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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Joe Gayo
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Neutron Transport

Post by Joe Gayo »

Stumbled upon this today ... https://www.seamplex.com/milonga/

Interested in using it to optimize moderator shape.
jeremy
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Re: Neutron Transport

Post by jeremy »

Hi.

Note that milonga has not been updated in a couple of years. I am working on an updated version here https://github.com/seamplex/feenox/
It should be usable by the end of the year.

About optimization, see "Geometric Optimization of Nuclear Reactor Cores" in https://seamplex.com/library.html
Let me know if I can be of any help.

Regards
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Finn Hammer
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Re: Neutron Transport

Post by Finn Hammer »

This looks interesting, like a FEMM for neutrons?

I have been wondering, since the fusor (BOT?) cubes are now so small, why not build the moderator as a box to put the fusor inside.
In that case it would be nice to know where the neutrons would go. Since most/all neutrons produced would then get thermalised, would they distribute evenly in the moderator material, or bunch together in one end. (Of course, they would have to exit at some point)
-But getting past the point where I feel like learning a new operating system.

Cheers, Finn Hammer
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Richard Hull
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Re: Neutron Transport

Post by Richard Hull »

Finn, you are assiduous in your quest to know and learn and that is a sign of a true amateur scientist. There is so much to learn, too.

Yes, the absolute ideal is to bury any small neutron producing device in a moderator. Waste not, want not! At thermalization, there is no bunching in a suitably sized moderator....No hot spots... Yes, most all neutrons not detected or slowed to super cool and absorbed will always exit the moderator, and again, assuming suitably sized, all will be thermalized.

Barring the ability to surround a fusor with a moderator, there is anisotropy in emission of fast neutrons. (beaming, variation in production geometry, etc.) It is at the ideal hot emission point of fast neturons that we seek to place our smallish moderator to intercept and thermalize the most fast neutrons as possible.

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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Joe Gayo
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Re: Neutron Transport

Post by Joe Gayo »

While in "timeout", I learned the barebones to perform simple simulations of fixed neutron source diffusion through a moderator (water).

Here is the code I used. Hopefully, this will be a good launching point for anyone interested.

reedWater.mil

Code: Select all

MESH FILE_PATH reedWater.msh
MILONGA_PROBLEM DIMENSIONS 1 GROUPS 1 FORMULATION diffusion

# some settings to improve cpu & memory usage
MILONGA_SOLVER EPS_TYPE jd ST_TYPE precond KSP_TYPE bcgs PC_TYPE asm

sn_alpha = 1.0

MATERIAL source        S 1  D 1
#Table 12 in page 18 of Los Alamos Report LA-13511, Water
MATERIAL moderator     S 0 SigmaT 0.32640 SigmaS 0.293760 SigmaA 0.032640 nuSigmaF 0.0

#If there physical entity named "external" or "source_mirror" gets the following BC
PHYSICAL_ENTITY external BC vacuum
PHYSICAL_ENTITY source_mirror BC mirror

MILONGA_STEP

PRINT_FUNCTION phi1 HEADER


reedWater.geo

Code: Select all

lc = 0.01;

Point(1) = {0, 0, 0, lc};
Point(2) = {0.1, 0, 0, lc};
Point(3) = {12.6, 0, 0, lc};

Point(11) = {0.05, 0, 0, lc};
Point(12) = {6.35, 0, 0, lc};

Line(1) = {1, 11};
Line(11) = {11, 2};
Line(2) = {2, 12};
Line(12) = {12, 3};

Physical Line("source")  = {1,11};
Physical Line("moderator") = {2,12};

Physical Point("source_mirror") = {1};
Physical Point("external") = {3};
WaterCube.PNG
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Richard Hull
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Re: Neutron Transport

Post by Richard Hull »

Water is a great moderator. I used it for my 3He tube neutron detection situation with fusor IV, 2003-2019. HDPE is a bit better for a small neutron oven which I used for activation 2006 to date.

The real beauty of water is, given a nice 2 foot diameter tub, two foot tall, and a very hot isotopic neutron source, you have an fabulous way to try variable activation distances. Jon Rosenstiel tried this early on with his fusor on a smaller scale, If I remember correctly. Sadly, galvanic action on his aluminum bodied 3He tube rather rapidly ate through and destroyed his costly tube!

I used an SS Reuter and Stokes 3He tube and suck it under water in a 1.25" diameter Lucite cylinder with a Lucite base. the tendency of extreme buoyancy of the Lucite tube allowed the fixed 3He tube to hold it under water jammed against the base. As my lab was unheated in winter, a bit of antifreeze in the water kept it from freezing at temperatures in the teens.

I am now totally sold on assembled HDPE block and slab moderators in all scenarios

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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Joe Gayo
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Re: Neutron Transport

Post by Joe Gayo »

Going even deeper down this rabbit hole ... if anyone wants to follow

Neutron Physics
Karl-Heinrich Beckurts; Karl Wirtz; Translator-L. Dresner
https://sci-hub.se/https://doi.org/10.1 ... 42-87614-1

and

The Theory of Neutron Slowing Down in Nuclear Reactors
International Series of Monographs in Nuclear Energy
By Joel H. Ferziger, P. F. Zweifel, J. V. Dunworth, D. J. Silverleaf · 2013

https://www.google.com/books/edition/_/ ... =en&gbpv=0

Fortunately, I have hard copies of both.
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Joe Gayo
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Re: Neutron Transport

Post by Joe Gayo »

Running experiments today to verify simulations. Here are the results:
fluence.PNG
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Finn Hammer
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Re: Neutron Transport

Post by Finn Hammer »

Joe,
There appears to be a sweet spot at around 38mm along the 152mm axis, but what does this curve show?
I am tempted to think that it shows that a foil lodged 38mm deep in a 152mm moderator cube, will experience a maximum of thermal neutron capture.
Is that it?
If so, would a 76mm ^2 moderator suffice?
Is it safe to assume you are using HDPE?
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Joe Gayo
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Re: Neutron Transport

Post by Joe Gayo »

Finn,

"Shows that a foil lodged 38mm deep in a 152mm moderator cube, will experience a maximum of thermal neutron capture." is correct. The material was HDPE (0.945g/cm^3). I simulated various depths and overall thicknesses and found that 1) always have a "reflector", depth of moderator behind the activation material, 2) the reflector should be at least 10cm (for D-D neutrons) and 12-15cm is more optimal (longer than 15cm provides diminishing gains), and 3) the ideal depth somewhat depends on the resonance capture energy of the activation material but generally, 3.5-4.0 cm is best. I studied hemispherical moderators as well, and it seems the simple cube is sufficient.

Based on my findings I would say that 7.6cm is too small. The moderator works like a bucket, converting fast to thermal neutrons, and the deeper the bucket (reflector dimensions) the higher the level (flux density) before it reaches equilibrium (fast -> thermal).

Joe
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Richard Hull
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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
Attachments
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
Posts: 2
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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