM tube operation

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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Michael Kaufmann
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Re: Is this neutron counter dead?

Post by Michael Kaufmann »

so, maybe my power supply isn't clean enough and that's the reason i am getting all those high number of counts? thanks for the tip - i will try microwave oven transformer instead and see what happens. or maybe it's because of bad wiring, just laying on the floor : )

as about overvolting - don't worry, please look here:
Operating voltage (recommended) V 2300 - 2500
Average current corona discharge μА 25
i do not exceed these values at the tube.

and lastly - about GM tube operation mode:

Untitled.png

so it's totally fine too.
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Michael Kaufmann
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Re: Is this neutron counter dead?

Post by Michael Kaufmann »

nah - MOT didn't help - still around 1 count per second.
Cai Arcos
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Re: Is this neutron counter dead?

Post by Cai Arcos »

Hi:

Are all the pulses "single" (i.e, do all have the shape shown in the first photo) or do some of them have small bumps after the start that prolong their duration?
Something like the attached photo
Attachments
FailedQuenching.jpeg
Rex Allers
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Re: Is this neutron counter dead?

Post by Rex Allers »

Michael, if you haven't read the paper Bob Higgins wrote on how he used these Russian 3He corona tubes, I highly suggest you should do that. He uses a much higher resistance in series from the HV supply (~100M). He also goes from tube specs about voltages to some simple calculations for how he arrived at values.

He also describes an input amp he designed.

Richard provided the link info here:
FAQ - More on those devilish Russian Corona tubes
viewtopic.php?f=31&t=13128#p85322

That gives a link to a google drive location of the pdf. I know I got it from there a few years back but I tried the link tonight and got a message that I had to request access. I did, but haven't gotten any reply to that request yet.

If that doesn't work I tried another link to an earlier version and that still works.
http://physicsopenlab.org/wp-content/up ... 1v10-1.pdf

So that is v1.10. The google drive link gave v1.20 when I got it earlier. I think 1.10 covers all the important stuff.
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Cai Arcos
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Re: Is this neutron counter dead?

Post by Cai Arcos »

Rex:

The previous screen capture is from the Higgins paper.
I suspected the discharges are not quenched properly, but my knowledge of this type of tube is extremely limited and thus did not want to make any calls before hand.
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Michael Kaufmann
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Re: Is this neutron counter dead?

Post by Michael Kaufmann »

@Cai Arcos

hola : )
all pulses are single ones, with an ideal form of RC discharge, the same duration, the difference is only in their amplitude - soon I will record a short video.
your capture looks very strange to me - I've never seen anything like it.

I'm starting to think that this counter is working "fine", the only problem is that its own "background" is about 10-15 times larger than it should be. maybe the counter just lost most of the helium - the tube was made in 1985.

@Rex Allers

my previous post contains a screenshot from a pdf document called "SlowNeutronCoronaCounterTubes_1v10-1", so...: D
by the way - this document contains some incorrect assumptions about the parameters of the tube - I think these are translation errors from Russian, but they seem strange to me.

Bob does not use "much higher resistance" - only 2 times more. and he uses an amplifier, probably because his tube is very weak - his tube is also about 10 times less sensitive to neutrons, I think this is a bad choice for counting neutrons, because there is only 0.5 bar of helium, and the diameter of the tube is also small.
Cai Arcos
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Re: Is this neutron counter dead?

Post by Cai Arcos »

Hola!!

The pulse shown is a GM tube that fails to quench the original discharge. When the electrons of the original discharge (the big pulse) reach the wall and release secondary electrons, the electric field is still high enough (due to a low RC time constant) to allow multiplication of the new electrons- and thus a new pulse.

If you only have clean shaped pulses it seems to me like quenching should be fine, which probably also means not too much gas has escaped.

I believe your reasoning about the Helium might be correct. I can recommend one thing, and please, take into account that my knowledge about this type of detectors is limited and incomplete: try lowering the supply voltage so that the tube operates firmly in the proportional region with low gas multiplication. That way, the difference between the alphas produced in the He3+n reaction and the other things you are not interested in should be clearly seen.

As you lower the voltage you should see how the counts decrease until you reach a plateau with only the neutron ones.

My best wishes and best of luck!
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Michael Kaufmann
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Re: Is this neutron counter dead?

Post by Michael Kaufmann »

i really hate to go proportional route at this time : )

the video - https://www.youtube.com/watch?v=VR98RR5Z080
Cai Arcos
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Re: Is this neutron counter dead?

Post by Cai Arcos »

I know, going proportional means having to deal with corona noise, and suddenly you don't just get an easy to use pulse. No fun.
However I believe it is the easiest way to at least at first check the tube.

In the first part of the video, you show the big pulses. Since there is still a big variety in heights, I belive you are not quite at the GM region; more likely in the end of the proportional region where the gas gain is huge but very nonlinear with particle energy.

Do you know how to use the persistence function on your scope? I believe this can make your proportional testing a lot more pain free. Try lowering the voltage in certain intervals, and then leave persistence on for about 10 minutes (just check it visually and leave in infinite persistence. It will reset once you touch any button). In that time, you should see some neutron pulses. Thus, you should have about *three* or *four* big pulses and the rest should be noticeably smaller. Try this for various intervals everytime going lower, and you should see how the big pulses get *relatively* a lot bigger (this is the nonlinearity of gas gain I mentioned previously. As you decresease the gain, it gets more linear, thus the HUGE energy difference between the alphas and the rest of the junk should be apparent).
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Richard Hull
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Re: Is this neutron counter dead?

Post by Richard Hull »

Most all neutron counters tend to have a background counts of around 6-10CPM MAX.

I suggest that you obtain a hot radium source such as an old clock or military meter face and see if your count rate goes up. If it does, you have a great GM counter and not a neutron counter. A healthy hot piece of uranium ore would substitute to a limited degree. Place it next to the tube any significant increase and you are not working in neutron counter mode.

The very fact that you have no neutron source means you will never know for sure if you are suitably discriminated against detection of all other forms of radiation.

Corona tubes are tricky to make work well. They always have been and always will be for the amateur who does not have the electronic gear and know how to hone them in to where they will not count any radiation from a very hot source of normal Beta and gamma radiation immediately at hand to tune out all detection of that form of radiation.

A careful reading of the Higgins paper, above, will note that the neutron pulse from a correctly discriminated corona tube is only about twice the amplitude of the constant coronal noise!! It is a critical balancing act, demanding the best of instrumentation. His circuitry is an example of that effort and any serious effort to turn the corona tube to good account.

The use of a boron lined neutron detection tube is also another study in avoiding the normal "all radiation counting" Geiger region. The boron lined tube is not a corona tube and works only at a critically held pure DC voltage just shy of the GM region. (500-700) volts and is experimentally, critically determined.

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Michael Kaufmann
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Re: Is this neutron counter dead?

Post by Michael Kaufmann »

this Ra-226 source is hot enough?

Untitled.JPG

measured with lead gamma filter ON, without the filter it shows 12 mSv and i hear a very loud hissing from the pancake GM detector : )
placing this right next to the tube does absolutely nothing, even at the 3600 volts, when the counts are at maximum.
Last edited by Michael Kaufmann on Sun Nov 28, 2021 3:08 pm, edited 1 time in total.
Cai Arcos
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Re: Is this neutron counter dead?

Post by Cai Arcos »

Then I must confess I'm at a loss. If the Count Rate is independent of whether you got any source or not, then the tube must be defective, and is producing discharges that when are low passed by your RC network makes it look like a particle has passed. No proportional tube or GM of your dimensions can be so sensitive as to generate so many pulses like that shown in your video simply by comic ray background.

Has the tube any signs of physical hits?
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Richard Hull
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Re: Is this neutron counter dead?

Post by Richard Hull »

More than hot enough! If the nearness of this source does not increase the tube count and you are still getting a natural one count per second, the tube might indeed be old or have leaked out a bit of the normal 3He fill gas. It still may count neutrons, but just at a higher than normal background that will need to be subtracted. You will need to moderate it and get a neutron source to be sure now.

I have seen some few ancient GM tubes where the normal Geiger plateau length has shrunk from 100 volts to 10 volts! This requires a critically held GM operating voltage if you wish to squeeze the last bit of life out of the GM tube. (should be thrown away - End of life) This was far more normal in the old organic quench tubes. (10e9 counts before death)

Assuming no leaking of original fill, most halogen quenched GM tubes have an effectively infinite life span.

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Michael Kaufmann
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Re: Is this neutron counter dead?

Post by Michael Kaufmann »

@Cai Arcos

no physical damage, not even single scratch.

ZY5yKubqrFg.jpg
Yj4b0Hds0Gk.jpg

there is a little dot-like mark on the bottom cap tho - like there was capacitive discharge to the tube body. but i can't find a second one anywhere.
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Michael Kaufmann
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Re: Is this neutron counter dead?

Post by Michael Kaufmann »

Richard - that's why i was asking in the first post of this topic - "so how exactly do they die? what happens at the end of their life?".
tho now i don't think the tube is EOL - more like defective or something.

there is something written on it with a pencil, but I cannot make out what exactly - something like "big", and the second word is not readable.
maybe it's a "big own background"?
i will continue to experiment with it - next step is a LDPE moderator (don't have HDPE), and then something like very small Pu-Be source.
Cai Arcos
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Re: Is this neutron counter dead?

Post by Cai Arcos »

Might be the discharge happened from inside the tube? That might reinforce the idea of internal damage causing spontaneous discharges.

In any case, I wish you the best of luck!
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Richard Hull
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Re: Is this neutron counter dead?

Post by Richard Hull »

I echo what Cai noted, best of luck.

Internal arcing in any gas detector is deleterious, damaging and possibly the end game.

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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