Neutron Detector - making more accurate

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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Dennis P Brown
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Neutron Detector - making more accurate

Post by Dennis P Brown » Thu Feb 23, 2017 11:03 am

I realize that in my fusor test and successful neutron run one possible issue for my my data not being as steady as I wanted/expected was related to my detector/moderator design/set-up. I had to removed and replace the neutron detector to get readings with and without the moderator. Getting the neutron detector back to its exact position and orientation wasn't really possible (close but there were variations.)

As such, I will hard mount the detector in a rigid manner and design the moderator as the only removable item. Also, I will reduce the height of the moderator - I will use a narrower piece of paraffin so that I preferentially collect fast neutrons just inline with my detector cross-section reducing off-angle fast neutron capture. Also, I will incorporate a moderator mount that allows that too, to be re-positioned in an reproducible manner.

In this way I hope my detector's neutron counts (for a steady current/voltage/pressure) becomes significantly more reproducible; thus allowing me a chance to attempt a rough calibration of the tube using "known" absorption values of boron for a series of thickness's.

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Richard Hull
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Re: Neutron Detector - making more accurate

Post by Richard Hull » Thu Feb 23, 2017 1:24 pm

If I could make a 12" cube moderator I would do it. The count only goes up with a huge moderator. Thermal neutrons in a moderator are just like air atoms they have no traceable origin and no tracable end point. A fast neutron skittering plane to a cubic moderator's edge is almost as likely to generate pulse as a neutron entering the moderator on a dead on impact with the detector tube. Thermal neutrons have no sense of direction in a moderator. They are all over the place!

The larger the moderator, the more counts will be tallied. The closer the moderator is to a fusor, the more neutrons will enter the moderator.

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Jim Kovalchick
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Re: Neutron Detector - making more accurate

Post by Jim Kovalchick » Thu Feb 23, 2017 4:53 pm

There is a point of diminishing and even degrading return on moderator thickness because no moderator has a zero cross section for absorbtion.

For a more sophisticated read on this, see Carl's paper:

http://www.qsl.net/k0ff/016%20Manuals/N ... 5B1%5D.doc

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Dennis P Brown
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Re: Neutron Detector - making more accurate

Post by Dennis P Brown » Thu Feb 23, 2017 10:15 pm

LOL, Richard. I happen to have two cubic feet of paraffin! (again, what one can get cheap, as surplus, is funny.) One block is 2' x 1' x 0.5'; and two others are 1' x 0.5' x 0.75'.

I will use one as a shield (for me) and I will try one for the detector; in this manner I will compare the performance between my original 10 cm and one that is 15 cm (0.5') thick. From a paper on the subject, 10 cm will "thermalize" about 62% of 1 - 2 MeV neutrons.

You are correct that the larger the "interaction" area, the more thermal neutrons that can 'randomly walk' into the detector increasing the signal (of course, thermalizing depth increases the number of thermal neutrons created just as absorption will, with enough thickness, start to seriously reduce those neutrons (random walking is a double edge sword!) However, a larger volume 'collector' does make calibration rather difficult for an unknown source even as I get more signal... . Guess that is an issue that is best addressed by using a bulk absorber like water with various concentrations of boron (as borax) maybe.

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Re: Neutron Detector - making more accurate

Post by Richard Hull » Fri Feb 24, 2017 9:58 pm

In general, whether paraffin or HDPE within 2-3 inches of solid moderator sourrounding a tube on all sides you will have a lot of fast neturons turned to thermals. I would think a one foot cube would be more than enough for fusor use. I use a 6 inch diameter, water filled, PVC pipe for my large 3He tube and it works great. For activation I use a 6 inch cube of HDPE which I personally think is a bit small. This "neutron oven" works fine, but a bigger surface area facing the fusor would be better.

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

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