Activation measurement technique

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.
User avatar
Doug Coulter
Posts: 1312
Joined: Sun May 27, 2007 7:18 pm
Real name: Doug Coulter
Location: Floyd, VA, USA
Contact:

Re: Activation measurement technique

Post by Doug Coulter » Tue May 18, 2010 9:33 pm

Oops, no file attached, but yes, I get you on this. I still think it may be worth a try -- as you are wont to say (and so am I) experiment rules, so I will do just that. Check up-thread for a picture of the other current project here -- a directional neutron sensor....hoping to get that tested in the next week or so.
Now, down to the shop to finish that puppy - can't do the experiment without making the gear first!

Never did I say I thought we could pack all the neuts into a 1v range around 5v, but we only have to do it a little better than now to see some of that 10k vs 100 barn improvement, eh? I was hoping at best to get 1/10 of that -- and if say, the rest are up there in the other 100 barn X section range, all the better. I was just trying to avoid the crummy 10 barn thermal range in the process. Should be quite easy to improve on that one. All those little peaks -- they do go well below 100 barns between them too, right?
Nothing is perfect, but I don't let the perfect be the enemy of the better than it is now.

And I do think you should try your last picture as a fun thing -- would make a great video demo like the one Carl did on moderation, would add nicely to that. I'd guess that you'd want really thin silver for that, as activating just one side of a thick piece, you'd never see the resulting betas on the back, or am I missing something there?

For thin, I'm really liking that real pure In from rotometals you turned us all on to (they are having a sale now, I may get more). It can be made very thin, it's amazingly ductile. Nice stuff. Also that's where I got the cadmium for the camera project and it seems great too. I made a Cd/Bi alloy, about 44% Cd for that -- safer than dealing with casting pure Cd by far, and I wanted it to stop X rays anyway. You have to chill it fast, though, or you get a mixture of large crystals of each element, eg places where there's no Cd or no Bi in thin castings. Very pretty but....not what you want I think.

I find that 2-3 mm of lead sheet will stop about all the X rays at 40-50kv just fine, it doesn't take that much, so that's a good start for your water box idea. In my case I just bit the bullet and also covered the entire tank with that stuff, a lot of work -- once, but now you can walk around the thing close while in full operation at the e6 n/sec or so range and above, and the geiger counter stays nice and quiet -- good for peace of mind if nothing else.
Why guess when you can know? Measure!

User avatar
Richard Hull
Moderator
Posts: 12811
Joined: Fri Jun 15, 2001 1:44 pm
Real name: Richard Hull

Re: Activation measurement technique

Post by Richard Hull » Wed May 19, 2010 2:42 pm

Activation occurs throughout the sheet's thickness up to a point, but the thinner the sheet the better, again, up to a point. It is all related to you cross section and how close you are to the ideal energy. This why thermals activate better, traditionally in ultra thin sections.

I doubt that you will ever get even 1/1000th of the moderator impinging neutrons in any form of a 1 ev window.

Tests will tell.

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.

User avatar
Doug Coulter
Posts: 1312
Joined: Sun May 27, 2007 7:18 pm
Real name: Doug Coulter
Location: Floyd, VA, USA
Contact:

Re: Activation measurement technique

Post by Doug Coulter » Wed May 19, 2010 3:25 pm

Yup, tests will tell.

On your water bath rig, which I hope you'll build. Not sure how it is now, but awhile back 5 or 10 gallon rectangular glass fishtanks were dirt cheap even new, and often show up at yard sales too. This would get rid of the carbon in the plexiglass so you'd be testing with a pure substance, or more so. The small ones were very thin soda glass in a metal or plastic frame.

Also, back in the day, I found it fairly easy to cut thin window glass (or you can have it cut at most full service hardware stores to pretty good precision) and make my own small tanks using RTV sealant, and rubber bands to hold together until the RTV cured. Although there is a runnier version of RTV meant for glass windshield seals, the plain old stuff was what I used and it was fine -- though that is becoming hard to find -- the new Ge "Silicon II" stuff -- avoid! It half cures in the tube on the shelf and that's it, it is just a defective product. The stuff in toothpaste tubes (autoparts store at higher price) is still the good stuff and you can sometimes find the old RTV in caulk tubes at the hardware store cheap. Of course, you should pay the $.50 extra for the kind with the cap you can replace...
I used this technique and still do to make tanks for nasty things like PCB etchant.


Sure, I'll never get the bulk of neutrons into a 1 ev window -- I said that already -- I also note that success for what I want doesn't require that amount of extreme perfection, either -- not even close. I might get more activation per input than now though, and I find it intellectually interesting to see just what the effect of using different moderator materials may have on the resulting distribution width at whatever energy. Seems to my intuition that materials that take less energy off per scattering event would have an effect on that resulting distribution width. You may lose more than you gain because with more scattering events needed to get down there, more may scatter out of the thing entirely.

That's what Carl's simulation should help to answer.

One might take a page from the nuke weapons designers here -- while I'm sure most of us don't agree with all their goals, I don't think any of us would claim they are stupid. And they often use a heavy substance on the outside layer of the device to reflect fast neutrons back into the reaction zone.
And various other techniques to control neutrons.

We know that works. Why ignore things that *have* been tested and are known to work?

This leads to the idea of a meta material, or composite design for *our* activation ovens, but optimized for our different goals. Perhaps certain substances work better in certain parts of the oven?

For example -- high H content in the area where most of the neutrons are way too fast, then once you get "closer" to desired energy, some heavier moderator (carbon or similar) to pull them down the rest of the way, or nearly. Behind the sample, a very heavy substance to "reflect" without taking much further energy off. At each stage, match the material to the situation and desired result.

I don't believe this has been explored even in the theoretical literature, much less experimental, and I see no theory/math that says this wouldn't be a possible large improvement on the current "simple and dumb" design. Whether it could be a *significant* improvement is something to be checked out before making some blanket statement. It doesn't need to be perfect -- I don't need 1000x gain, but I bet that 10x is pretty easy and 100x is possible.

Just another example (from a paid-professional inventor, me) == not long ago, things having negative index of refraction were beneath consideration -- only a couple natural substance exhibited this at all, in small amounts and over very narrow bands -- a curiosity with no real uses. Now metamaterials that exhibit far better NIR properties are all over the place in science news, and are finding real world applications. Never safe to bet something can't be done or won't be useful if you try it.

So far, in the literature on narrow band neutron work, all I see is what amounts to diffraction gratings that effectively select a narrow band, and simply ditch the rest (or choppers with time of flight, same result). Useful if you've got a very loud source, not so much useful for us, but even that technique might be useful to help keep medium energy neutrons from scattering completely out of the oven....It is already used "in reverse" to get sub thermal (~18k if i recall right) neutrons by scattering the hotter ones *out* of the moderator column using poly-crystalline graphite with random crystal orientation to accomplish that. The super slow neutrons have too long a DeBroglie wavelength to see this effect and just continue on through, kind of a cool effect (pun intended ;~).

I see work here as a potential "low hanging fruit" that has been skipped over by others who have different goals than we do -- something where a person like one of us might make a really new contribution to the art.

This could be important. Radio Hams got (and still do, but to a lesser extent now since they no longer lead the way) a lot of special treatment from the regulating authorities because it was recognized that they were improving the art in ways that had uses for others. We could certainly benefit from that kind of thing, as a group. But first we have to lead the way.

Edit:

After about 24 more hours of intense searching, I AM finding evidence that yes, indeed you can moderate neutrons into fairly narrow bands above thermal, using some non-conventional materials and constructions. The magic term is "BNCT" for boron neutron capture therapy, a newly hot topic, and sadly, most of the info is in pay-for papers, but thank god for grad students who publish theses. One paper describes using a combination of AlF3 and teflon, and another a combo of MgF, Al2O3, and teflon again. Who'da thunk it. The need for above thermal neutrons there is to precompensate for further moderation in the patient on the way to the boron laced tumor.

Along the way I realized that putting boron in my wax and Cd washers in my fast neutron camera was a big mistake. One that will be easily fixed by melting the wax etc back out of there and replacing with plain old wax or HDPE as a moderator for the neutrons I don't want to detect (here we go to McMaster again, the 4th or 5th time this week) -- thermal neutrons won't make light in the plastic scintillator, but the very high energy gammas off the boron and Cd will, so no point even testing that in its current form. Sigh.
Why guess when you can know? Measure!

User avatar
Carl Willis
Posts: 2841
Joined: Thu Jul 26, 2001 11:33 pm
Real name: Carl Willis
Location: Albuquerque, New Mexico, USA
Contact:

Re: Activation measurement technique

Post by Carl Willis » Sat May 22, 2010 7:32 am

As promised I have some results from this calculation. Please refer to the post above for details on the geometry. Keep in mind that sweeping conclusions can't be drawn from this kind of calculation; the specific geometry matters a lot.

I'll upload the "pretty pictures" when I get home from the trip I am on, and I'll upload the data graphs now.

There are three graphs with results. Two illustrate the relative neutron capture rates, in units of capture probability per metal (Ag, In) atom per source neutron, as a function of depth in various moderators. One compares Ag and In capture rates in HDPE with slightly more accessible units on the vertical axis of capture rate per gram metal per source neutron. The moderators considered are HDPE, graphite, beryllium, heavy water, and Teflon. The densities and compositions in the model may deviate significantly from some real products. Most notably, commercial graphite has a very wide range of densities and equivalent boron concentrations. The model considers graphite to have a density of 1.88 g / cc and no boron, high-end material that costs a few thousand dollars for a standard bar.

What can be said from this? Well, for starters, the HDPE is hands-down the best of these five choices for a moderator in this application. The ideal depth in the moderator is 4.0 cm for indium, very slightly less for silver. As an example, with a one-gram, 2"-diameter sheet of indium properly located 4 cm from the irradiated face of the moderator, a fusor source rate of 1,000,000 n / s would generate 15 nCi of total activity (In-116, In-116m1, In-116m2) after several hours of irradiation. Why is HDPE the best? Because it can slow down the most neutrons in the shortest path length to the useful epithermal and thermal energies. In the other moderators, which are actually less lossy than the hydrogen in HDPE, the energy decrement per collision is so much lower that the neutrons tend to escape before they reach useful energies. The source is very close to the activation material, so a small change in distance from the source comes at the price of a large loss in flux just due to the divergence of the neutron field around the source. The right way to mitigate the material losses and the geometric losses in this case is to use the hydrogenous moderator.

Image of the as-modeled geometry and the "mesh tallies" will show up when I get the opportunity.

EDIT: A couple images added.

The second-to-last image shows the geometry, with the source represented in blue and the moderator in red. At left is the view toward the moderator front face in the "YZ" plane; at right is the view of the side of the moderator and the line source in the "XY" plane. For reference, the source is 4" long.

The last image is a color contour plot of neutron capture rate in Ag in an HDPE moderator in the "XZ" plane. Coloring in logarithmic, with red being highest. The length of moderator in the X direction in this depiction is 20 cm.

-Carl

[attachment=4]7445.geom1.jpg[/attachment]


[attachment=3]7430.doug_3.jpg[/attachment]
[attachment=1]7428.doug_1.jpg[/attachment]
[attachment=0]dc_1.doc[/attachment]
Attachments
dc_1.doc
(37 KiB) Downloaded 43 times
7428.doug_1.jpg
7429.doug_2.jpg
7430.doug_3.jpg
7445.geom1.jpg
7446.meshtally.jpg
Carl Willis
http://carlwillis.wordpress.com/
TEL: +1-505-412-3277

User avatar
Frank Sanns
Site Admin
Posts: 1753
Joined: Fri Jun 14, 2002 6:26 pm
Real name: Frank Sanns
Location: Pittsburgh, PA USA

Re: Activation measurement technique

Post by Frank Sanns » Sat May 22, 2010 3:38 pm

Very nice Carl!

So the conclusion seems clear. 4 cm of HDPE on the source side of the sample and 1 cm to 2 cm on the far side of the sample with a higher Z backing like graphite behind it to kick some neutrons back in the direction of the sample. A cup or V shape adding a bit more gain from the high Z backing.

Frank Sanns

User avatar
Doug Coulter
Posts: 1312
Joined: Sun May 27, 2007 7:18 pm
Real name: Doug Coulter
Location: Floyd, VA, USA
Contact:

Re: Activation measurement technique

Post by Doug Coulter » Sat May 22, 2010 7:24 pm

Carl, you are truly a gentleman and a scholar! Thank you very much for this, it had been bugging me, and it looks like I'd been doing it not quite right -- so these will give me some improvement, not to mention a better understanding. Thanks!

What do you think of the idea of a C reflector behind this, as Frank mentioned? Seems reasonable to me, but hey -- only intuition which doesn't have quite have as good an error rate as real math/science.

I don't have nuclear grade graphite, (or at least I sure didn't pay that much for it) but I do have some stuff I think might be pretty good for that use.
Why guess when you can know? Measure!

User avatar
Frank Sanns
Site Admin
Posts: 1753
Joined: Fri Jun 14, 2002 6:26 pm
Real name: Frank Sanns
Location: Pittsburgh, PA USA

Re: Activation measurement technique

Post by Frank Sanns » Sun May 23, 2010 12:36 am

I am certain Carl will have substantial to say about this but let me expound upon my conclusion.

Atomic collisions can be considered elastic so there will be conservation of both enery and momentum. When similar masses collide, then half of the energy and momentum are carried away by both particles.

As the mass differences become greater between two particles this relationship changes. In the extreme, take the case of a particle and an entire wall (or a much more massive particle). When an atom hits a wall, the energy will remain the same as the input energy but the momentum will change direction causing a 2mv transfer of momentum to the wall or massive particle. The recoil particle then has changed direction and comes back with the same energy it had before the collision.

These are the extremes and 12 amu vs 1 amu is not quite a wall but it is getting there. From m1v1=m2v2 it is clear that the 1 amu will rebound with a much larger velociy in the reverse direction than in a 1 amu vs 1 amu collision (neutron vs proton).

The protons come from the hydrogen that is ubiquitous in the HDPE so it is a good moderator with good braking efficiency. Every neturon can lose up to half of its energy in each collision but most collsions will be far less than half due to angular scattering from off center impacts. The overall momentum remains in the forward direction but not in a straight line. This is due to the momentum at right angles to the collisions being small and their direction is easily changed. This is manifested by diffusion or conical widening out of the neutron beam as it enters a moderator.

In the case of a more massive Z like 12 for carbon, the change in velocity of an incident neutron is greater than with a single proton. BUT, it will only change direction to those neutrons that have already been slowed down enough so the collision with the high Z actually changes the direction of the neutron. That is why I suggested 1 to 2 cm of HDPE after the sample but before for high Z material. The key would be to slow down the neutron enough in the moderator to get rid of enough energy to have a direction change in a few number of collisions near the surface of the high Z material. If too great an energy neutron were to hit the high Z material, it would penetrate too deeply and would not make it back out since high Z material would self shield and the neutrons would reflect back into it.

Using a high Z backing reflector is a tightrope that may be tamed but is not going to double the neutron flux by any means. It may increase the flux at the sample by 5% to 10% but so every little bit helps but may not be worth the cost to do so. At the least, I would have double the thickness of HDPE on the backside of the sample and I think this is what most of the activation people are currently doing.

Frank Sanns

User avatar
Doug Coulter
Posts: 1312
Joined: Sun May 27, 2007 7:18 pm
Real name: Doug Coulter
Location: Floyd, VA, USA
Contact:

Re: Activation measurement technique

Post by Doug Coulter » Sun May 23, 2010 4:24 pm

Right Frank, I check you on all but the details of the math, as the literature gives another equation for equal weight particles in the lab frame (and for other ratios), but the idea remains the same.

You should get more high angle scattering with high weight stuff, so it should "in theory" make a better reflector. Although the force here is the strong force, not coulomb, it should be roughly analogous to coulomb scattering. Low weight stuff doesn't give much high angle scattering, but higher weight stuff does.

I am a little surprised at not seeing better results from things that do inelastic scattering - as long as a neutron is left over after that, it should take them down in energy just fine, thanks -- perhaps with some X ray production, which wouldn't matter in this application too much.

Which might mean surrounding the main moderator column with high weight material is also worth a little more -- those neutrons would otherwise be completely lost, so scattering *any* back into the column at least gives you another chance with them. The question there is would we already be so far into diminishing returns with that would it still be worth it at all?

I'm going to ask Carl a couple of other questions, to which I think I know the answers, but they'd be good to have up here (and he should probably start another thread beginning with his data to get it up front and out of all the irrelevant stuff -- this is too long and the good answers are in the middle!).

1. Did his model take into account the full cross section vs energy of each material to be activated? Answer almost certainly yes.

2. Did it account for the fact that once a neutron has been absorbed, it's gone and can't be reflected back etc? Maybe not. Maybe these plots are just density of absorb able neuts vs position in column with no absorber present.

3. Did it use the math for random walk that says distance from original source tends to average to sq root of path segment count? That's not the same as falling off square law. Answer, I have no clue!
It's not obvious to me that it makes a ton of difference anyway, but it should be different than falling off as 1/r^2. I'd assume any good modelling program would "know" that but you never know.
Why guess when you can know? Measure!

User avatar
Richard Hull
Moderator
Posts: 12811
Joined: Fri Jun 15, 2001 1:44 pm
Real name: Richard Hull

Re: Activation measurement technique

Post by Richard Hull » Mon May 24, 2010 3:44 pm

At all events here, the original and now ancient pronouncement posts of Carl and superlative experiments by Jon remain the law. The calcs recently done by Carl just throw another nail in this long dead issue.

A sandwich of a large area face of ~1.5" thick HDPE - silver of no greater area than our detector face - lots of HDPE backing and surrounding it is the ticket to max activation in any given neutron field of fast D-D neutrons.

Lets face it, if the ideal for the 1-2 ev resonance band is virtually achieved for whatever microscopic fraction of the neutrons impinging on the surface of the moderator's first or presentation face, at the silver or indium, you will only be dealing with lesser capable, off resonance activation neutrons thumping the silver from any reflector. Thus, most reflected neutrons are of the 1/v law type providing only a tiny gain, at best.

As Don Lancaster says....."the game is rigged, you can't win, you will never break even and it is the only game in town".

For those of us who have always obeyed the second, single sentence paragrah above, we have done it about as good as it is going to be done with any given flux of D-D neutrons.

Here endeth the lesson, yet again.

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.

User avatar
Doug Coulter
Posts: 1312
Joined: Sun May 27, 2007 7:18 pm
Real name: Doug Coulter
Location: Floyd, VA, USA
Contact:

Re: Activation measurement technique

Post by Doug Coulter » Mon May 24, 2010 4:23 pm

You may choose to believe, like Lord Kelvin, that everything important has already been discovered, or like Rutherford, that anyone talking about getting energy from this atomic stuff is talking moonshine.

GoodLuckWithThat ;~)

I differ. And history has shown that going negative on people for trying to improve things makes one look not so good later on when they succeed.

Nice thing about the forum -- if you don't like the topic or aren't interested no one is forcing you to listen.
Why guess when you can know? Measure!

Post Reply