Gamma Spectacular Neutron Counting

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.
Tom McCarthy
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Gamma Spectacular Neutron Counting

Post by Tom McCarthy » Sun Dec 10, 2017 6:23 pm

Hi all,

I've been playing around with my neutron detection gear. It's a Gamma Spectacular GS-2000-PRO made and sold by Steven Sesselmann. I have my own CHM 17 Soviet He3 tube, 13" long but have been using one of Steven's own, easier to use He3 tubes. He's been extremely generous as he sent the tube to me in 2014 and I've had it since. I'm putting together a coax C to SHV connector to try out the CHM tube.

As the GS-2000-PRO is more for PMTs than He3 tubes, Steven made a pre-amp to go with the supply that he tested on his own tube before sending. Pictures below of the setup. The GS-2000-PRO provides up to 2000v bias, and the He3 tube is sweet around 1700V. I'm using the PRA software package. So far I've been trying to figure out the proper settings, been getting a lot of noise, and I'm unable to make a judgement on when I'm seeing cosmic neutrons, and not gammas or other undesirables. After pulse shape acquisition, I did a 20 minute run, which was quite clean, but had very few counts - less than 1 cpm. Picture attached.

I'm going to do some more reading and see if I can get my settings right.

Tom
3chart_pracounting1.jpg
3 charts from one run. Bottom 2 are count rate and pulse height histogram.
3chart_pracounting1.jpg
3 charts from one run. Bottom 2 are count rate and pulse height histogram.
pracounting1settings.jpg
PRA settings
Attachments
IMG-0397.JPG
GS-2000-PRO with red screen. Custom pre-amp linked. He3 tube in HDPE moderator.

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Richard Hull
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Re: Gamma Spectacular Neutron Counting

Post by Richard Hull » Sun Dec 10, 2017 8:52 pm

With a smaller volume 3-5 CPM background might be about right. The real test at 1 CPM background you have now is to use the system with your source.

What makes up the background background is fairly unimportant as long as it is low and your count with a known good source is 10-20 times more.

Run a test with your source for 20 minutes.

Richard Hull
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Silviu Tamasdan
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Re: Gamma Spectacular Neutron Counting

Post by Silviu Tamasdan » Sun Dec 10, 2017 8:59 pm

I have several of these Russian corona tubes (the SNM-17 is 3He, I have a few 10B-lined tubes, and a 3He SNM-32) which I have been using so far with another detection system; it just so happens that I have received yesterday in the mail my order of GS-PREAMP from Steven, and I already have a GS2000Pro. So I'm about to build a system that will be very similar to yours. Been already using the GS2000Pro with PRA and a scintillation detector for some gamma spectroscopy experiments.

My observations regarding the corona tubes: the 3He ones are very noisy, with a very low S/N ratio of about 3. It is difficult to use them to detect neutrons because the signal is fairly well drowned in noise. I'm much happier with the boron-lined tubes (I use SNM-11) which have about the same magnitude of signal output, but a much cleaner baseline and thus a better S/N ratio. They are much easier to setup for neutron detection. I've tested 3 of them and it's fairly consistent across all.

with the tube you have, I think 1 cpm is actually high if you're just looking at background neutrons. With my tubes (which are the same size as yours) I count in my location 0.1-0.3 cosmic neutrons per minute at max. I'm afraid that most of your counts may be noise.

I'm still working on my system, but things that I did so far to optimize my system are:
-made a neutron source from polonium/beryllium. This is more or less indispensable to have in order to properly adjust these tubes
-used gamma sources to ensure that I'm not detecting environmental photons instead of neutrons; I have a 10uCi 137Cs and a 1uCi 60Co source to use for that
-shielded the tube by encasing it into a larger copper tube that is grounded to reject as best as possible EM interference
I have all of the above scattered in several threads around this forum.
viewtopic.php?f=13&t=11954
viewtopic.php?f=18&t=12016

(edit) but yeah, as Richard said above you won't know if what you're detecting are neutrons until you use a neutron source.
Last edited by Silviu Tamasdan on Sun Dec 10, 2017 11:13 pm, edited 1 time in total.
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Tom McCarthy
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Re: Gamma Spectacular Neutron Counting

Post by Tom McCarthy » Sun Dec 10, 2017 10:01 pm

Thanks for the responses guys.

Point to note: The tube I used above is not a Soviet tube. I have a Soviet tube (as described), but haven’t used it yet. I’ve been using Steven’s Reuter-Stokes RS-P4-0404-253 3He tube. Your points are very good for when I get my own Soviet tube connected though, Silviu.

Next step then is to do long runs and get neutron and gamma check sources. Will check around, I know I have a little bit of Beryllium somewhere.

Tom

Silviu Tamasdan
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Re: Gamma Spectacular Neutron Counting

Post by Silviu Tamasdan » Sun Dec 10, 2017 11:15 pm

Then your 1cpm may be OK; the Russian tubes are not very sensitive. I do recommend the SNM-11 though just because of the superior noise, and it's still available at nice prices. And it just works.

If you need an affordable source of SHV cables and connectors let me know, I'll tell you where I'm getting mine.
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Steven Sesselmann
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Re: Gamma Spectacular Neutron Counting

Post by Steven Sesselmann » Sun Dec 10, 2017 11:30 pm

Hi Tom,

It has taken a while, but I'm glad you are closing in on those pesky neutrons..

The preamplifier mentioned by Silviu had not yet been developed when I made yours, but in any case what you have there should work.

I recommend you download the latest version of PRA here: http://www.gammaspectacular.com/marek/pra/index.html

Finding the right settings should be easy, since you effectively have a pulse height spectrometer.

Start off by opening the "View Audio Input" window, which is like a simple oscilloscope, use this to identify which way your pulses are going, positive or negative?

Then set your pulse height threshold to a very low value -0.01 for negative pulses or 0.01 for positive pulses.

Collect a spectrum and try to figure out where gamma ends and where neutron starts.

It's not a perfect science with He3 detectors, because there is some overlap, but generally the neutron counts are stronger than the gamma.

Move your pulse height threshold to a point where you are counting a reasonable background for that tube. (Can't recall what the NVTH is)

Here is a link to a calculator where you can estimate the cosmic neutron flux at your location: http://www.seutest.com/cgi-bin/FluxCalculator.cgi

If the NVTH of your tube is 1.0 then that should be your ballpark figure.

Steven
http://www.gammaspectacular.com - Gamma Spectrometry Systems
https://www.researchgate.net/profile/Steven_Sesselmann - Various papers and patents on RG

Silviu Tamasdan
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Re: Gamma Spectacular Neutron Counting

Post by Silviu Tamasdan » Mon Dec 11, 2017 12:23 am

Hmm, that calculator gives a relative neutron flux as compared to the flux at sea level in NYC.
Given that I am at sea level and a few miles away from NYC, I guess the relative flux here is 1. :)
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Steven Sesselmann
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Re: Gamma Spectacular Neutron Counting

Post by Steven Sesselmann » Mon Dec 11, 2017 1:32 am

It's actually pretty hard to find any real information on what a reasonable neutron flux is at the earth surface. The neutron monitoring stations have huge tube arrays and counts are in the thousands.

The best I could find is that neutrons per cubic cm per minute range from 1 to 9, and as is often the case a long googling session takes you right back to Fusor.net for the best advise :)

viewtopic.php?t=5386&p=33830

Steven
http://www.gammaspectacular.com - Gamma Spectrometry Systems
https://www.researchgate.net/profile/Steven_Sesselmann - Various papers and patents on RG

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Re: Gamma Spectacular Neutron Counting

Post by Tom McCarthy » Tue Dec 12, 2017 12:33 am

Thanks Silviu, Steven. Did some long run this evening, ~22 and ~30 minutes.

I have to change my settings a bit more as per Steven's advice to get rid of a lot of the noise. It's hard to tell where gammas could end and neutrons start. You can see in the attached pics (labelled 20 min run and 30 min run) that there's a definite small group of strong pulses in the pulse height histograms. However, the transition isn't clear cut. I'm going to have to wait until I get a neutron source to get a good idea of where neutrons begin, I should be able to bring my gear to a local college to test things there.

Steven, for the audio input, I've attached a pic of a typical pulse that I get. I took it as positive.

I've also attached the dataset for my 30 minute run, you can open PRA (Steven linked it) to check it out.

I got a 1.04 for neutron flux here (Mayo, Ireland) relative to NYC.

Tom
111216_20mpulseheightandpulseview.png
20 minute run
111217_30minruncharts.png
30 minute run
111217_audioinput.png
Audio input
111217_30minrun.zip
Data for 30 minute run
(21.33 KiB) Downloaded 46 times

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Steven Sesselmann
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Re: Gamma Spectacular Neutron Counting

Post by Steven Sesselmann » Tue Dec 12, 2017 1:40 am

Tom,

I opened up the attached file, and I suspect most of what you see there is gamma, and the chances are your neutrons start way up at 50 arb.u.

Once you start seeing some real neutrons it will become very obvious where to discriminate.

As a general rule it is a good idea to record the whole spectrum, and then use the Advanced pulse filter to clean up the spectrum post recording.

The Advanced filter doesn't alter anything in your file, it only changes whet you see, so set the threshold in the advanced filter to 50 arb.u and then see what's left ...

If your gamma is going out to 50 arb.u, I suggest lowering the volume, so the gamma stops at around 20 arb.u. this should also get rid of the tall black peak on the left which is most likely just noise.

PS: The pulse you captured doesn't look very good, it's very small and dips below the line, looks like a bit of ringing, are you using a long cable?

Steven
Neutrons.png
Tom's BG File
Filtered.png
Filtered Spectrum
http://www.gammaspectacular.com - Gamma Spectrometry Systems
https://www.researchgate.net/profile/Steven_Sesselmann - Various papers and patents on RG

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