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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Frank Sanns
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Re: Activation measurement technique

Post by Frank Sanns » Tue May 18, 2010 4:28 am

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.

Frank

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Re: Activation measurement technique

Post by Richard Hull » Tue May 18, 2010 3:29 pm

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.

Richard Hull

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

Post by Doug Coulter » Tue May 18, 2010 5:34 pm

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?
Why guess when you can know? Measure!

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Re: Activation measurement technique

Post by Richard Hull » Tue May 18, 2010 6:52 pm

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.

Richard Hull
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Re: Activation measurement technique

Post by Chris Bradley » Tue May 18, 2010 7:21 pm

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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Re: Activation measurement technique

Post by Richard Hull » Tue May 18, 2010 7:29 pm

A better mouse trap....... attached image. Click image to view details

In this iteration you can actually work for best moderation and thus activation on the fly while running allowing a minute or two for the activation to build as you move the slider detector tank back and forth in the moderator.

I gotta' build this.

Richard Hull

[attachment=0]7422.activationtank.JPG[/attachment]
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Re: Activation measurement technique

Post by Doug Coulter » Tue May 18, 2010 8:01 pm

Thanks for the plot, which btw agrees with your idea that absolute measurement is hard. That first peak you show at ~15 eV, a K barn or so -- look at the plots I got from the other literature and see just how much they disagree....20 something barns vs a thousand? Wonder which one is the correct number?

Both are no doubt from the "vetted" literature (mine is from a conference in honor of T Bonner), which should inculcate a healthy disrespect for "authority", I think.

But of course it's area under the curve intersections (between the curve of distribution of neutron energies and that of cross sections) that matters here -- a bunch of 10-100 barn stuff merely means an average of 10+ barns with random distribution of energies, which has been good enough for most people in the past....

Your plot doesn't show down to the ~10k barn peak at 5 ev, though, since it starts at double that energy.

I very much realize that no way we can get a "narrow & tight" distribution right there. But better is better if we can have it, so why not try a little harder, that's all I'm asking.

I regularly shoot .223 bullets into chicken eggs at 500 & 1000 meters (it's somewhat easier with .308 of course, but a lot harder on my skinny shoulder at the bigger calibers) -- I am a competitive shooter, and last I checked held the all time record at hunter benchrest at Roanoke Rifle and Revolver club, having shot a perfect "possible" at 100,200, 300 yards, 120 shots total through X rings the size of pencil erasers at all three ranges -- less than one caliber absolute error in the whole string. Never been done before in 160 years of club history. But I did it. I admit, I didn't do that off-hand, I used a rest...and the best gun and ammo I could build here in the same shop I now make fusors in.
Why use inferior tools if you have a choice?

And I should add, I didn't stand out all that much at that rifle match, it's not that I'm all that special.
It was the X's not the score that decided. A couple other guys were right there with me.
I don't win them all either, just many of them. Impressions of difficulty formed a long time ago may not stay valid over long time periods, is the point of that anecdote.

Which, by the way, is better accuracy than needed to make one nucleus hit another in a beam on target device...in a fusor or a beam on target, there need be no wind, no barrel vibrations, no unevenness in the bullet weight or balance, case capacity, muzzle blast turbulence (which gets ahead of the bullet for a little while), and so on.... this fusion thing *should* be easier than that was, "in theory". Vastly oversimplified, I know, but clearly, advantage => fusion.

A lot of times the impossible gets done because someone said, "there's no way you can do that" to someone else who then said, oh yeah? Watch my smoke! I will take your various negative remarks on the forum in that spirit -- I know you in person too, so I know you're not actually all that negative! Just very conservative, and like all other humans, sometimes miss the point of the discussion.

And I AM that someone else...Never tell me something can't be done unless you want to see it done.

Hopefully soon ;~)
Why guess when you can know? Measure!

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Re: Activation measurement technique

Post by Doug Coulter » Tue May 18, 2010 8:29 pm

Again, I'm trying to skip the H moderator in favor of something hopefully better for getting a more tight and uniform distribution in the results.

I don't see the need for the extra step in what you describe here though. For a fast response fast neutron detector, a simple scintillator plastic does quite well, the main objection being keeping electrical noise out of the signal from the phototube takes some real good care. I have one here that works pretty well, actually, and the tube response is in the 5 ns range, the scintillator and any neutron straggling add to that somewhat, with no need to wait for something activated to decay. It's what I'm planning to use in the neutron "camera" as the detector, as it's small, light, and doesn't need moderation so I can have a definite field of view via a small hole through a large moderator it sits behind. I'm within a few days of testing that lashup, so expect some reports on it soon.

I normally set the threshold of my big 3he tube about double the norm, as I only want to see multiple hits on it, mainly to keep the count rate down to audibility on the audio amp I listen to it on. With a more normal threshold on this device I get about the same count rate -- and it only "sees" about a square inch, rather than the 2 feet by 6" moderator picking up neutrons for the 3He tube. So it's actually not bad at all. This is with 3mm lead wrapping the thing to keep all the X rays out, but it probably also sees the rare high energy gammas from that 1:1000 reaction path of DD->He + y.
For my purposes, that doesn't matter -- still means fusion when I get a count.

For grins, since I have it all torn down to finish it up, here's a pic of the device parts. Sigh, so many almost-finished projects around here, I'm sure you can relate.

The first is a look down the bore, which is 1/2" hole surrounded by borated wax and interspersed with cast Cd washers, about 6" long for that part. The back half of the lead tubing holds the phototube and scint plastic shown in the next pic, with a little UV present to show the plastic glowing. In front of the tube is 1" length of cast cerroshield, which also surrounds the tube about 3/8"thick to keep stray X rays out of it all - including the ones from the boron and Cd. There will of course be a lead end cover to keep light from going down the bore, and to stop low energy (eg power supply volts level) X rays from the fusor out of things.

While this will have no where near the spatial resolution that John Hendron was wishing for on another, older, thread, it should actually *work* -- get enough neutrons to get an idea where they are coming from to some extent as it is aimed around. Should I find that it is sensitive enough or has some to spare, I'll just make another tube with a smaller hole in the moderator/neutron eater. At the neutron rates coming out of the current fusor lashup, it should in fact have plenty sensitivity to spare, but it pays to be conservative at first -- to keep the poor guy who builds all this highly motivated (eg me ;~).

But even this isn't fast enough to resolve the bursts in burst mode...they come faster than 10ns or so per during a pulse. Hence the desire that started this thread in the first place.
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Re: Activation measurement technique

Post by Doug Coulter » Tue May 18, 2010 8:35 pm

Yes, you definitely should build that -- I am glad I'm getting things started here, that is always some of my intent when I post on the forum. I am an enthusiasm-junkie, I suppose.

Now if we could figure out how to build that with some non-H substance as the moderator, it would have answered all my questions, including the original one I asked, which was -- "how does the type of moderating substance affect the energy distribution of the resulting neutrons", rather than how much water or hydrocarbon would be ideal -- I knew that answer already and so did you -- you told me, and it was correct, I did check!

Heck, all I want is more....you know.
Why guess when you can know? Measure!

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Re: Activation measurement technique

Post by Richard Hull » Tue May 18, 2010 8:44 pm

Here is the endif plot for Ag109 which includes the wonderment spot you are agog about. To actually realize the value of the 10,000 barn peak you will have to pack as many neuts as possible into a totally warranted range of 4.5-5.5 ev window. If you are willing to suffer a loss range where most activations would be in the 1000 barn range with a few in the 10kilobarn range you will have to warrant a tight window of only 2 ev from about 4 ev to 6 ev. Tighter than a 26 inch girdle on a 250 pound lady. All the best on this using any scheme imaginable or at least cost effective for the amateur.

With no such thing or even possibility of a neutron beam from the fusor, the classic chopper is just not possible.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
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