Neutron Transport

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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Richard Hull
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Real name: Richard Hull

Re: Neutron Transport

Post by Richard Hull »

Carl Willis posted the optimum moderator distance for water, paraffin, and HDPE back in 2004 time frame. (Activation was getting popular then as more fusors were coming on line) This distance is the point at which to place material for moderation in activation or a detector tube and it agreed precisely with the graph presented here. Everything I have done since then has been based on this figure. The reflector is important and can be of infinite size, but as noted, a back reflector beyond 4X the ideal moderation distance in HDPE is of little added value in detection or moderation. One might note that the ideal is just that, and that placement from 25mm to almost 50mm will result in near maximal activation and detection. This allows for a bit of slop for complex forms. Never forget the reflector as it plays an important roll, especially with weak sources of neutrons most often found in amateur experimentation using the fusor.

I used water for detection after 2004, but have always used HDPE for activation. Water demands only a slight increase in moderation point distance. Jon Rosentstiel, if I remember correctly, made a small water moderator whereby he could slide the item to be activated to varying distances for optimum moderation. All of this work was in the 2003-2006 time frame. I now, since 2019, prefer HDPE for both detection and moderation.

For those who are cash poor and might wish to monkey with a water moderator, I attach an image I submitted here in the early 2000's for activation based on my expansion idea of what Jon originally reported on. The one issue with water moderators is evaporation. You lose water to the air over time if the moderator is open to air. Mine was open to air and before each run, often months apart 2004-2019, I had to fill my moderator with distilled water to its fill line or risk losing my calibration factor so hard won, and costly, using 2 different BTI bubble dosimeters.

The time and relatively minor expense for using HDPE for both purposes is repaid in the ability to tune a system for best detection and moderation. This is especially true in research setups where blocks of HDPE of suitable smaller, variable sizes can be assembled in many ad hoc setups around detection and activation efforts in bench top test scenarios.

I suggest that the graph that joe has provided be printed out and retained in any reference work or lab note book. I have Carl's old graph in my handy reference book.

Richard Hull
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Water tank activator.  Dimensions are left to the user.  Size to suit.
Water tank activator. Dimensions are left to the user. Size to suit.
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
jeremy
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Joined: Thu Jul 08, 2021 7:19 am
Real name: Jeremy Theler

Re: Neutron Transport

Post by jeremy »

Joe Gayo wrote: Thu Jul 15, 2021 1:34 pm Running experiments today to verify simulations. Here are the results:
I think this is the very first time that a comparison between experimental results with milonga prediction.
Congratulations. You can already write a paper Joe!
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