Neutron Detector and Moderator Test

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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Kai Kaletsch
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Neutron Detector and Moderator Test

Post by Kai Kaletsch » Sat Oct 27, 2018 6:04 pm

I put my neutron detector about 50 cm from a soil moisture gauge, containing 40 mCi AmBe, and recorded the results with various moderator thicknesses and got between 38 and 166 cpm. I don't know if there is any neutron shielding along the side of the gauge and didn't think of it when I had the chance. (Could have just put the gauge on its side because the source certainly isn't shielded to the bottom.)

Anyway, here is the video (no sound): https://www.youtube.com/watch?v=AY54sNDQRoE

Any feedback would be appreciated, especially wrt sensitivity vs size (~0.25L, 155g), as compared to other detectors.

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Rich Feldman
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Re: Neutron Detector and Moderator Test

Post by Rich Feldman » Mon Oct 29, 2018 5:51 am

Nice work. Thanks for sharing. Is the black mark on the edge of your work surface (was going to say, your counter) a reference for putting detector in the same place each time?

For people new to neutron physics, how about demonstrating some other geometries? Instead of surrounding the detector with different thicknesses of polyethylene, leave the bare detector in place. How much does the count change when you place a container of plastic pellets in line between source and detector? Or right behind the detector, or off to the side?

That kind of soil moisture gauge is new to me, but easy to learn about on the Internet.
nmm.png
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Do you know if the 40 mCi rating of your source is the neutron activity or the internal Am-241 activity? I guess your experiment had the source in its retracted position, substantially shielded. Can you tell us about being trained to use the instrument, or not to sleep next to it?

Suppose a hypothetical detector counted 100% of the incident neutron flux, at your experimental distance from a bare 40 mCi source. What would be the detector area, to match your highest count rate? :-)
Richard Feldman

Kai Kaletsch
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Re: Neutron Detector and Moderator Test

Post by Kai Kaletsch » Mon Oct 29, 2018 4:33 pm

I believe the gauge is a Troxler 3430 ( https://www.troxlerlabs.com/downloads/p ... manual.pdf ). Radiologically, it is pretty benign. Radiation levels are given on page A9 of the pdf. According to the manual, operators are usually not considered radiation workers.

The 40 mCi (1.48 GBq) is the Am-241 activity, not the neutron generation rate. They also make a model (3430M) with a Cf source with 1000 times less activity. According to pg 12 of https://www.nrc.gov/docs/ML1122/ML11229A704.pdf , we get ~ 6E4 n/s per GBq AmBe, or 90 000 n/s for our 1.48 GBq source.

At 50 cm, we have 90000n/s / (4*pi*(50cm)^2) = 2.86 n/s/cm2.

I measured 166 cpm or 2.77 cps. Essentially, my whole detector saw as many neutrons as would have passed through 1 cm2 of cross sectional area. The cross sectional area of my detector is 5.3 cm diameter x 12.5 cm height = 66 cm2.

That means that, in this configuration, I see about 1 in every 66 neutrons that pass through. (That’s not exactly right because neutrons are complicated and the moderator does stuff to the geometry. But, it should be in the right ball park.)

The neutron source and detector in this gauge are in fixed locations and always above the soil surface. The moisture content is determined from neutron back scatter. The gauge also has a gamma source, for density determination. That’s what gets lowered into the ground.

Yes, I’d love to take all the measurements that you suggested. But, I don’t want to wear out my welcome. (It’s not my source and not my lab.)

I’m still not quite sure about any neutron shielding the gauge may have. The manual mentions cadmium. This wouldn’t do anything to fast neutrons. My guess is that’s to keep background neutrons out, rather than to keep gauge neutrons in. I’d still like to go back and quickly check if there is a different neutron rate with the gauge on its side.

Nice observational skills on your part! Yes, the piece of electrical tape is ~ 50 cm from the center of the gauge, which is approximately where the source is. In the video, it looks like I put the detector much closer to the source. But, that’s mainly the camera angle.

Kai Kaletsch
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Re: Neutron Detector and Moderator Test

Post by Kai Kaletsch » Thu Nov 01, 2018 12:28 am

I'd like to get some feedback from people who have looked into neutron detectors on how this compares to other neutron detectors out there. Is it good, bad or ugly, compared to other methods?

I've looked around and I think that it compares pretty well. But, this is my baby and I might be deluding myself.

BTW, the method of detection is not one of the ones described in the FAQs and doesn't involve rare, expensive or toxic materials.

I would really appreciate a critical review on the performance before I spend $20,000 on a patent lawyer. Don't worry about hurting my feelings. They will heal. Spending $20,000 protecting technology that is not worth protecting would really suck!

So, please don't hold back.

Kai

Dan Knapp
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Re: Neutron Detector and Moderator Test

Post by Dan Knapp » Thu Nov 01, 2018 2:15 am

Kai
I can’t offer any input on your numbers, but the fact that the moderator increases your counts suggests you are indeed counting neutrons. You haven’t demonstrated selectivity for neutrons over gammas (the Troxler gauge also contains a cesium-137 gamma source), so some of your counts could be from gammas. That would be a critical issue in meeting the criteria of “new, nonobvious, and useful” to be granted a patent.
Unless you think you have something that is so good that it is going to make you lots of money and needs top notch patent protection expertise, you can get a patent for a lot less than $20K. Pick up a copy of the book “Patent it Yourself.” I used this book and successfully obtained a patent on a new IEC reactor geometry. I had to go back a forth with the examiner and concede a few claims, but my primary claims prevailed. As a first step, you can file a US Provisional Patent application for one year of protection a very little cost. Doing it yourself with the book clearly is not as good as having a good patent attorney do it, but you need to have something pretty good to warrant spending $20K.
Dan

Kai Kaletsch
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Re: Neutron Detector and Moderator Test

Post by Kai Kaletsch » Thu Nov 01, 2018 3:21 am

Thanks for the suggestions, Dan. All good points.

Yes, the video doesn't show gamma response. But, I'm pretty sure we are fairly blind to gamma. We tested it on some of our own sources and also had the neutron detector right up to the gamma source on the Troxler gauge and opened the shield and it made no difference to the count rate.

I took the neutron detector on an air plane and got about 20 or 30 times background. I'm still trying to figure out if that is only due to neutrons or if it sees something else. I don't think that it sees gamma, electrons or fast protons. It might count direct hits from heavy ions and it may count protons in the Bragg Peak. Anyway, I'll post the results in a different thread, even though I'm not 100% sure how to interpret them, yet.

Thanks for the suggestions on the patent process. It's really valuable to know that you were successful. Congratulations!

I have 1 patent granted and another one that just went through the PCT process. It's pretty cool. Its for a directional radiation detector. See https://patentimages.storage.googleapis ... 8817A1.pdf . It always cost lots of money. I've been tempted to try and do it myself. Maybe this time I'll go for it.

It all depends on how this neutron detector stacks up to the competition in terms of performance. If we beat a He-3 tube and come in at a fraction of the price, then I'll probably go through a lawyer. If we are not even in the same order of magnitude, then I'll probably just walk away. If it is somewhere in between, maybe I'll try to do it myself (or hire an Intern to write it).

Kai

John Futter
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Re: Neutron Detector and Moderator Test

Post by John Futter » Thu Nov 01, 2018 5:44 am

Kai

I do this sort of thing for a day job.
you need to have a couple of differing brand detectors preferrably from differnt vendors that are in calibration.
I have used Am /Be neut source 70mega Bq (that is the Alpha output from the Am) and just like you have done I put the detectors 1.00 metres away from the source.
Last time I did this i was surprised to get within a power of 2 agreement on all three detectors, I would have been happy with a power of ten due to the difficulty dealing with neutrons in general.
We have a 3MeV accelerator and I could muster up a D beam onto a carbon target but I have not needed to do this, it is much better to use the Am/Be source as I can have it on the table in the lab for a few hours and not put too much on my dosimeter

Justin Fozzard
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Re: Neutron Detector and Moderator Test

Post by Justin Fozzard » Thu Nov 01, 2018 9:24 am

Kai
You've done some good experiments and I would be interested to find out more about your detector. I hope that you apply for a patent for it.
Could you tell us the publication number for the patent that you were granted? I note that the application that you have linked was rejected on page 32 for, amongst other things, not being considered novel for many of the claims.

Thanks,

Justin Fozzard

Kai Kaletsch
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Re: Neutron Detector and Moderator Test

Post by Kai Kaletsch » Thu Nov 01, 2018 12:54 pm

Justin,

Thank you very much for pointing out the rejection on pg 32. I haven't had time to look through the whole thing and was used to just breezing through the PCT process with my other patent ( https://patents.google.com/patent/US8766201 ). I'll have to look at this in more detail.

Again, thanks a lot! I had completely missed this and that's pretty important.

Kai

Kai Kaletsch
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Re: Neutron Detector and Moderator Test

Post by Kai Kaletsch » Thu Nov 01, 2018 1:18 pm

John,

I totally agree that that's how it should be done. The problem is that I don't have access to relevant reference instruments and was hoping to get feedback on whether I'm in the right ballpark before I spend too much money.

Around here, everyone has several Rem Balls but nobody has just a bare tube. Here is a comparison to a Rem Ball that we did on an earlier prototype of the neutron detector: https://www.youtube.com/watch?v=w9Oh5tYXqgA . That test gave us cps per uSv/h values. This is somewhat informative because it can be compared to other instruments' data sheets. But, it is not really a relevant comparison for this type of instrument.

John, can you post your results from your 70 MBq test? Was this on tubes or Rem balls?

Kai

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