Neutron detector calibration

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.

Neutron detector calibration

Postby Peter Schmelcher » Fri Feb 17, 2017 4:29 am

A simple and easy method of detector calibration would sure be nice. I wonder if cosmic rays might be part of the answer. They are ubiquitous and uniform day and night, throughout our solar system, as far as I know. Probabilistically, short term they vary greatly but over a longer time span an average should converge to a small stable number. I was thinking a few hours count with no moderator and a few hours count with a water jacket moderator several inches thick (plastic bucket full of water) and Bob's your uncle.

Messing up the traceable ;) calibration?

Curious what the old metrologists in the group think.

-Peter
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Re: Neutron detector calibration

Postby John Futter » Fri Feb 17, 2017 5:23 am

Peter
depends on latitude and altitude for sure,
and probably whether you are left or right handed
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Re: Neutron detector calibration

Postby Richard Hull » Fri Feb 17, 2017 10:15 pm

Very altitude/humidity dependent, as noted. The number of neutrons that reach the ground are pretty much thermalized by the water vapor in the atmosphere. Most neutrons are created 11 miles or more up via spallation.

There is no harm in the use of the moderator /no moderator test to see if fast neutrons are present, but it would be pyhsically and scientifically impossible to glean any sort of calibration figure from such a test.

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Re: Neutron detector calibration

Postby Peter Schmelcher » Sat Feb 18, 2017 8:50 am

John at first I thought you were having a go at me about latitude. I guess my memory is not what it was. I had a look at some reports I downloaded and read 5 years ago and sure enough the magnetic poles play a role, and of course it would. These reports must have been lurking around in my mind making me think about calibration.

I verified my detector system functioned using this information (specifically the charge amp that I designed). I’m partial to fast neutron detection because you can measure the cosmic energy spectrum as a test.

The first file is an EU aircraft pilot exposure report (has some graphs).
over my file attachment limit, search google for: Cosmic spectrum aircraft 140.pdf

The second file is older and has a similar theme plus it has a graph of moderator thickness vs count rate.
Cosmic neutron spectrum Hess_1959.pdf
(2.19 MiB) Downloaded 13 times


The third file is about closer to the ground neutrons. It was done using an elevator in a 1175’ tall aerial, and also has another moderator thickness graph.

The last file is newer and more of the same and also describes a few moderator configurations.
cosmic neutron spectrum aircraft 14119538.pdf
(295.77 KiB) Downloaded 14 times

Richard and John I still wonder if the error bars would be acceptable. Calibrate at midnight favoring galactic cosmic rays, and not during a northern or southern lights show.

-Peter
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Re: Neutron detector calibration

Postby John Futter » Sat Feb 18, 2017 6:38 pm

Peter
At work we used some very big neutron detectors to measure the neutron level at sealevel and on some of our countries highest peaks in a world wide study.
In fact I recently put up the teardown of one of these on this forum.
As far as I recall we counted for months at each site to get statistical ok results
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Re: Neutron detector calibration

Postby Peter Schmelcher » Mon Feb 20, 2017 8:48 am

Two pages that I find interesting from the EU report.
page7 Cosmic spectrum aircraft 140.pdf
(38.52 KiB) Downloaded 8 times

page 8 Cosmic spectrum aircraft 140.pdf
(43.25 KiB) Downloaded 6 times

Plus some old scope captures from my charge amp and 10 Bar LND 25160 He3 tube with and without moderating water.
water mod.jpg


NO Moderator 1 hour capture and my only capture of pulse pile up event
1hr no mod.JPG


With Moderator 1 hour capture
1hr mod water.JPG


With Moderator 90 minute capture
90min moderator water.JPG


My scope was configured to be an 8 channel event counter (actually it is a telecommunication waveform template compliance option). When the signal trace cross a numbered line segments the total for that segment gets incremented. Vertical (well actually the area under the pulse) is the kinetic energy of the neutron event. I figured the signal to noise of the trace was adequate to just use the pulse height and so I interpret at the display as a multi channel analyzer.

From memory the scope display was reasonably repeatable from hour to hour and day to day at least at sea level.

10 hours with moderator. The vertical scale was changed for this capture which effects the scope trigger sensitivity so do not scale it to the 1 hour capture. I included it so the younger guys can see the banding. The moderator strips away most of the initial neutron kinetic energy, then you get a He3 atom capturing the slow neutron and becoming an unstable atom. The band is the 0.764MeV of energy released and allows some calibration of the charge amp.
10hr mod water.JPG


Another paper about neutrons near the earth with some error discussion.
HE3 tube nhess-3-637-2003.pdf
(546.74 KiB) Downloaded 15 times


I think if one could be content with +-50% error bars in count totals this could work.
-Peter
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Re: Neutron detector calibration

Postby Peter Schmelcher » Fri Feb 24, 2017 7:30 am

I see a reasonable supply of $58.85 neutron counting tubes on ebay item # 161944724166. A different seller has tubes made in 2011 for $70 item # 272226474491 and he has a good picture where you can see the untranslated Russian part number.
The seller does not state the fill gas and his recommended operating voltage is questionable based on measurements I made almost 5 years ago, my tubes are from a different seller plus have an older date code 9105. Mine run as proportional neutron detectors with 1,000V bias.

The research paper in my last post above used these tubes. The author discusses tube details on page 638 under experimental methods.

It might be a very good time to learn a bit and chance your arm.

Have fun
-Peter
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Re: Neutron detector calibration

Postby Richard Hull » Fri Feb 24, 2017 9:47 pm

Peter has the right idea. If you are an adroit electronics guy and are willing to do a bit of work, the Russian tubes can and have been turned to good purpose in neutron counting. After the right voltages are applied and the charge amps made, it is a matter of biasing properly to avoid the tube just becoming a fancy x-ray/gamma detector and not a neutron, specific, counter.
This usually means having a significant gamma source on hand to find the bias point by adjusting out the moderate energy gamma counts, first with the correct bias and then, if needed, a windowing module to set a threshold to avoid higher energy gamma counts.

It must be remembered that just because a tube is called a neutron detector tube it is not going to count only neutrons. It is up to you to make it count only neutrons via electronic wizardy and careful attention to force it not to count all the other stuff it is quite capable of counting other than neutrons.

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