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.
tligon
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Re: Activation measurement technique

Post by tligon » Mon May 17, 2010 5:20 pm

There was a criticality accident during a lab demo back in the early nuclear days, where a researcher brought two pieces of fissible material too close together and then the apparatus would not allow them to be pulled apart. He reached in with his hands and pulled the two masses apart, saving the day but absorbing a fatal dose of radiation.

They were able to tell the exact dose by measuring the activation of a gold belt buckle worn by a researcher across the lab. Dr. Bussard knew the wearer ... said he showed no ill effects at that distance, although the activation was unambiguous.

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Carl Willis
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Re: Activation measurement technique

Post by Carl Willis » Mon May 17, 2010 9:00 pm

Geometry matters greatly to the questions of optimum thickness and composition, and if you want case-specific guidance on what to use and how thick to make it in order to get the highest specific activity in a particular activation sample, then a detailed problem has to be specified. Otherwise, one can merely only suggest in the most qualitative terms what the relevant tradeoffs are.

I have sometimes looked at other materials and other geometries in MCNP, e.g.:

viewtopic.php?f=13&t=5384#p33828
viewtopic.php?f=6&t=2998#p18567

With some specific input from you, the same code package will likely answer most detailed questions about moderator design for the purposes of activation. Without more detail, I just concur with Richard that a few cm of a dense hydrogenous moderator and a thick reflector are likely "where it's at," that graphite is too low-lethargy for the situation, beryllium and D2O too expensive in the necessary quantities and likely not as good as HDPE, and so on as I wrote previously. All this just reflects on my knowledge of apparatus that I presume is similar to yours in layout, but may be different in some critical respects.

Most people would probably say these problems are beyond the domain of paper-and-pencil mathematics. In lieu of direct experiment, a multigroup transport equation has to be solved deterministically, or a Monte-Carlo simulation performed, in order to calculate the energy-dependent flux and the activation rate, and realistically that's done on a computer.

-Carl
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Chris Bradley
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Re: Activation measurement technique

Post by Chris Bradley » Mon May 17, 2010 9:41 pm

Being the inventive soul that I am; I am picturing in my head a series of HDPE discs spinning on a common axis, each with a series of radial slits (sectors) cuts out of them and displaced by some given distance, according to the speed of the rotation and that of the neutrons you want. If a neutron comes by and hits the HDPE part of the first disc, so it is slowed down a little. Then again at the next, etc., etc., until eventually it passes through the slit in one of the discs and if all is set correctly then it will then pass through the next slit in the next disc and so on, unimpeded provided its energy has dropped to the desired energy. If it is still a little too high, so it will get to the next disc just ahead of the cutout slit and so be slowed down a little further [each time], just enough that it then matches the slit in the next disc.

Each disc would need to have slits fine enough that the slits in the next disc are offset so as to obscure fast neutrons, but close enough (azimuthally) that an achievable rotation speed would bring the slit into alignment timed for neutrons of the desired velocity to get to the next disc as a slit comes into alignment.

To do some maths, say we have discs containing 120 slits of 1 degree coverage each, with 2 degrees of HDPE between each slit and each disc is 5cm apart from the previous. So if we want to discriminate 5eV neutrons at 30,000m/s, they cover the 5cm in 2us, so the rotation would have to be once every 240us or 4,000Hz = 250,000rpm.

Seems like it might be a bit quick for a rotor made out of HDPE, but I'll leave that one for some engineers to solve, to optimise for number of slits and rotation speed! (e.g. 1000 laser-cut slits would mean only 25,000 rpm is required)...Doug, I am sure you could knock a working prototype up!!!

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Doug Coulter
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Re: Activation measurement technique

Post by Doug Coulter » Mon May 17, 2010 10:11 pm

Carl,
Thanks for those, we are now getting somewhere with this! Still all mostly hydrogen based, no pure carbon (or other somewhat heavier element than H that could be good -- you'd know better than I what that might be).

Of course, if it was an easy paper/pencil question I'd be posting the results instead of asking the question! I appreciate that this is hard, and I'm for sure asking a big favor here, to explore a thing that no one else seems to have bothered with so far (or at least published about where I can find it).


For purposes of the assumptions you have to make with your model, let's suppose these conditions:

4" round moderator column (length TBD), starting at 3" from source (grid) center, in line with the main neutron output, which I assume for now is straight out from the grid (in my case a line source about 4" long, which might be a little better than a point source in a spherical arrangement, but shouldn't be a real big deal).

We can forget for now that I'm actually using a piece of HDPE there that is cut to match the curve of the 6" pipe the grid is inside of -- any "wings" that provides are just bonus after all -- the chances of neuts caught by that scattering up into the column are probably not real great - mechanically it helps the thing stay put is all, and the reason I made it that way. If I get a little more from the thin part that wraps the pipe sides some, fine. Some materials you wouldn't be willing to waste that cut out of anyway.

So let us assume (because other theory says we should, and it's probably correct) that we have a line source ~4" long, ~3" away from the start of the moderator column, which is 4" round (more or less, I could make it a little bigger if needed but there's not a lot of room close in). We assume the source is isotropic, that is, there's no preferred direction that the neutrons come out, no beams or anything, also what theory says. (I am about to test that assumption with a directional neutron detector, but for now, we assume it's correct) I am using 2" diameter foils placed inside the column, flat to the direction the neuts are presumably coming from (before scattering, by the time they reach my sample I assume they're going in all directions pretty much). The big picture question is of course, what to make the column out of to get the best net neutron flux at a couple eV and how far down the column to put the sample.

This means we're catching neutrons that are coming both straight at us, and also some that are at a slight angle, perhaps +/- 20 degrees max that our column base subtends from the POV of the source, and try with some things NOT H containing and see what the energy distributions along the length of the column would be. Or rather, what distributions have the bulk of the neutrons in the desired range for resonance activation.

My gut is really telling me that more gradual moderation than you get with H will win out for single digit eV neutron flux at some distance, even though it also should increase the number of neutrons scattered all the way out of the column (because more scattering would be needed to get to the energy). I just can't find any data in the literature I have access to.

In this case, the things that made carbon such a pain for non enriched fission reactors don't apply -- if a percent or so of neutrons are captured in this moderator column by some impurity, it would be in the noise here compared to scattering right out of it anyway (which means all the calx done so far for fission reactors are meaningless for this problem). So I don't need the fancy nuclear grade of carbon (or whatever else) if it just maximizes the net neutrons at 2-5 ev in a moderator I can actually make.

For example, I'm unlikely to be able to make one of that scale from Be....hard to machine and kinda expensive. But it *would* be cool to know if that would be best!

Again, making the bottom of the thing form-fit to my cylinder, or someone's sphere would just be bonus-extra from this POV, so need not be modeled. Just use a flat plate/end touching the chamber should be fine to get a feel for what material would be best at whatever net thickness gets to the required speeds/energies, here. I'd assume that any moderator behind the sample would mostly reflect neutrons that by then would be thermal, but I'd love to stand corrected on that one too -- could it be that a really high Z moderator behind the sample that would tend to reflect without much further energy loss improve things further (kind of a metamaterial)? A tantalizing thought, at least to me.
Why guess when you can know? Measure!

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Frank Sanns
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Re: Activation measurement technique

Post by Frank Sanns » Mon May 17, 2010 10:14 pm

It is not a one dimensional problem but a three dimensional problem. Nearly as many neutrons arrive at the detector from past the detector. To capture the greatest number of slow neutrons, the moderator surrounds the detector and is not just between the dector and the source.

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Chris Bradley
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Re: Activation measurement technique

Post by Chris Bradley » Mon May 17, 2010 10:26 pm

Frank S. wrote:
> It is not a one dimensional problem but a three dimensional problem. Nearly as many neutrons arrive at the detector from past the detector.
Ordinarily, because of the distrubution of energies. But if you can slow down neutrons to a non-normal and more mono-energetic 5eV, hitting the peak cross-section of the activation material, then why would the same assumptions apply?

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Carl Willis
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Re: Activation measurement technique

Post by Carl Willis » Tue May 18, 2010 12:18 am

OK Doug, I have the following noted:

Line source, 2.5 MeV isotropic, length 4"

Right cylindrical moderator, 4" diameter, effectively infinite length, face normal to a source radius and displaced 3", each of these materials:
-HDPE @ 0.945 g/cc, composition CH2
-Graphite @ 1.88 g/cc, composition C (< 1ppm B-10 eq.)*
-Beryllium metal @ 1.85 g/cc, composition Be
-Heavy water @ 1.11 g/cc, composition D2O
-Anything else?

Activation targets are 2" dia. thin (non-self-shielding) foils of Ag or In at their typical densities and isotopic compositions, disposed co-axially in the moderator.

Deliverable results: specific radiative-capture rate in the foils per source particle, as a function of axial position in the moderator. Capture in silver produces Ag-108 in two states and Ag-110 in two states; capture in indium produces In-116 in three states. (I can't provide detail on the breakdown from this calculation, just total specific radiative capture rate, which equals the saturation activity. You can estimate the product ratios to within 5-10% by hand.)

If that all looks interesting to you I will write and run this problem in the next couple days, time permitting.

-Carl
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Doug Coulter
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Re: Activation measurement technique

Post by Doug Coulter » Tue May 18, 2010 2:55 am

Yes, Carl, beautiful, and more than I had any right to ask for -- but that is precisely what I want, and you have the specs I think I want precisely.

Those plots you provided in that first link you gave above were quite beautiful themselves, and helped me get some insight on those materials as well -- good stuff. It's now in the library here, thanks.

As to other moderator materials? Well, it's pretty unlikely I'll ever get a mass of Be that big and then dare to try and machine it (I can do without lung failure), but what else would be "interesting" as regards the most of us?
(goes and looks at periodic table) I guess there's not much there going down from carbon, and until we see carbon, no idea if there's any point in going up farther I guess. Li is available, cheap and moderately easy to deal with (and a pound is a large lot of Li) so maybe that one. Boron is obviously out as it would react with anything getting slow and make gammas -- Li may have the same troubles I suppose if there's significant 6Li in it, dunno about 7Li if that has the same problems. Sulfur? I don't know what I'm asking here as I don't know how hard it is to plug in a new element into your model, so don't take that all that seriously if it's a pain to do or too many computer cycles. And I don't have reaction cross sections for a lot of the nearby elements, or whether they do nasty things with neutrons (like capture them and make hot gammas) in my literature collection in any detail worth using.

I think just a comparison between H and C will tell what's needed about which direction I should be looking in. We have all this nice data on hydrocarbons due to you already....so we also know what the mixture looks like and only need to tease apart the contributions of each part.

Covering Ag and In pretty much spans the range of interest, as most of the other elements we'd activate also have resonances in there someplace close to or in between those -- and correct me if I'm wrong, but aren't those about the "easiest" to activate and get a nice loud resulting count from?
I'd heard that manganese might also be a candidate (and the price is right for MnO2), but I'm thinking that I want something with pretty short half life so I "get all the radiation back I can" during a fairly short measurement after each run. Not to mention not having to wait weeks for a sample to cool back down for re-use.

This isn't for the more exotic stuff I (and I hope others) someday might activate, it's just going to be part of a measurement system I am hoping to increase the sensitivity of, over what I have now.


As I mentioned, the thing here that's killing me in getting reasonably accurate measurements here is my wildly varying cosmic counts, which I'm also setting up an anti-coincidence rig for, but as we know, those things are kind of a band-aid and never perfect. Here, in ten second counts, normalized to CPM, I will see anything from 6 to 90 cpm from cosmics alone -- sometimes both extremes in two consecutive counts, which kinda puts a rather big error bar on things that only count a few hundred CPM in the first interval and decay fast after that. So that's the motivation here.

So yes, please do as you describe, I'll be waiting gratefully for the eventual results.
Why guess when you can know? Measure!

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Doug Coulter
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Re: Activation measurement technique

Post by Doug Coulter » Tue May 18, 2010 3:00 am

Sounds like what you are describing here is kind of like the chopper used in time of flight neutron energy spectrometers, but with built in variable-moderation ability. Quite clever, and I might indeed figure a way to make one at some point. Yeah, I don't think I'd try spinning HDPE that fast, and as Sam Barrows once found out, even CD's can't quite take 20k rpm and explode in a most satisfying way (if that's what you wanted) when you try. I'll do some skull sweat on that one and see what I come up with -- that actually might be something not just I am interested in, you know -- could have value to others with (much) more money than we have...Surprised no one else has mentioned that one. Perhaps a carbide of something that would take the forces would do....have to look into that.

Thanks for the idea -- you get the credit, I'll do the fab when the time comes around for doing it.
Why guess when you can know? Measure!

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Doug Coulter
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Re: Activation measurement technique

Post by Doug Coulter » Tue May 18, 2010 3:11 am

Yes, of course Frank, you're right.

I knew this and it's why I tossed in the idea that what goes behind the sample in the moderator column might ideally be something different than what goes in the front -- something in the back that tends to just reflect the neutrons without slowing them much would seem ideal in a perfect world, eh? In other words, something pretty high atomic weight for that part.

It may not be so 3d though -- if we're hitting those fantastic-big resonances, the sample will capture virtually all the neutrons on one pass at reasonable sample thicknesses, and of course side-on it will get them all at 2" thick in that dimension, as I am assuming 2" diameter foil samples here.

So the only stuff that comes back from the "reflector" would be either stuff that went around the sides of the sample, or was too fast or too slow to get captured on the first go around. If too slow, well, nothing will fix that up, if too fast we might get another crack at that one, but my gut says that would be relatively minor -- although my gut has been known to be wrong now and again ;~)
Why guess when you can know? Measure!

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