Using the neutron cosmic background to calibrate detector

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.
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Rich Gorski
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Using the neutron cosmic background to calibrate detector

Post by Rich Gorski »

Hi everyone,

Has anyone tried to calibrate their home assembled neutron detector using the cosmic neutron background (around 0.01nv)? Is this a valid way?
Below is the calculation I performed to come up with a sensitivity value of 1.16 CPS/nv for my B10 based detector. One interesting fact is that the base background count I am getting is 50 CPH with or without the HDPE moderator. Does this mean I'm only detecting the thermalized portion of the background or is this method just not valid?

I would appreciate any thoughts and advice.

Thanks,
Rich G.
n-bck-cal.jpg
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Richard Hull
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Re: Using the neutron cosmic background to calibrate detector

Post by Richard Hull »

50cpm background on any well adjusted neutron counter is just not possible. As always place a very hot source of gamma radiation. near the GE boron tube and adjust the discriminator to near zero cpm. Remove the source. The normal background for a 22 inch long, high efficiency 3He counter in moderator is on the order of 5-10 cpm. The GE tube should be in this range as well.

During high solar activities, like a major CME, these counts can nearly double.

First do what you need to do electronically to limit background to under 10 cpm. Then place a significant gamma source near the tube. You should never see even a 5% increase in counts. Ideally, no increase in counts.

You should never need a neutron source to "set up" or test a neutron detection tube.
You will need a very hot gamma source! (old large radium dialed WWII aircraft instrument, very large piece of uranium or thorium ore with a GM count over 100,000 CPM)
Tune the voltage of the tube and the electronics first to a background of about 5-12 CPM.
Place the hot source next to the tube. No real significant increase in counts should be seen. If they are seen, tune the detector back to a lower background and repeat.

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
JoeBallantyne
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Re: Using the neutron cosmic background to calibrate detector

Post by JoeBallantyne »

Richard, he was reporting 50 counts per hour, not counts per minute. Which depending on the sensitivity of the tube, is certainly reasonable. A little less than 1cpm.

Rich, yes, you are counting thermalized neutrons from the cosmic background radiation. Which is why it doesn't matter whether you use a moderator or not. The neutrons get thermalized by the atmosphere. (Water vapor with its hydrogen atoms certainly helps.)

Joe.
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Richard Hull
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Re: Using the neutron cosmic background to calibrate detector

Post by Richard Hull »

That is a very low background for such a large volume tube as the GE tube. He may now need to show neutron detection by obtaining or exposing the tube to a real, known source of neutrons. In short, he is on his own from this point. Without a gamma source of suitable intensity, setting by some background alone is a crap shoot.

With a hot gamma source, he better not see an increase in counts or the tube is shot with a 1 cpm background.
I would put the source near the tube and turn the bias UP until counting really begins and then back off until no counts were seen.
The GE tubes are tough to bring on line. They are all many years old now.

With a known good, properly biased GE boron lined tube, you can have a very sensitive and rather efficient neutron detector.

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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Rich Gorski
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Re: Using the neutron cosmic background to calibrate detector

Post by Rich Gorski »

Yes, gone through all that. With Joe's help we determined that the best bias for this particular B10 tube is 742V. This was determined under a hot gamma source (X-ray tube at 40kV) and with the bias at 742V it showed only about 1 CPM. In addition this tube set at 742 volts detected around 300CPM of fusion neutrons from my ring ion accelerator at 30kV moderated and only 4CPM unmoderated. This was the data that promoted me into the neutron club. My best neutron count so far is up to 750CPM at 32kV with this ring accelerator fusor.

So, back to the original question. Is it reasonable to use the cosmic neutron background (0.012nv at my location) as a calibration source for my B10 setup as I described above?

Thanks
Rich G.
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Richard Hull
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Re: Using the neutron cosmic background to calibrate detector

Post by Richard Hull »

I would say an emphatic no! Cosmics vary. You are not only detecting cosmic rays but secondary neutron events. X-ray sources, especially at a rather puny 40kv are far too weak to rule out the contributions of the 500kev to 2Mev gammas so common from local radioactives from thorium and radium found in the earth. These whistle through many meters of air and buildings. This is why I note an intense source of the 500kev to 2.6mev gammas from radium or ore.

However at 1cpm if that is a true background and you are detecting 300cpm in a moderator around a fusor, I would let this sleeping dog lie.

I just hope you aren't cheating yourself out of neutron numbers. Again a hot source of gammas might allow you to raise that voltage to a no gamma detection of 750 volts, increasing your background a tiny bit and your neutron numbers significantly. No way to know without a hot gamma source.

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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Rich Gorski
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Re: Using the neutron cosmic background to calibrate detector

Post by Rich Gorski »

Richard,

Then, if my 50 CPH background value includes some number of high energy gammas then the sensitivity number I calculated is too high and I might indeed have more fusion neutrons than I think. I guess the best way to get calibrated is to spend a few bucks and rent a Ludlam counter for a few weeks. Then I have a direct comparison of my B10 system with a known calibrated instrument.

Rich G.
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Richard Hull
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Re: Using the neutron cosmic background to calibrate detector

Post by Richard Hull »

Different neutron detectors detect in different ways and have different efficiencies and different sensitive volumes. Much depends on the detector in the Ludlum, should you rent one, so beware of apples and oranges.

Here is a table of neutron detectors and relative efficiencies.

The 10B lined chambers are relatively low efficiency 10% compared to the best BF3 and 3He tubes 50-77%. I note they are relatively insensitive to a huge gamma flux. With this, I think you are OK as is. Let the sleeping dog lie.


Richard Hull
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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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