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.
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Doug Coulter
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Activation measurement technique

Post by Doug Coulter »

Let us suppose I want to make a more-sensitive neutron flux integration measurement via activation.
For the moment, I am not worried about absolute calibration, but would simply like to make the current process more efficient than it now is, so lower fluxes can be measured with decent repeatability.

The current method I'm using is to put silver or indium into my neutron oven, which is an affair made of 4" HDPE rod split up so a foil can be placed between the pieces. With about 1.5" of HDPE between the foil and the fast neutron source, and several more inches behind it, the process works quite well with my existing setup, particularly when the fusor output is high -- I have a lashup to count the resulting activation as the sample decays, uniform sample sizes and all the rest -- that's handled fine. But it takes a pretty good flux for a pretty long time to get the samples as hot as I'd like them to be compared to background on the sensitive 2" diameter pancake detector -- I want to extend the useful range downward from where it is, and be able to follow the decay curve longer out without getting into too much noise from background cosmic rays and so on.

Now, looking at the absorption cross sections of silver and indium I see that they have some really big capture resonances at energy levels way above the usual .025 eV thermal neutrons. In the case of silver, for ideal sensitivity it seems you want about 5 eV neutrons, and for indium you'd want about 1.9 eV assuming I'm reading these plots correctly. For silver this means I want energies around 200 times thermal. This shouldn't be a huge problem as we are starting with DD neutrons that are plenty hot by comparison to either -- it's just that I don't want to slow them all the way to thermal for this.

What I am wondering is has anyone worked out what would be the best moderation scheme to get the original neutrons down to these levels more efficiently, rather than all the way to thermal, where the cross sections of my samples are much lower than they are at resonance? Seems in general that many of the other things we'd use would be in a similar range for a large capture cross section. As you can see in the plots, this is a pretty big deal compared to thermal cross sections, so in theory the sensitivity for activation could be raised immensely.

Would one use a non hydrogen containing moderator for this, to lower the possibility that neutrons would be lowered all the way down to thermal in only a couple of collisions? Would carbon be better, or perhaps some oxide or carbide or sulphide of something that has a negligible neutron cross section itself (meaning the other element in there would be more or less inert for this process)? If so, how thick would it want to be between the source and the sample? Since thermal neutrons are still useful (just not as much so) of course you'd back this with some more moderator, and getting some thermals is still better than nothing if the goal is ultimate sensitivity.

You can see where this could be really useful particularly for cases where then neutron flux is either small, or in short bursts that the detector tube types cannot time-resolve, and also good for people just starting out and getting low fluxes at first, since we hope they have a geiger counter before they start out anyway, for their own safety. It would of course be good if whatever moderator substance didn't itself activate of course, so carbon looks good -- less neutron energy loss per scattering event, and perhaps some other medium-light elements would be good too. Most of those have the nice property of being available cheap, and most are fairly easy to form to some shape, but if I had to go to a container full of some powder that would do if nothing better can be devised.

Does someone know how to calculate/predict this so I am not reduced to "trying everything"?
I realize there is no way to get them all down to just this or that particular energy, but it would be nice to get a larger fraction to this range than a normal H-containing moderator would.

Since even a moderator designed to take neutrons to thermal isn't perfect, and has some fraction that escape that -- but an unpredictable fraction it seems, or has to be so big to get them all to thermal that too many are scattered out of the moderator in the process, this seems something worth looking into.
Even a factor of 10 increase in sensitivity would be a boon, especially if it were accompanied with better repeatability, and from these plots, it looks to me like threre's one heck of a lot more than that possible. Since a few here like to activate other, more difficult elements, a plan to get the bulk of neutrons into their resonance ranges (if any) would have good usefulness too.

I believe Carl has some of this info and some software that may help, but anyone is of course invited to comment on this.
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Steven Sesselmann
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Re: Activation measurement technique

Post by Steven Sesselmann »

Doug,

When you moderate neutrons with HDPE at room temperature, you get room temperature neutrons right?

How about using hot oil as a moderator, you can heat the oil to 200 C˚ then activate your silver sample in the oil before removing it and counting.

This might almost double the temperature right?

There might be other materials that could work as a moderator if they were hot enough.

Steven
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Doug Coulter
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Re: Activation measurement technique

Post by Doug Coulter »

Yes, or even below room temp in some cases (polycrystalline graphite will scatter all the but the coldest neutrons out of the column via a Bragg type phenomenon and make super cold ones.

A single collision with a proton (hydrogen) and reduce the neutron energy to essentially zero, so anything containing hydrogen is out. Works like the pool shot where the cue ball just stops, imparting all its energy to the struck ball.

So, you certainly want something that won't stop them cold (or to room temp more or less) in just one collision. D would of course be better than H -- but heavy water isn't cheap or that easy to deal with for making a a neutron oven out of.

I note that here I am lookng for neutrons roughly 200 times room temperature, lessee....call that 291k * 200 or 58,200 degrees K. Nope, don't have any medium weight elements that can do that one.

IE thermal neutrons are roughly .025 ev energy and I want 5 volts more or less.

Things like carbon, oxygen, sulfer, and other stuff in that weight range only take about 20 something percent off a neutron per collision. Of course, some of those things might also absorb the neutrons I want to use, so some are right out of consideration. Or they do complex things with resonance energy neutrons (like Al).

I am right now thinking carbon or some chemical with carbon and other stuff might do, but I once heard a certain very smart guy on the forum (Carl) knew how to calculate things like this feedforward, and I was kind of hoping he'd chime in here so we all can benefit -- else I'd have just mailed him direct.

The idea would be to get a gaussian spread (the best you can do anyway) around the resonance region for all these cool things we want to activate, instead of thermal. If you look at those plots there's rather a large advantage if you can do that without losing too many -- the cross sections are nearly 1000x higher at resonance than for thermal....now that's an advantage worth trying for I think.

I like activating things for certain classes of measurements simply because you can't fool it with electrical noise, gamma rays, or anything else, and it can handle huge short bursts and still catch them all, unlike any other detection method yet mentioned on this board. And since I'm getting real high Q in pulse mode these days, all the 3he tubes etc do is inform me that one pulse happened, as all the neutrons in that pulse come out inside a single resolution time of any gas tube device.

Even my very short pulse scintillator that sees knock-on neutrons in a scint plastic also sees some stragglers....the tube can see 5 ns or better, but the neutrons may farkle around in the scint plastic and time-smear things much worse than that -- so this is the single candidate method that can work for short, intense bursts. Ideally, I'd like to be able to measure the output of just a few pulses so I can tune without making my lab too radioactive...and though I realize I won't get any 1000x (theoretical best possible) 100x would sure be nice....and probably possible.

See, going thin with something like HDPE -- many neutrons don't hit anything so they are still at full energy -- and the ones that do mostly get to thermal in 1 or a few collisions, so just making the HDPE thinner isn't going to work. Or so I think -- I was asking a question I actually don't know the answer to, hoping someone did.
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Steven Sesselmann
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Re: Activation measurement technique

Post by Steven Sesselmann »

Doug,

Yes, I agree it would be out of the question to heat the moderator to 52,000 K˚.

Would it be an option to cool the specimen sample instead?

Not sure if cooling changes the resonance...

Steven
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Re: Activation measurement technique

Post by tligon »

Doug,

We played around with indium at EMC2 a little, while using a fusor running at 10 kV. Our neutron flux was too low to produce good results, so we stuck with a He-3 detector.

I had no idea what I was doing when I put ours together. I bought some indium wire and draped short lengths of it over rods and suspended it in water in a Tupperware container. After activation, I gathered the lengths together and dumped them on a GM counter, but the geometry would have been useless for calibration.

Dr. Bussard crunched some numbers and said the wire was the wrong form. What was better was foil about 0.002 inches thick. I have not checked his calculations, but apparently at that prominent resonant capture cross section a neutron is unlikely to make it more than about 0.002" before being soaked up. His conclusion was that a sandwich of indium foil and a moderator such as polyethelene would be ideal.

Just thinking about the math needed to accurately predict the sensitivity of such a detector from basic physics makes my head hurt. But if you can get a readibly detectable count, tracking the decay rate over an hour or so should be convincing proof that neutron activation occurred. Thus, the technique would prove that fast neutrons had been produced.

Overall, bubble dosimeters probably make more sense, but I've always wanted to get a second chance at indium neutron activation. There is something cool about making new elements at will.
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Doug Coulter
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Re: Activation measurement technique

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Yes, it seems foils are the best form for activation. For one thing, with a thick piece activated all through, only the radiation (assuming beta decay is the result) from the outermost layer/area can ever get out to be seen on your detector, so the rest is wasted. So that is what we use here. We got silver as thin sheets, and roll our own indium in a metal rolling tool -- it's real easy to do even with just a small hammer. We use fixed size foils that just cover the sensitive area of a pancake detector for post-activation counting. I even bought a piece of very pure gold sheet to fool with, but compared to the others, it's relatively "numb" (in agreement with theory). And in this case, changing gold to mercury is kind of going the wrong direction if one is primarily interested in transmutation!

What I am attempting to do here is make what amounts to a better BTI -- more sensitive and more reliable and accurate. We get such disparate results with those compared to other measurement gear that we got kind of disgusted with them. Not to mention short lifetimes and how much they cost.
I think we're about at the $1000 level with those -- just the ones that have hit the trashcan at end of life. Not a great deal.

Example: Cylinder fusor, inner grid about 1" diameter, and about 4" long in a 6" ID pipe with BTI's side by side standing on end about 4" from fusor grid center, centered on grid center axis (eg in the middle of the 4" length). One BTI 8.6 bub/mrem, the other 26.6 bub/mrem. Take two runs, where all of the other gear (in this case, silver activation, 3He tube, BF3 tube) all read within 10% of the same for the duration of the two runs. In one orientation, the two BTIs have *the same* number of bubbles as each other, switch them for the other run, and they *almost* show the correct sensitivity ratio between them as the labels -- but not quite, and they vary 50% comparing the BTI's to the other gear between the runs. According to all the literature and math I can find, the output of DD fusion should result in uniform neutron output across all the steradians -- perfect istotropy, not small beams....which is about the only way to explain that that doesn't indicate serious problems with the BTI's.

That is not acceptable at all -- I don't even have consistency run to run with BTI's much less absolute accuracy, and it's far easier to believe that the stuff that doesn't even agree with *itself* is the problem, not the other gear. I know of things that can fool the tubes -- fast bursts and such; sometimes you can see this with a scope on the raw signal from the tubes, which beats a counter in some cases -- a counter will count one neutron or a million that happen in a few uS the same -- one pulse, but at least the scope shows a larger pulse for the latter case of a burst.

I know of *nothing* that can fool the activation oven which is fixed in place and consistent from run to run, and tracks the tubes quite accurately when we're not bursting. Both of those, of course, collect neutrons from a fairly large angular area -- so "beams" would explain that -- but the theory doesn't seem to indicate the possibility of beams at these energy levels, so the game's afoot. In fact, we just spent the last day starting to build a directional fast-neutron "camera" to test for the existence of beams, just in case, and will report if/when we have anything interesting to report on that. I will personally be pretty surprised if that much of the literature is wrong, though.

So, it seems activation is the "gold standard" if there is one, for repeatability, though absolute calibration is going to be fairly suspect, also true of the tubes if you want absolute accuracy better than some double digit percent (I don't care about that for now, myself -- "Am I doing better this time than last time" is enough for what we are doing now.). As the curves I show indicate, tiny differences in the neutron energy spectrum from the moderator can have enormous effects on net activation for a certain number of neutrons entering -- nearly 1000::1 in some cases.

So what I *am* looking for here is a moderator design that results in mainly neutrons centered around a few eV, rather than thermal, starting of course with neutrons of DD reaction energies. This should make the measurement both much more sensitive and much more repeatable. If in a thermal moderator, the "tails" are doing all the "work" then that's also where the variability is going to happen according to basic statistics - fewer samples generally means an inferior signal to noise ratio in results.

It occurs to the engineer in me (what I started out as) that moderators that can take 2/3 of the neutron energy on average per collision would not be ideal for meeting this goal. EG hydrogen based, and that one ought to be able to do better with something (carbon?) that takes more collisions to get to thermal. In that case, it seems more likely there ought to be a spot along the moderator column that satisfies this condition, and I am hoping to find that someone knows how to calculate that. It would be far nicer to have a reasonable starting point than having to simply "try everything".

I am sure that I could get fairly close with some sort of sandwich affair that may (but as you point out, due to absorption by other sandwich layers, may not) give me a clue in one test where the right thickness is of whatever the moderator part of the sandwich should be for the final design.

But I'm only well off, not insanely wealthy - and that much gold/silver/indium and the issues of getting many samples counted simultaneously right after a run are well, daunting, so for me it's not practical.
(very pure gold foils we obtained cost the equivalent of about $4k/oz, silver sheet was more like twice the bullion price, and indium is $99/.25lb at rotometals). But getting say 20 pancake detectors hooked to counters and cross calibrated isn't trivial either -- not to mention getting them all loaded and started together quickly after a run.

So I'm after something very particular here -- a moderator design that optimizes several eV neutrons as well as can be done, rather than just going all the way to thermal + tails. Or worse -- some thermals plus the rest full or nearly full energy, which is what I'd predict from a thin, mostly H based moderator (but only based on my gut). So I'm looking for someone who has the math to get me closer in a feedforward fashion so I can just tweak from there.
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Re: Activation measurement technique

Post by Rich Feldman »

Remember the fatal criticality accident in a Japanese uranium-processing plant, about 15 years ago? Every time the froth subsided in their acid tank, it would go critical again.

According to one technical report, the best estimate of total fission yield was from measuring activation of some coins found in the office.
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Re: Activation measurement technique

Post by Richard Hull »

The ideal is about 1.5"- 2" of moderator to place your neutrons in the resonance range for silver. Jon Rosenstiel did a complete exposition on this in the radiation forum some years back. This is what I use for silver to gain the max activation. It puts you a bit closer to the source, too.

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Re: Activation measurement technique

Post by Carl Willis »

Hi Doug

I last considered the silver activation problem in this reply to Robert Tubbs last year:

viewtopic.php?f=13&t=6021#p39812

The best thickness of moderator depends quite a bit on geometry. Localized sources have a lot of geometric attenuation of the flux close to the source, so ideal moderators are thinner if you're close to localized sources.

Sometimes other ideas for moderation arise in the context of improving neutron economy. Light moderators with low cross-sections, like heavy water or beryllium or graphite, where the moderation is due to elastic scattering, can sometimes replace hydrogenous media for better results. Inelastic scattering can sometimes be exploited to preferentially cause the loss of energy from high-speed neutrons; such moderators may contain fluorine, magnesium, or aluminum. But for neutron activation with a fusor, the most effective moderators will be made out of dense hydrogenous materials because of the geometry of the source and its neutron field.

Heating the activation target can cause Doppler broadening of the narrow capture resonances in the epithermal band, leading to a higher effective cross-section. I doubt this would have a noticeable effect for indium or silver activation with a fusor, but the phenomenon is reliably modeled in computer codes (see link) and can be checked easily. Similarly, the geometry in the problem shown can be changed to whatever you have in order to get a highly-accurate result. But in general, I concur with what Richard said earlier about the optimum moderator.

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Re: Activation measurement technique

Post by Doug Coulter »

Ah, finally a response to the actual question asked!

Yes, I agree that for many things about 1-1.5" of good old HDPE is great, and it's what I'm using now.

It works, but my best guess is that it's mainly the tail off the mainly thermal neutrons (and of course nearly all from the reflector side of the oven are going to be thermal) that are doing all the "work" on silver or indium, as their thermal cross sections are pretty small compared to their resonances, nearly 1k::1 ratio -- big enough to look seriously at getting some good out of.

But lets do a little reductio ad absurdium here to get at what I'm really asking/trying to find out.

Let us suppose we have a moderator so thin that we either see neutrons with one scattering event, or zero. For H type things, the math I have means we either have full energy, or ~2/3 energy neutrons at that point -- some completely unaffected, some greatly (but of course still nowhere near taking a 2.xxx MeV neutron down to 5v). That's the highest moderation per scattering you can get, and after not many, you have primarily thermal neutrons -- with a tail going upwards in energy, still some hotter ones.

So the tradeoff I'm interested in exploring here is more or less, would carbon (pure) vs hydrogen be better at getting a narrower net neutron distribution around 2-5eV than something lighter.

The lighter (H or D containing) moderator would need fewer scattering events to reduce the neutron energy, but since each removes so much energy, it might be harder to hit the target energy without going past it to thermal, or getting scattering at large angles (out of the moderator and lost entirely).

The heavier (carbon or similar that does moderate but not absorb) would take more scattering events so there "ought" to be a smoother drop to energy level desired, and a smaller distribution of energies at any point in the column length, so perhaps you could pick a spot in the colum where you got a more decent distribution of energies (for silver and its ilk) someplace -- but due to more scattering events needed to get to 5 eV or so, may also scatter more neutrons out of the moderator entirely.

This could "in theory" be improved by some kind of bragg scattering if the moderator column walls were made of the "right stuff in the right crystal orientation" for neutrons in the 1 to about 20ev range, you can get pretty decent reflections back into the moderator, again "in theory" in the literature.
I have seen neutron energy spectrometers working in this range using a crystal of something just like a prism would work for light (the internal mechanism is more like a diffraction grating, of course, but the results are the same).

So the real question is:
Is is possible, by any means described above, or any I've left out, to do a much better job of getting a fixed amount of 2-something MeV neutrons down to roughly 2-5ev than the HDPE at 1.5" or so?

My gut says "yes". My experimentalist side says "that's one heck of a job of testing to find out", therefore the request for prior data or good math.

Make this a little thicker so we have several scatterings per -- and we still have a "wide" distribution, as the loss per event is so high. But for some purposes maybe we don't want such a wide distribution,and of course the other side of the tradeoff is that the more scatterings there are (and the more energy lost in each) the more there are scattered right out of a finite volume moderator.

FWIW for those who haven't seen it, my moderator/oven looks like this:
viewtopic.php?f=13&t=6069&hilit=neutron+oven#p34513

One end is a curved cut in an HDPE rod that fits over the cylinder outside the grid, with a reflector over the top, between which I put samples. The whole thing is coated in lead to help cut down the X rays that would otherwise leak out through the plastic. Due to close proximity, it does a pretty good job of capturing neutrons, as the entrance subtends a fairly wide angle of the fusor output.
I have cut various HDPE spacers to put the samples either closer or farther from the entrance point, with the range mentioned by Richard being about the best -- in HDPE. The real question is, is HDPE really best for above-thermal results, or would something else (and maybe a different geometry) be better?

It seems well worth asking due to that 1000::1 ratio shown in the cross section plots -- and of course once you have a bunch of single digit eV neutrons, going the rest of the way to thermal for other tests isn't hard at all..

Carl's link above does not address this -- all the moderators used are hydrocarbons, no pure carbon or anything else tried. Are we really sure that those are the only, or best, usable substances for things like this?

I want to point out again, that when working with lower total neutron inputs (lab safety and all that) I can't just count silver or indium for longer to get a better estimate in the wildly varying cosmic background I have here, unless I first make it very hot. So, the point is getting more hot for less neutrons.
Which speeds up other testing as each run could be say one minute vs 5 or more (assuming you have a batch of fresh cold samples to use each run, which I do).
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Re: Activation measurement technique

Post by tligon »

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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Re: Activation measurement technique

Post by Carl Willis »

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.

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Re: Activation measurement technique

Post by Chris Bradley »

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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Re: Activation measurement technique

Post by Doug Coulter »

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.
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Re: Activation measurement technique

Post by Frank Sanns »

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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Re: Activation measurement technique

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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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Re: Activation measurement technique

Post by Carl Willis »

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.

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Re: Activation measurement technique

Post by Doug Coulter »

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.
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Re: Activation measurement technique

Post by Doug Coulter »

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.
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Re: Activation measurement technique

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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 ;~)
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Re: Activation measurement technique

Post by Frank Sanns »

Chris,

I was applying it to your chopper approach. One slit would be open but just behind it would be a solid that would destroy the linearity of the path of a neutron if it would happen to interact with it. The path then of a neutron through a series of spinning slits would not be a simple linear problem because no slit would be in "clear air" if you will and any collisions.

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Re: Activation measurement technique

Post by Richard Hull »

It is always nice to have a kind heart like Carl run the numbers at his level of knowledge and experience. However, even the big boys tend to tune for minimum smoke on these things. The best calcs only ball park and you will never ever know to 1% the number of neutrons you have there.

Since 1999 I came to the realization and instruction from neutron metrologists in conversations, as well as readings, that neutron counting and acquistion is a crap shoot where 10% precision is quite good in the sense of absolute numbers.

As we are looking at improvements in and modifications of the fusor environment, relative measurements based on fixed measurement scenarios can, indeed afford +/-2% or better RELATIVE neutron readings if the counts are high enough and instrumentation is bolted to the floor.

To tune for minimum smoke here, (maximum response), from a given moderator and detector geometry, we need a rigidly fixed neutron source so that relative improvements in moderator-detector design can be witnessed. Unfotunately, the fusor is not a good candidate here as no two runs, no matter how well regulated, warrant any reasonable degree of precision in the source.

Here is where even a crude but satisfactory neutron source would be of value. While not potent enough to rival a fusor's "numbers", you do have all the time in the world to collect data from each iteration and alteration of the activation-moderator-detector setup. Again, you are source limited and statisitcally bound, mathematically.

Probably playing with the moderator as Jon did way back when would the be the answer. He used a water tank moderator and could slide his silver and indium back and forth in the tank to increase or decrease his activation. (older posting)

Activation in fusor operation is proof positive beyond all doubt as to fusion taking place, but it is never going to be a quick and ready, non-statistical method for gauging operation the way an 3He tube will. While I might imagine a doubling of activation numbers between resonance-thremal activation and a delicately adjusted, fixed, resonance activator system, the effort would not compare to the numbers and relative indication of system improvements announced by a good He3 system.

Still, intellectually and construction wise, it would be an interesting effort, but Jon has already trod some of the ground long ago.

I attach an image of a simple test rig that might serve well for experimental location of ideal resonance activation for silver. The drawing is not to scale. I doubt if backing this 6 X 6 cube or even surrounding it with more moderator would increase the activation to any genuinely significant degree. Naturally, placing this almost in contact with the fusor would be the ideal. Quick removal and measurement is key to this operation.

Finally, if you did not mind counting some proton recoils with the activation, embedment, from the rear of a 2" pancake GM tube facing the foil in situ might also be an idea provided you do not have "shine through" x-rays from the fusor shell. If you did, the thinest piece of lead that would stop them might be employed between the fusor and the moderator assembly.

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[attachment=0]7419.resonance moderator.JPG[/attachment]
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Re: Activation measurement technique

Post by Doug Coulter »

Yes, indeed, I am quite grateful for Carl's willingness to help here.

As you say absolute measurements to a few percent are tough indeed, and this is acknowledged all over the literature.

As I stated above, either theory of isotropy in low energy DD collisions is dead wrong or the BTI's we've compared to one another are off by far more than that -- factor of 3 or more -- compared to each other on the *same* run, sitting touching side by side at the same distance from the action. That's not acceptable for that money and in my eyes, makes any "absolute" calibration of them or via them a complete waste of time. This was done with two brand new certified BTI's. Either they forgot how to make them right, or something else interesting is going on. I intend to find out which, and prove it beyond any question.

However, run-to-run measurements should be a lot better (with other techniques than BTI) if, as you say the "equipment is bolted to the floor", and relative measurement is very much more important to me, personally, than absolute, at this point. When things stop getting better as I make mods, and it seems I've reached some kind of plateau or am banging my head against some wall, I'll get more interested in absolute numbers to perhaps get a better clue as to why I am stuck and where I'm stuck.
But that hasn't happened yet -- each new thing I learn and mod I make based on that has improved the results so far.

I think you are missing a couple of important points here, Richard. One is that I am *not* looking for the ideal thickness/geometry of an H-containing moderator here at all; if I were, I already know the answers, found out the same way as you describe -- try a few things with a known reasonable starting range, which of course I got from information here (including yours) and from the literature.

What I *am* interested in here is something where very little, if any, work has *ever* been done (or at least published). Most all moderator work has been for fission reactors, where this energy region of a few eV is one they avoid like the plague, to keep from getting resonance neutron captures in the U238, so very little work has ever been done on the best design for producing this *particular* energy level from a fast neutron source -- it's normally an avoided range of energy for very good reasons if one is making a fission pile, unless you are working with the "leftover neutrons" to do breeding of plutonium in one (in which case the desired range would be determined by the U238 resonance cross section plot, not the ones shown above). After considerable search of the literature I was unable to find out some of the things Carl has generously offered to help investigate.

So I'm thanking him again in advance.

Why do I care? Again, I am trying to get a more sensitive measurement of neutron production for run to run comparisons. A cursory look at the plots above shows that compared to thermal (what everyone goes for in moderator design, more or less) is maybe almost 1000 times less effective to activate the common silver or indium we use for this than a narrow neutron energy distribution centered on the few eV range would be -- that's doggone significant, and implies that perhaps 1/10 of that factor may be possible to improve the sensitivity we get, or in other words, a lot more activation per neutron input. It may not be as good as 10x or it could be better than that, depending on the resulting energy distribution width we can achieve right where we (or the silver or indium) want it.

If I can get more good data for less radiation exposure for me and my lab, isn't that a good thing?
Getting it faster seems good too, as with other data logging going on, there's more chance that we find those good spots rather than average over a bunch of varying conditions -- so we know what to try and stabilize on "for the real thing". Far better info than "conditions changed all over during this 5 minutes and some average output from them all was X".

The implications of that are better statistical accuracy (at both activation and measurement ends of the chain), need for less neutrons in a run to get a decent reading, and just plain old making of a decent incremental advance in the art, which is always gratifying to certain types (me, at least).

Being able to make shorter runs and quicker measurements is a good thing, I believe, and as someone who runs a fusor often and long, you must know that keeping one right on the "sweet spot" for long periods is problematic even with DC input and a good pilot at the controls, so an activation test is more a measure of that -- than how good the sweet spot was for the couple moments you were right on it.

Further, despite the 3He and BF3 tubes being more or less ideal for a situation where the neutrons come out at random times (as in normal DC/equilibrium fusor operation), they fail miserably if the source is doing short, intense bursts, which mine happens to do in some modes I've found desirable while making *another* significant advance in our art -- much higher Q. The tubes mentioned (we have both here) fail miserably when all the neutrons come out in a couple of microseconds at most -- we measure some of the pulses as sub uS of power input. The simply detect the pulse rate, not the number of neutrons per pulse, as they cannot time-resolve that well. The net result is that in normal fusor operation, the tubes do track activation numbers really well here, but when in pulse mode, the activation numbers indicate that far more neutrons were produces than the tubes counted. In pulse modes, the BTI's track the activation results far better than the tubes do and probably for the same reasons -- fast bursts don't fool them like they can a proportional gas tube with long drift times.

In the pulsed case, the "tracking error" between the tubes and activation goes from something better than a couple percent to factors of 10 off and more, utterly unacceptable, the tubes are useless for this mode of operation, other than to count the pulse rate which is easily measured in other ways (like counting the pulse generator output which needs no radiation detector at all to do, but tells me nothing of the success rate of fusion).

So this is a fairly significant and new line of inquiry, and I think, worth doing, if it can give us the hoped for result, or any decent fraction thereof. Heck, it's worth doing even if it proves we can't get any improvement, as one of my mantras is "why guess, when you can know". If it's a dead end, why not prove it and be done with it? I don't think so, but that's what Carl will reveal when he gets a chance to do the math and enlighten us. Which I will then test experimentally (and which will probably agree with Carl's results) and it will then be a locked-down closed subject.

But not before -- until then it's just guessing.

As an adjunct to this, I am also constructing a beam-selecting one pixel neutron camera (unmoderated fast neutron detector with limited angular purview) just in case I'm seeing non-isotropy from the main reaction zone, which would explain some of the bizarre BTI results we've seen even in DC modes. But it would also indicate that a lot of the theory is wrong (or that some ions are getting to MeV energies someway), both of which I kind of doubt -- but mean to find out once and for all.

Besides, being able to see where the neutrons are really being produced in a fusor might be enlightening as its own thing -- so again, why guess?
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Re: Activation measurement technique

Post by Richard Hull »

Being able to hum in a fast neutron beam to a useful rather precise area of activation to within a few EV would be like shooting, off hand, a 22 bullet through a keyhole at 1000 yards. Just not really doable. yes you can optimise to a horribly generalized degree, but you will never warrant a truly hot usable zone so narrow, especially with the output of a fusor.

I attach a combo plot of silver 107 1nd 109 from the endif 300k library.

Viewing this plot we see you are getting decent, tight activation from 5000ev all the way to 10ev. Instead of trying to hum in on a tiny peak in the single units ev range, it would be interesting to use maybe 1/2" to 1" of HDPE with decent side and rear backing to compare against a deeper and more complete moderation. There appears to be thousands of resonances in a tight wad in the 3kev to 100ev range all at 100++ barns. Lots of slop room for all kinds of decent activation at a vast range of energies rather that trying for a 10X better cross section at the biggest, narrow peak. Only experiment would tell here.

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Re: Activation measurement technique

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OK - so , fantastic neutron detecting invention #2; this invention depends on maybeunobtanium, so I'm not sure if you can get it, but let's say there is a high-H content scintillating material with a reasonable light output in response to betas within the thickness of the maybeunobtanium layers.

So, what you do is build up a sandwich of layers of silver foil with thin sheets of this maybeunobtanium between them. As you have a block, the probability of neutrons bouncing around and slowing down to just the right levels for neutron activation of the silver foil will be very high (they will just keep on bouncing till they hit the right energy, or leave the block). The measurement of activation is taken from the light emitted from the polished edges of the sheets of maybeunobtanium, where the emissions from the many-surfaces of the silver foils will stimulate light emissions in the sheets which'll be guided, waveguide-like, through those sheets to their edges.

...A silver-foil-scintillation sandwich, with as many layers of silver as possible between sheets of maybeunobtanium. Is there any such thing as maybeunobtanium?
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